1//===- Writer.cpp ---------------------------------------------------------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "Writer.h"
10#include "AArch64ErrataFix.h"
11#include "ARMErrataFix.h"
12#include "CallGraphSort.h"
13#include "Config.h"
14#include "InputFiles.h"
15#include "LinkerScript.h"
16#include "MapFile.h"
17#include "OutputSections.h"
18#include "Relocations.h"
19#include "SymbolTable.h"
20#include "Symbols.h"
21#include "SyntheticSections.h"
22#include "Target.h"
23#include "lld/Common/Arrays.h"
24#include "lld/Common/CommonLinkerContext.h"
25#include "lld/Common/Filesystem.h"
26#include "lld/Common/Strings.h"
27#include "llvm/ADT/STLExtras.h"
28#include "llvm/ADT/StringMap.h"
29#include "llvm/Support/BLAKE3.h"
30#include "llvm/Support/Parallel.h"
31#include "llvm/Support/RandomNumberGenerator.h"
32#include "llvm/Support/TimeProfiler.h"
33#include "llvm/Support/xxhash.h"
34#include <climits>
35
36#define DEBUG_TYPE "lld"
37
38using namespace llvm;
39using namespace llvm::ELF;
40using namespace llvm::object;
41using namespace llvm::support;
42using namespace llvm::support::endian;
43using namespace lld;
44using namespace lld::elf;
45
46namespace {
47// The writer writes a SymbolTable result to a file.
48template <class ELFT> class Writer {
49public:
50 LLVM_ELF_IMPORT_TYPES_ELFT(ELFT)
51
52 Writer() : buffer(errorHandler().outputBuffer) {}
53
54 void run();
55
56private:
57 void addSectionSymbols();
58 void sortSections();
59 void resolveShfLinkOrder();
60 void finalizeAddressDependentContent();
61 void optimizeBasicBlockJumps();
62 void sortInputSections();
63 void sortOrphanSections();
64 void finalizeSections();
65 void checkExecuteOnly();
66 void setReservedSymbolSections();
67
68 SmallVector<PhdrEntry *, 0> createPhdrs(Partition &part);
69 void addPhdrForSection(Partition &part, unsigned shType, unsigned pType,
70 unsigned pFlags);
71 void assignFileOffsets();
72 void assignFileOffsetsBinary();
73 void setPhdrs(Partition &part);
74 void checkSections();
75 void fixSectionAlignments();
76 void openFile();
77 void writeTrapInstr();
78 void writeHeader();
79 void writeSections();
80 void writeSectionsBinary();
81 void writeBuildId();
82
83 std::unique_ptr<FileOutputBuffer> &buffer;
84
85 void addRelIpltSymbols();
86 void addStartEndSymbols();
87 void addStartStopSymbols(OutputSection &osec);
88
89 uint64_t fileSize;
90 uint64_t sectionHeaderOff;
91};
92} // anonymous namespace
93
94template <class ELFT> void elf::writeResult() {
95 Writer<ELFT>().run();
96}
97
98static void removeEmptyPTLoad(SmallVector<PhdrEntry *, 0> &phdrs) {
99 auto it = std::stable_partition(
100 first: phdrs.begin(), last: phdrs.end(), pred: [&](const PhdrEntry *p) {
101 if (p->p_type != PT_LOAD)
102 return true;
103 if (!p->firstSec)
104 return false;
105 uint64_t size = p->lastSec->addr + p->lastSec->size - p->firstSec->addr;
106 return size != 0;
107 });
108
109 // Clear OutputSection::ptLoad for sections contained in removed
110 // segments.
111 DenseSet<PhdrEntry *> removed(it, phdrs.end());
112 for (OutputSection *sec : outputSections)
113 if (removed.count(V: sec->ptLoad))
114 sec->ptLoad = nullptr;
115 phdrs.erase(CS: it, CE: phdrs.end());
116}
117
118void elf::copySectionsIntoPartitions() {
119 SmallVector<InputSectionBase *, 0> newSections;
120 const size_t ehSize = ctx.ehInputSections.size();
121 for (unsigned part = 2; part != partitions.size() + 1; ++part) {
122 for (InputSectionBase *s : ctx.inputSections) {
123 if (!(s->flags & SHF_ALLOC) || !s->isLive() || s->type != SHT_NOTE)
124 continue;
125 auto *copy = make<InputSection>(args&: cast<InputSection>(Val&: *s));
126 copy->partition = part;
127 newSections.push_back(Elt: copy);
128 }
129 for (size_t i = 0; i != ehSize; ++i) {
130 assert(ctx.ehInputSections[i]->isLive());
131 auto *copy = make<EhInputSection>(args&: *ctx.ehInputSections[i]);
132 copy->partition = part;
133 ctx.ehInputSections.push_back(Elt: copy);
134 }
135 }
136
137 ctx.inputSections.insert(I: ctx.inputSections.end(), From: newSections.begin(),
138 To: newSections.end());
139}
140
141static Defined *addOptionalRegular(StringRef name, SectionBase *sec,
142 uint64_t val, uint8_t stOther = STV_HIDDEN) {
143 Symbol *s = symtab.find(name);
144 if (!s || s->isDefined() || s->isCommon())
145 return nullptr;
146
147 s->resolve(other: Defined{ctx.internalFile, StringRef(), STB_GLOBAL, stOther,
148 STT_NOTYPE, val,
149 /*size=*/0, sec});
150 s->isUsedInRegularObj = true;
151 return cast<Defined>(Val: s);
152}
153
154// The linker is expected to define some symbols depending on
155// the linking result. This function defines such symbols.
156void elf::addReservedSymbols() {
157 if (config->emachine == EM_MIPS) {
158 auto addAbsolute = [](StringRef name) {
159 Symbol *sym =
160 symtab.addSymbol(newSym: Defined{ctx.internalFile, name, STB_GLOBAL,
161 STV_HIDDEN, STT_NOTYPE, 0, 0, nullptr});
162 sym->isUsedInRegularObj = true;
163 return cast<Defined>(Val: sym);
164 };
165 // Define _gp for MIPS. st_value of _gp symbol will be updated by Writer
166 // so that it points to an absolute address which by default is relative
167 // to GOT. Default offset is 0x7ff0.
168 // See "Global Data Symbols" in Chapter 6 in the following document:
169 // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
170 ElfSym::mipsGp = addAbsolute("_gp");
171
172 // On MIPS O32 ABI, _gp_disp is a magic symbol designates offset between
173 // start of function and 'gp' pointer into GOT.
174 if (symtab.find(name: "_gp_disp"))
175 ElfSym::mipsGpDisp = addAbsolute("_gp_disp");
176
177 // The __gnu_local_gp is a magic symbol equal to the current value of 'gp'
178 // pointer. This symbol is used in the code generated by .cpload pseudo-op
179 // in case of using -mno-shared option.
180 // https://sourceware.org/ml/binutils/2004-12/msg00094.html
181 if (symtab.find(name: "__gnu_local_gp"))
182 ElfSym::mipsLocalGp = addAbsolute("__gnu_local_gp");
183 } else if (config->emachine == EM_PPC) {
184 // glibc *crt1.o has a undefined reference to _SDA_BASE_. Since we don't
185 // support Small Data Area, define it arbitrarily as 0.
186 addOptionalRegular(name: "_SDA_BASE_", sec: nullptr, val: 0, stOther: STV_HIDDEN);
187 } else if (config->emachine == EM_PPC64) {
188 addPPC64SaveRestore();
189 }
190
191 // The Power Architecture 64-bit v2 ABI defines a TableOfContents (TOC) which
192 // combines the typical ELF GOT with the small data sections. It commonly
193 // includes .got .toc .sdata .sbss. The .TOC. symbol replaces both
194 // _GLOBAL_OFFSET_TABLE_ and _SDA_BASE_ from the 32-bit ABI. It is used to
195 // represent the TOC base which is offset by 0x8000 bytes from the start of
196 // the .got section.
197 // We do not allow _GLOBAL_OFFSET_TABLE_ to be defined by input objects as the
198 // correctness of some relocations depends on its value.
199 StringRef gotSymName =
200 (config->emachine == EM_PPC64) ? ".TOC." : "_GLOBAL_OFFSET_TABLE_";
201
202 if (Symbol *s = symtab.find(name: gotSymName)) {
203 if (s->isDefined()) {
204 error(msg: toString(f: s->file) + " cannot redefine linker defined symbol '" +
205 gotSymName + "'");
206 return;
207 }
208
209 uint64_t gotOff = 0;
210 if (config->emachine == EM_PPC64)
211 gotOff = 0x8000;
212
213 s->resolve(other: Defined{ctx.internalFile, StringRef(), STB_GLOBAL, STV_HIDDEN,
214 STT_NOTYPE, gotOff, /*size=*/0, Out::elfHeader});
215 ElfSym::globalOffsetTable = cast<Defined>(Val: s);
216 }
217
218 // __ehdr_start is the location of ELF file headers. Note that we define
219 // this symbol unconditionally even when using a linker script, which
220 // differs from the behavior implemented by GNU linker which only define
221 // this symbol if ELF headers are in the memory mapped segment.
222 addOptionalRegular(name: "__ehdr_start", sec: Out::elfHeader, val: 0, stOther: STV_HIDDEN);
223
224 // __executable_start is not documented, but the expectation of at
225 // least the Android libc is that it points to the ELF header.
226 addOptionalRegular(name: "__executable_start", sec: Out::elfHeader, val: 0, stOther: STV_HIDDEN);
227
228 // __dso_handle symbol is passed to cxa_finalize as a marker to identify
229 // each DSO. The address of the symbol doesn't matter as long as they are
230 // different in different DSOs, so we chose the start address of the DSO.
231 addOptionalRegular(name: "__dso_handle", sec: Out::elfHeader, val: 0, stOther: STV_HIDDEN);
232
233 // If linker script do layout we do not need to create any standard symbols.
234 if (script->hasSectionsCommand)
235 return;
236
237 auto add = [](StringRef s, int64_t pos) {
238 return addOptionalRegular(name: s, sec: Out::elfHeader, val: pos, stOther: STV_DEFAULT);
239 };
240
241 ElfSym::bss = add("__bss_start", 0);
242 ElfSym::end1 = add("end", -1);
243 ElfSym::end2 = add("_end", -1);
244 ElfSym::etext1 = add("etext", -1);
245 ElfSym::etext2 = add("_etext", -1);
246 ElfSym::edata1 = add("edata", -1);
247 ElfSym::edata2 = add("_edata", -1);
248}
249
250static void demoteDefined(Defined &sym, DenseMap<SectionBase *, size_t> &map) {
251 if (map.empty())
252 for (auto [i, sec] : llvm::enumerate(First: sym.file->getSections()))
253 map.try_emplace(Key: sec, Args&: i);
254 // Change WEAK to GLOBAL so that if a scanned relocation references sym,
255 // maybeReportUndefined will report an error.
256 uint8_t binding = sym.isWeak() ? uint8_t(STB_GLOBAL) : sym.binding;
257 Undefined(sym.file, sym.getName(), binding, sym.stOther, sym.type,
258 /*discardedSecIdx=*/map.lookup(Val: sym.section))
259 .overwrite(sym);
260 // Eliminate from the symbol table, otherwise we would leave an undefined
261 // symbol if the symbol is unreferenced in the absence of GC.
262 sym.isUsedInRegularObj = false;
263}
264
265// If all references to a DSO happen to be weak, the DSO is not added to
266// DT_NEEDED. If that happens, replace ShardSymbol with Undefined to avoid
267// dangling references to an unneeded DSO. Use a weak binding to avoid
268// --no-allow-shlib-undefined diagnostics. Similarly, demote lazy symbols.
269//
270// In addition, demote symbols defined in discarded sections, so that
271// references to /DISCARD/ discarded symbols will lead to errors.
272static void demoteSymbolsAndComputeIsPreemptible() {
273 llvm::TimeTraceScope timeScope("Demote symbols");
274 DenseMap<InputFile *, DenseMap<SectionBase *, size_t>> sectionIndexMap;
275 for (Symbol *sym : symtab.getSymbols()) {
276 if (auto *d = dyn_cast<Defined>(Val: sym)) {
277 if (d->section && !d->section->isLive())
278 demoteDefined(sym&: *d, map&: sectionIndexMap[d->file]);
279 } else {
280 auto *s = dyn_cast<SharedSymbol>(Val: sym);
281 if (sym->isLazy() || (s && !cast<SharedFile>(Val: s->file)->isNeeded)) {
282 uint8_t binding = sym->isLazy() ? sym->binding : uint8_t(STB_WEAK);
283 Undefined(ctx.internalFile, sym->getName(), binding, sym->stOther,
284 sym->type)
285 .overwrite(sym&: *sym);
286 sym->versionId = VER_NDX_GLOBAL;
287 }
288 }
289
290 if (config->hasDynSymTab)
291 sym->isPreemptible = computeIsPreemptible(sym: *sym);
292 }
293}
294
295static OutputSection *findSection(StringRef name, unsigned partition = 1) {
296 for (SectionCommand *cmd : script->sectionCommands)
297 if (auto *osd = dyn_cast<OutputDesc>(Val: cmd))
298 if (osd->osec.name == name && osd->osec.partition == partition)
299 return &osd->osec;
300 return nullptr;
301}
302
303// The main function of the writer.
304template <class ELFT> void Writer<ELFT>::run() {
305 // Now that we have a complete set of output sections. This function
306 // completes section contents. For example, we need to add strings
307 // to the string table, and add entries to .got and .plt.
308 // finalizeSections does that.
309 finalizeSections();
310 checkExecuteOnly();
311
312 // If --compressed-debug-sections is specified, compress .debug_* sections.
313 // Do it right now because it changes the size of output sections.
314 for (OutputSection *sec : outputSections)
315 sec->maybeCompress<ELFT>();
316
317 if (script->hasSectionsCommand)
318 script->allocateHeaders(phdrs&: mainPart->phdrs);
319
320 // Remove empty PT_LOAD to avoid causing the dynamic linker to try to mmap a
321 // 0 sized region. This has to be done late since only after assignAddresses
322 // we know the size of the sections.
323 for (Partition &part : partitions)
324 removeEmptyPTLoad(phdrs&: part.phdrs);
325
326 if (!config->oFormatBinary)
327 assignFileOffsets();
328 else
329 assignFileOffsetsBinary();
330
331 for (Partition &part : partitions)
332 setPhdrs(part);
333
334 // Handle --print-map(-M)/--Map and --cref. Dump them before checkSections()
335 // because the files may be useful in case checkSections() or openFile()
336 // fails, for example, due to an erroneous file size.
337 writeMapAndCref();
338
339 // Handle --print-memory-usage option.
340 if (config->printMemoryUsage)
341 script->printMemoryUsage(os&: lld::outs());
342
343 if (config->checkSections)
344 checkSections();
345
346 // It does not make sense try to open the file if we have error already.
347 if (errorCount())
348 return;
349
350 {
351 llvm::TimeTraceScope timeScope("Write output file");
352 // Write the result down to a file.
353 openFile();
354 if (errorCount())
355 return;
356
357 if (!config->oFormatBinary) {
358 if (config->zSeparate != SeparateSegmentKind::None)
359 writeTrapInstr();
360 writeHeader();
361 writeSections();
362 } else {
363 writeSectionsBinary();
364 }
365
366 // Backfill .note.gnu.build-id section content. This is done at last
367 // because the content is usually a hash value of the entire output file.
368 writeBuildId();
369 if (errorCount())
370 return;
371
372 if (auto e = buffer->commit())
373 fatal(msg: "failed to write output '" + buffer->getPath() +
374 "': " + toString(E: std::move(e)));
375
376 if (!config->cmseOutputLib.empty())
377 writeARMCmseImportLib<ELFT>();
378 }
379}
380
381template <class ELFT, class RelTy>
382static void markUsedLocalSymbolsImpl(ObjFile<ELFT> *file,
383 llvm::ArrayRef<RelTy> rels) {
384 for (const RelTy &rel : rels) {
385 Symbol &sym = file->getRelocTargetSym(rel);
386 if (sym.isLocal())
387 sym.used = true;
388 }
389}
390
391// The function ensures that the "used" field of local symbols reflects the fact
392// that the symbol is used in a relocation from a live section.
393template <class ELFT> static void markUsedLocalSymbols() {
394 // With --gc-sections, the field is already filled.
395 // See MarkLive<ELFT>::resolveReloc().
396 if (config->gcSections)
397 return;
398 for (ELFFileBase *file : ctx.objectFiles) {
399 ObjFile<ELFT> *f = cast<ObjFile<ELFT>>(file);
400 for (InputSectionBase *s : f->getSections()) {
401 InputSection *isec = dyn_cast_or_null<InputSection>(Val: s);
402 if (!isec)
403 continue;
404 if (isec->type == SHT_REL) {
405 markUsedLocalSymbolsImpl(f, isec->getDataAs<typename ELFT::Rel>());
406 } else if (isec->type == SHT_RELA) {
407 markUsedLocalSymbolsImpl(f, isec->getDataAs<typename ELFT::Rela>());
408 } else if (isec->type == SHT_CREL) {
409 // The is64=true variant also works with ELF32 since only the r_symidx
410 // member is used.
411 for (Elf_Crel_Impl<true> r : RelocsCrel<true>(isec->content_)) {
412 Symbol &sym = file->getSymbol(symbolIndex: r.r_symidx);
413 if (sym.isLocal())
414 sym.used = true;
415 }
416 }
417 }
418 }
419}
420
421static bool shouldKeepInSymtab(const Defined &sym) {
422 if (sym.isSection())
423 return false;
424
425 // If --emit-reloc or -r is given, preserve symbols referenced by relocations
426 // from live sections.
427 if (sym.used && config->copyRelocs)
428 return true;
429
430 // Exclude local symbols pointing to .ARM.exidx sections.
431 // They are probably mapping symbols "$d", which are optional for these
432 // sections. After merging the .ARM.exidx sections, some of these symbols
433 // may become dangling. The easiest way to avoid the issue is not to add
434 // them to the symbol table from the beginning.
435 if (config->emachine == EM_ARM && sym.section &&
436 sym.section->type == SHT_ARM_EXIDX)
437 return false;
438
439 if (config->discard == DiscardPolicy::None)
440 return true;
441 if (config->discard == DiscardPolicy::All)
442 return false;
443
444 // In ELF assembly .L symbols are normally discarded by the assembler.
445 // If the assembler fails to do so, the linker discards them if
446 // * --discard-locals is used.
447 // * The symbol is in a SHF_MERGE section, which is normally the reason for
448 // the assembler keeping the .L symbol.
449 if (sym.getName().starts_with(Prefix: ".L") &&
450 (config->discard == DiscardPolicy::Locals ||
451 (sym.section && (sym.section->flags & SHF_MERGE))))
452 return false;
453 return true;
454}
455
456bool lld::elf::includeInSymtab(const Symbol &b) {
457 if (auto *d = dyn_cast<Defined>(Val: &b)) {
458 // Always include absolute symbols.
459 SectionBase *sec = d->section;
460 if (!sec)
461 return true;
462 assert(sec->isLive());
463
464 if (auto *s = dyn_cast<MergeInputSection>(Val: sec))
465 return s->getSectionPiece(offset: d->value).live;
466 return true;
467 }
468 return b.used || !config->gcSections;
469}
470
471// Scan local symbols to:
472//
473// - demote symbols defined relative to /DISCARD/ discarded input sections so
474// that relocations referencing them will lead to errors.
475// - copy eligible symbols to .symTab
476static void demoteAndCopyLocalSymbols() {
477 llvm::TimeTraceScope timeScope("Add local symbols");
478 for (ELFFileBase *file : ctx.objectFiles) {
479 DenseMap<SectionBase *, size_t> sectionIndexMap;
480 for (Symbol *b : file->getLocalSymbols()) {
481 assert(b->isLocal() && "should have been caught in initializeSymbols()");
482 auto *dr = dyn_cast<Defined>(Val: b);
483 if (!dr)
484 continue;
485
486 if (dr->section && !dr->section->isLive())
487 demoteDefined(sym&: *dr, map&: sectionIndexMap);
488 else if (in.symTab && includeInSymtab(b: *b) && shouldKeepInSymtab(sym: *dr))
489 in.symTab->addSymbol(sym: b);
490 }
491 }
492}
493
494// Create a section symbol for each output section so that we can represent
495// relocations that point to the section. If we know that no relocation is
496// referring to a section (that happens if the section is a synthetic one), we
497// don't create a section symbol for that section.
498template <class ELFT> void Writer<ELFT>::addSectionSymbols() {
499 for (SectionCommand *cmd : script->sectionCommands) {
500 auto *osd = dyn_cast<OutputDesc>(Val: cmd);
501 if (!osd)
502 continue;
503 OutputSection &osec = osd->osec;
504 InputSectionBase *isec = nullptr;
505 // Iterate over all input sections and add a STT_SECTION symbol if any input
506 // section may be a relocation target.
507 for (SectionCommand *cmd : osec.commands) {
508 auto *isd = dyn_cast<InputSectionDescription>(Val: cmd);
509 if (!isd)
510 continue;
511 for (InputSectionBase *s : isd->sections) {
512 // Relocations are not using REL[A] section symbols.
513 if (isStaticRelSecType(type: s->type))
514 continue;
515
516 // Unlike other synthetic sections, mergeable output sections contain
517 // data copied from input sections, and there may be a relocation
518 // pointing to its contents if -r or --emit-reloc is given.
519 if (isa<SyntheticSection>(Val: s) && !(s->flags & SHF_MERGE))
520 continue;
521
522 isec = s;
523 break;
524 }
525 }
526 if (!isec)
527 continue;
528
529 // Set the symbol to be relative to the output section so that its st_value
530 // equals the output section address. Note, there may be a gap between the
531 // start of the output section and isec.
532 in.symTab->addSymbol(sym: makeDefined(args&: isec->file, args: "", args: STB_LOCAL, /*stOther=*/args: 0,
533 args: STT_SECTION,
534 /*value=*/args: 0, /*size=*/args: 0, args: &osec));
535 }
536}
537
538// Today's loaders have a feature to make segments read-only after
539// processing dynamic relocations to enhance security. PT_GNU_RELRO
540// is defined for that.
541//
542// This function returns true if a section needs to be put into a
543// PT_GNU_RELRO segment.
544static bool isRelroSection(const OutputSection *sec) {
545 if (!config->zRelro)
546 return false;
547 if (sec->relro)
548 return true;
549
550 uint64_t flags = sec->flags;
551
552 // Non-allocatable or non-writable sections don't need RELRO because
553 // they are not writable or not even mapped to memory in the first place.
554 // RELRO is for sections that are essentially read-only but need to
555 // be writable only at process startup to allow dynamic linker to
556 // apply relocations.
557 if (!(flags & SHF_ALLOC) || !(flags & SHF_WRITE))
558 return false;
559
560 // Once initialized, TLS data segments are used as data templates
561 // for a thread-local storage. For each new thread, runtime
562 // allocates memory for a TLS and copy templates there. No thread
563 // are supposed to use templates directly. Thus, it can be in RELRO.
564 if (flags & SHF_TLS)
565 return true;
566
567 // .init_array, .preinit_array and .fini_array contain pointers to
568 // functions that are executed on process startup or exit. These
569 // pointers are set by the static linker, and they are not expected
570 // to change at runtime. But if you are an attacker, you could do
571 // interesting things by manipulating pointers in .fini_array, for
572 // example. So they are put into RELRO.
573 uint32_t type = sec->type;
574 if (type == SHT_INIT_ARRAY || type == SHT_FINI_ARRAY ||
575 type == SHT_PREINIT_ARRAY)
576 return true;
577
578 // .got contains pointers to external symbols. They are resolved by
579 // the dynamic linker when a module is loaded into memory, and after
580 // that they are not expected to change. So, it can be in RELRO.
581 if (in.got && sec == in.got->getParent())
582 return true;
583
584 // .toc is a GOT-ish section for PowerPC64. Their contents are accessed
585 // through r2 register, which is reserved for that purpose. Since r2 is used
586 // for accessing .got as well, .got and .toc need to be close enough in the
587 // virtual address space. Usually, .toc comes just after .got. Since we place
588 // .got into RELRO, .toc needs to be placed into RELRO too.
589 if (sec->name == ".toc")
590 return true;
591
592 // .got.plt contains pointers to external function symbols. They are
593 // by default resolved lazily, so we usually cannot put it into RELRO.
594 // However, if "-z now" is given, the lazy symbol resolution is
595 // disabled, which enables us to put it into RELRO.
596 if (sec == in.gotPlt->getParent())
597 return config->zNow;
598
599 if (in.relroPadding && sec == in.relroPadding->getParent())
600 return true;
601
602 // .dynamic section contains data for the dynamic linker, and
603 // there's no need to write to it at runtime, so it's better to put
604 // it into RELRO.
605 if (sec->name == ".dynamic")
606 return true;
607
608 // Sections with some special names are put into RELRO. This is a
609 // bit unfortunate because section names shouldn't be significant in
610 // ELF in spirit. But in reality many linker features depend on
611 // magic section names.
612 StringRef s = sec->name;
613
614 bool abiAgnostic = s == ".data.rel.ro" || s == ".bss.rel.ro" ||
615 s == ".ctors" || s == ".dtors" || s == ".jcr" ||
616 s == ".eh_frame" || s == ".fini_array" ||
617 s == ".init_array" || s == ".preinit_array";
618
619 bool abiSpecific =
620 config->osabi == ELFOSABI_OPENBSD && s == ".openbsd.randomdata";
621
622 return abiAgnostic || abiSpecific;
623}
624
625// We compute a rank for each section. The rank indicates where the
626// section should be placed in the file. Instead of using simple
627// numbers (0,1,2...), we use a series of flags. One for each decision
628// point when placing the section.
629// Using flags has two key properties:
630// * It is easy to check if a give branch was taken.
631// * It is easy two see how similar two ranks are (see getRankProximity).
632enum RankFlags {
633 RF_NOT_ADDR_SET = 1 << 27,
634 RF_NOT_ALLOC = 1 << 26,
635 RF_PARTITION = 1 << 18, // Partition number (8 bits)
636 RF_LARGE_ALT = 1 << 15,
637 RF_WRITE = 1 << 14,
638 RF_EXEC_WRITE = 1 << 13,
639 RF_EXEC = 1 << 12,
640 RF_RODATA = 1 << 11,
641 RF_LARGE = 1 << 10,
642 RF_NOT_RELRO = 1 << 9,
643 RF_NOT_TLS = 1 << 8,
644 RF_BSS = 1 << 7,
645};
646
647unsigned elf::getSectionRank(OutputSection &osec) {
648 unsigned rank = osec.partition * RF_PARTITION;
649
650 // We want to put section specified by -T option first, so we
651 // can start assigning VA starting from them later.
652 if (config->sectionStartMap.count(Key: osec.name))
653 return rank;
654 rank |= RF_NOT_ADDR_SET;
655
656 // Allocatable sections go first to reduce the total PT_LOAD size and
657 // so debug info doesn't change addresses in actual code.
658 if (!(osec.flags & SHF_ALLOC))
659 return rank | RF_NOT_ALLOC;
660
661 // Sort sections based on their access permission in the following
662 // order: R, RX, RXW, RW(RELRO), RW(non-RELRO).
663 //
664 // Read-only sections come first such that they go in the PT_LOAD covering the
665 // program headers at the start of the file.
666 //
667 // The layout for writable sections is PT_LOAD(PT_GNU_RELRO(.data.rel.ro
668 // .bss.rel.ro) | .data .bss), where | marks where page alignment happens.
669 // An alternative ordering is PT_LOAD(.data | PT_GNU_RELRO( .data.rel.ro
670 // .bss.rel.ro) | .bss), but it may waste more bytes due to 2 alignment
671 // places.
672 bool isExec = osec.flags & SHF_EXECINSTR;
673 bool isWrite = osec.flags & SHF_WRITE;
674
675 if (!isWrite && !isExec) {
676 // Among PROGBITS sections, place .lrodata further from .text.
677 // For -z lrodata-after-bss, place .lrodata after .lbss like GNU ld. This
678 // layout has one extra PT_LOAD, but alleviates relocation overflow
679 // pressure for absolute relocations referencing small data from -fno-pic
680 // relocatable files.
681 if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64)
682 rank |= config->zLrodataAfterBss ? RF_LARGE_ALT : 0;
683 else
684 rank |= config->zLrodataAfterBss ? 0 : RF_LARGE;
685
686 if (osec.type == SHT_LLVM_PART_EHDR)
687 ;
688 else if (osec.type == SHT_LLVM_PART_PHDR)
689 rank |= 1;
690 else if (osec.name == ".interp")
691 rank |= 2;
692 // Put .note sections at the beginning so that they are likely to be
693 // included in a truncate core file. In particular, .note.gnu.build-id, if
694 // available, can identify the object file.
695 else if (osec.type == SHT_NOTE)
696 rank |= 3;
697 // Make PROGBITS sections (e.g .rodata .eh_frame) closer to .text to
698 // alleviate relocation overflow pressure. Large special sections such as
699 // .dynstr and .dynsym can be away from .text.
700 else if (osec.type != SHT_PROGBITS)
701 rank |= 4;
702 else
703 rank |= RF_RODATA;
704 } else if (isExec) {
705 rank |= isWrite ? RF_EXEC_WRITE : RF_EXEC;
706 } else {
707 rank |= RF_WRITE;
708 // The TLS initialization block needs to be a single contiguous block. Place
709 // TLS sections directly before the other RELRO sections.
710 if (!(osec.flags & SHF_TLS))
711 rank |= RF_NOT_TLS;
712 if (isRelroSection(sec: &osec))
713 osec.relro = true;
714 else
715 rank |= RF_NOT_RELRO;
716 // Place .ldata and .lbss after .bss. Making .bss closer to .text
717 // alleviates relocation overflow pressure.
718 // For -z lrodata-after-bss, place .lbss/.lrodata/.ldata after .bss.
719 // .bss/.lbss being adjacent reuses the NOBITS size optimization.
720 if (osec.flags & SHF_X86_64_LARGE && config->emachine == EM_X86_64) {
721 rank |= config->zLrodataAfterBss
722 ? (osec.type == SHT_NOBITS ? 1 : RF_LARGE_ALT)
723 : RF_LARGE;
724 }
725 }
726
727 // Within TLS sections, or within other RelRo sections, or within non-RelRo
728 // sections, place non-NOBITS sections first.
729 if (osec.type == SHT_NOBITS)
730 rank |= RF_BSS;
731
732 // Some architectures have additional ordering restrictions for sections
733 // within the same PT_LOAD.
734 if (config->emachine == EM_PPC64) {
735 // PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections
736 // that we would like to make sure appear is a specific order to maximize
737 // their coverage by a single signed 16-bit offset from the TOC base
738 // pointer.
739 StringRef name = osec.name;
740 if (name == ".got")
741 rank |= 1;
742 else if (name == ".toc")
743 rank |= 2;
744 }
745
746 if (config->emachine == EM_MIPS) {
747 if (osec.name != ".got")
748 rank |= 1;
749 // All sections with SHF_MIPS_GPREL flag should be grouped together
750 // because data in these sections is addressable with a gp relative address.
751 if (osec.flags & SHF_MIPS_GPREL)
752 rank |= 2;
753 }
754
755 if (config->emachine == EM_RISCV) {
756 // .sdata and .sbss are placed closer to make GP relaxation more profitable
757 // and match GNU ld.
758 StringRef name = osec.name;
759 if (name == ".sdata" || (osec.type == SHT_NOBITS && name != ".sbss"))
760 rank |= 1;
761 }
762
763 return rank;
764}
765
766static bool compareSections(const SectionCommand *aCmd,
767 const SectionCommand *bCmd) {
768 const OutputSection *a = &cast<OutputDesc>(Val: aCmd)->osec;
769 const OutputSection *b = &cast<OutputDesc>(Val: bCmd)->osec;
770
771 if (a->sortRank != b->sortRank)
772 return a->sortRank < b->sortRank;
773
774 if (!(a->sortRank & RF_NOT_ADDR_SET))
775 return config->sectionStartMap.lookup(Key: a->name) <
776 config->sectionStartMap.lookup(Key: b->name);
777 return false;
778}
779
780void PhdrEntry::add(OutputSection *sec) {
781 lastSec = sec;
782 if (!firstSec)
783 firstSec = sec;
784 p_align = std::max(a: p_align, b: sec->addralign);
785 if (p_type == PT_LOAD)
786 sec->ptLoad = this;
787}
788
789// A statically linked position-dependent executable should only contain
790// IRELATIVE relocations and no other dynamic relocations. Encapsulation symbols
791// __rel[a]_iplt_{start,end} will be defined for .rel[a].dyn, to be
792// processed by the libc runtime. Other executables or DSOs use dynamic tags
793// instead.
794template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
795 if (config->isPic)
796 return;
797
798 // __rela_iplt_{start,end} are initially defined relative to dummy section 0.
799 // We'll override Out::elfHeader with relaDyn later when we are sure that
800 // .rela.dyn will be present in the output.
801 std::string name = config->isRela ? "__rela_iplt_start" : "__rel_iplt_start";
802 ElfSym::relaIpltStart =
803 addOptionalRegular(name, sec: Out::elfHeader, val: 0, stOther: STV_HIDDEN);
804 name.replace(pos: name.size() - 5, n1: 5, s: "end");
805 ElfSym::relaIpltEnd = addOptionalRegular(name, sec: Out::elfHeader, val: 0, stOther: STV_HIDDEN);
806}
807
808// This function generates assignments for predefined symbols (e.g. _end or
809// _etext) and inserts them into the commands sequence to be processed at the
810// appropriate time. This ensures that the value is going to be correct by the
811// time any references to these symbols are processed and is equivalent to
812// defining these symbols explicitly in the linker script.
813template <class ELFT> void Writer<ELFT>::setReservedSymbolSections() {
814 if (ElfSym::globalOffsetTable) {
815 // The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention usually
816 // to the start of the .got or .got.plt section.
817 InputSection *sec = in.gotPlt.get();
818 if (!target->gotBaseSymInGotPlt)
819 sec = in.mipsGot ? cast<InputSection>(Val: in.mipsGot.get())
820 : cast<InputSection>(Val: in.got.get());
821 ElfSym::globalOffsetTable->section = sec;
822 }
823
824 // .rela_iplt_{start,end} mark the start and the end of .rel[a].dyn.
825 if (ElfSym::relaIpltStart && mainPart->relaDyn->isNeeded()) {
826 ElfSym::relaIpltStart->section = mainPart->relaDyn.get();
827 ElfSym::relaIpltEnd->section = mainPart->relaDyn.get();
828 ElfSym::relaIpltEnd->value = mainPart->relaDyn->getSize();
829 }
830
831 PhdrEntry *last = nullptr;
832 OutputSection *lastRO = nullptr;
833 auto isLarge = [](OutputSection *osec) {
834 return config->emachine == EM_X86_64 && osec->flags & SHF_X86_64_LARGE;
835 };
836 for (Partition &part : partitions) {
837 for (PhdrEntry *p : part.phdrs) {
838 if (p->p_type != PT_LOAD)
839 continue;
840 last = p;
841 if (!(p->p_flags & PF_W) && p->lastSec && !isLarge(p->lastSec))
842 lastRO = p->lastSec;
843 }
844 }
845
846 if (lastRO) {
847 // _etext is the first location after the last read-only loadable segment
848 // that does not contain large sections.
849 if (ElfSym::etext1)
850 ElfSym::etext1->section = lastRO;
851 if (ElfSym::etext2)
852 ElfSym::etext2->section = lastRO;
853 }
854
855 if (last) {
856 // _edata points to the end of the last non-large mapped initialized
857 // section.
858 OutputSection *edata = nullptr;
859 for (OutputSection *os : outputSections) {
860 if (os->type != SHT_NOBITS && !isLarge(os))
861 edata = os;
862 if (os == last->lastSec)
863 break;
864 }
865
866 if (ElfSym::edata1)
867 ElfSym::edata1->section = edata;
868 if (ElfSym::edata2)
869 ElfSym::edata2->section = edata;
870
871 // _end is the first location after the uninitialized data region.
872 if (ElfSym::end1)
873 ElfSym::end1->section = last->lastSec;
874 if (ElfSym::end2)
875 ElfSym::end2->section = last->lastSec;
876 }
877
878 if (ElfSym::bss) {
879 // On RISC-V, set __bss_start to the start of .sbss if present.
880 OutputSection *sbss =
881 config->emachine == EM_RISCV ? findSection(name: ".sbss") : nullptr;
882 ElfSym::bss->section = sbss ? sbss : findSection(name: ".bss");
883 }
884
885 // Setup MIPS _gp_disp/__gnu_local_gp symbols which should
886 // be equal to the _gp symbol's value.
887 if (ElfSym::mipsGp) {
888 // Find GP-relative section with the lowest address
889 // and use this address to calculate default _gp value.
890 for (OutputSection *os : outputSections) {
891 if (os->flags & SHF_MIPS_GPREL) {
892 ElfSym::mipsGp->section = os;
893 ElfSym::mipsGp->value = 0x7ff0;
894 break;
895 }
896 }
897 }
898}
899
900// We want to find how similar two ranks are.
901// The more branches in getSectionRank that match, the more similar they are.
902// Since each branch corresponds to a bit flag, we can just use
903// countLeadingZeros.
904static int getRankProximity(OutputSection *a, SectionCommand *b) {
905 auto *osd = dyn_cast<OutputDesc>(Val: b);
906 return (osd && osd->osec.hasInputSections)
907 ? llvm::countl_zero(Val: a->sortRank ^ osd->osec.sortRank)
908 : -1;
909}
910
911// When placing orphan sections, we want to place them after symbol assignments
912// so that an orphan after
913// begin_foo = .;
914// foo : { *(foo) }
915// end_foo = .;
916// doesn't break the intended meaning of the begin/end symbols.
917// We don't want to go over sections since findOrphanPos is the
918// one in charge of deciding the order of the sections.
919// We don't want to go over changes to '.', since doing so in
920// rx_sec : { *(rx_sec) }
921// . = ALIGN(0x1000);
922// /* The RW PT_LOAD starts here*/
923// rw_sec : { *(rw_sec) }
924// would mean that the RW PT_LOAD would become unaligned.
925static bool shouldSkip(SectionCommand *cmd) {
926 if (auto *assign = dyn_cast<SymbolAssignment>(Val: cmd))
927 return assign->name != ".";
928 return false;
929}
930
931// We want to place orphan sections so that they share as much
932// characteristics with their neighbors as possible. For example, if
933// both are rw, or both are tls.
934static SmallVectorImpl<SectionCommand *>::iterator
935findOrphanPos(SmallVectorImpl<SectionCommand *>::iterator b,
936 SmallVectorImpl<SectionCommand *>::iterator e) {
937 // Place non-alloc orphan sections at the end. This matches how we assign file
938 // offsets to non-alloc sections.
939 OutputSection *sec = &cast<OutputDesc>(Val: *e)->osec;
940 if (!(sec->flags & SHF_ALLOC))
941 return e;
942
943 // As a special case, place .relro_padding before the SymbolAssignment using
944 // DATA_SEGMENT_RELRO_END, if present.
945 if (in.relroPadding && sec == in.relroPadding->getParent()) {
946 auto i = std::find_if(first: b, last: e, pred: [=](SectionCommand *a) {
947 if (auto *assign = dyn_cast<SymbolAssignment>(Val: a))
948 return assign->dataSegmentRelroEnd;
949 return false;
950 });
951 if (i != e)
952 return i;
953 }
954
955 // Find the most similar output section as the anchor. Rank Proximity is a
956 // value in the range [-1, 32] where [0, 32] indicates potential anchors (0:
957 // least similar; 32: identical). -1 means not an anchor.
958 //
959 // In the event of proximity ties, we select the first or last section
960 // depending on whether the orphan's rank is smaller.
961 int maxP = 0;
962 auto i = e;
963 for (auto j = b; j != e; ++j) {
964 int p = getRankProximity(a: sec, b: *j);
965 if (p > maxP ||
966 (p == maxP && cast<OutputDesc>(Val: *j)->osec.sortRank <= sec->sortRank)) {
967 maxP = p;
968 i = j;
969 }
970 }
971 if (i == e)
972 return e;
973
974 auto isOutputSecWithInputSections = [](SectionCommand *cmd) {
975 auto *osd = dyn_cast<OutputDesc>(Val: cmd);
976 return osd && osd->osec.hasInputSections;
977 };
978
979 // Then, scan backward or forward through the script for a suitable insertion
980 // point. If i's rank is larger, the orphan section can be placed before i.
981 //
982 // However, don't do this if custom program headers are defined. Otherwise,
983 // adding the orphan to a previous segment can change its flags, for example,
984 // making a read-only segment writable. If memory regions are defined, an
985 // orphan section should continue the same region as the found section to
986 // better resemble the behavior of GNU ld.
987 bool mustAfter = script->hasPhdrsCommands() || !script->memoryRegions.empty();
988 if (cast<OutputDesc>(Val: *i)->osec.sortRank <= sec->sortRank || mustAfter) {
989 for (auto j = ++i; j != e; ++j) {
990 if (!isOutputSecWithInputSections(*j))
991 continue;
992 if (getRankProximity(a: sec, b: *j) != maxP)
993 break;
994 i = j + 1;
995 }
996 } else {
997 for (; i != b; --i)
998 if (isOutputSecWithInputSections(i[-1]))
999 break;
1000 }
1001
1002 // As a special case, if the orphan section is the last section, put
1003 // it at the very end, past any other commands.
1004 // This matches bfd's behavior and is convenient when the linker script fully
1005 // specifies the start of the file, but doesn't care about the end (the non
1006 // alloc sections for example).
1007 if (std::find_if(first: i, last: e, pred: isOutputSecWithInputSections) == e)
1008 return e;
1009
1010 while (i != e && shouldSkip(cmd: *i))
1011 ++i;
1012 return i;
1013}
1014
1015// Adds random priorities to sections not already in the map.
1016static void maybeShuffle(DenseMap<const InputSectionBase *, int> &order) {
1017 if (config->shuffleSections.empty())
1018 return;
1019
1020 SmallVector<InputSectionBase *, 0> matched, sections = ctx.inputSections;
1021 matched.reserve(N: sections.size());
1022 for (const auto &patAndSeed : config->shuffleSections) {
1023 matched.clear();
1024 for (InputSectionBase *sec : sections)
1025 if (patAndSeed.first.match(S: sec->name))
1026 matched.push_back(Elt: sec);
1027 const uint32_t seed = patAndSeed.second;
1028 if (seed == UINT32_MAX) {
1029 // If --shuffle-sections <section-glob>=-1, reverse the section order. The
1030 // section order is stable even if the number of sections changes. This is
1031 // useful to catch issues like static initialization order fiasco
1032 // reliably.
1033 std::reverse(first: matched.begin(), last: matched.end());
1034 } else {
1035 std::mt19937 g(seed ? seed : std::random_device()());
1036 llvm::shuffle(first: matched.begin(), last: matched.end(), g);
1037 }
1038 size_t i = 0;
1039 for (InputSectionBase *&sec : sections)
1040 if (patAndSeed.first.match(S: sec->name))
1041 sec = matched[i++];
1042 }
1043
1044 // Existing priorities are < 0, so use priorities >= 0 for the missing
1045 // sections.
1046 int prio = 0;
1047 for (InputSectionBase *sec : sections) {
1048 if (order.try_emplace(Key: sec, Args&: prio).second)
1049 ++prio;
1050 }
1051}
1052
1053// Builds section order for handling --symbol-ordering-file.
1054static DenseMap<const InputSectionBase *, int> buildSectionOrder() {
1055 DenseMap<const InputSectionBase *, int> sectionOrder;
1056 // Use the rarely used option --call-graph-ordering-file to sort sections.
1057 if (!config->callGraphProfile.empty())
1058 return computeCallGraphProfileOrder();
1059
1060 if (config->symbolOrderingFile.empty())
1061 return sectionOrder;
1062
1063 struct SymbolOrderEntry {
1064 int priority;
1065 bool present;
1066 };
1067
1068 // Build a map from symbols to their priorities. Symbols that didn't
1069 // appear in the symbol ordering file have the lowest priority 0.
1070 // All explicitly mentioned symbols have negative (higher) priorities.
1071 DenseMap<CachedHashStringRef, SymbolOrderEntry> symbolOrder;
1072 int priority = -config->symbolOrderingFile.size();
1073 for (StringRef s : config->symbolOrderingFile)
1074 symbolOrder.insert(KV: {CachedHashStringRef(s), {.priority: priority++, .present: false}});
1075
1076 // Build a map from sections to their priorities.
1077 auto addSym = [&](Symbol &sym) {
1078 auto it = symbolOrder.find(Val: CachedHashStringRef(sym.getName()));
1079 if (it == symbolOrder.end())
1080 return;
1081 SymbolOrderEntry &ent = it->second;
1082 ent.present = true;
1083
1084 maybeWarnUnorderableSymbol(sym: &sym);
1085
1086 if (auto *d = dyn_cast<Defined>(Val: &sym)) {
1087 if (auto *sec = dyn_cast_or_null<InputSectionBase>(Val: d->section)) {
1088 int &priority = sectionOrder[cast<InputSectionBase>(Val: sec)];
1089 priority = std::min(a: priority, b: ent.priority);
1090 }
1091 }
1092 };
1093
1094 // We want both global and local symbols. We get the global ones from the
1095 // symbol table and iterate the object files for the local ones.
1096 for (Symbol *sym : symtab.getSymbols())
1097 addSym(*sym);
1098
1099 for (ELFFileBase *file : ctx.objectFiles)
1100 for (Symbol *sym : file->getLocalSymbols())
1101 addSym(*sym);
1102
1103 if (config->warnSymbolOrdering)
1104 for (auto orderEntry : symbolOrder)
1105 if (!orderEntry.second.present)
1106 warn(msg: "symbol ordering file: no such symbol: " + orderEntry.first.val());
1107
1108 return sectionOrder;
1109}
1110
1111// Sorts the sections in ISD according to the provided section order.
1112static void
1113sortISDBySectionOrder(InputSectionDescription *isd,
1114 const DenseMap<const InputSectionBase *, int> &order,
1115 bool executableOutputSection) {
1116 SmallVector<InputSection *, 0> unorderedSections;
1117 SmallVector<std::pair<InputSection *, int>, 0> orderedSections;
1118 uint64_t unorderedSize = 0;
1119 uint64_t totalSize = 0;
1120
1121 for (InputSection *isec : isd->sections) {
1122 if (executableOutputSection)
1123 totalSize += isec->getSize();
1124 auto i = order.find(Val: isec);
1125 if (i == order.end()) {
1126 unorderedSections.push_back(Elt: isec);
1127 unorderedSize += isec->getSize();
1128 continue;
1129 }
1130 orderedSections.push_back(Elt: {isec, i->second});
1131 }
1132 llvm::sort(C&: orderedSections, Comp: llvm::less_second());
1133
1134 // Find an insertion point for the ordered section list in the unordered
1135 // section list. On targets with limited-range branches, this is the mid-point
1136 // of the unordered section list. This decreases the likelihood that a range
1137 // extension thunk will be needed to enter or exit the ordered region. If the
1138 // ordered section list is a list of hot functions, we can generally expect
1139 // the ordered functions to be called more often than the unordered functions,
1140 // making it more likely that any particular call will be within range, and
1141 // therefore reducing the number of thunks required.
1142 //
1143 // For example, imagine that you have 8MB of hot code and 32MB of cold code.
1144 // If the layout is:
1145 //
1146 // 8MB hot
1147 // 32MB cold
1148 //
1149 // only the first 8-16MB of the cold code (depending on which hot function it
1150 // is actually calling) can call the hot code without a range extension thunk.
1151 // However, if we use this layout:
1152 //
1153 // 16MB cold
1154 // 8MB hot
1155 // 16MB cold
1156 //
1157 // both the last 8-16MB of the first block of cold code and the first 8-16MB
1158 // of the second block of cold code can call the hot code without a thunk. So
1159 // we effectively double the amount of code that could potentially call into
1160 // the hot code without a thunk.
1161 //
1162 // The above is not necessary if total size of input sections in this "isd"
1163 // is small. Note that we assume all input sections are executable if the
1164 // output section is executable (which is not always true but supposed to
1165 // cover most cases).
1166 size_t insPt = 0;
1167 if (executableOutputSection && !orderedSections.empty() &&
1168 target->getThunkSectionSpacing() &&
1169 totalSize >= target->getThunkSectionSpacing()) {
1170 uint64_t unorderedPos = 0;
1171 for (; insPt != unorderedSections.size(); ++insPt) {
1172 unorderedPos += unorderedSections[insPt]->getSize();
1173 if (unorderedPos > unorderedSize / 2)
1174 break;
1175 }
1176 }
1177
1178 isd->sections.clear();
1179 for (InputSection *isec : ArrayRef(unorderedSections).slice(N: 0, M: insPt))
1180 isd->sections.push_back(Elt: isec);
1181 for (std::pair<InputSection *, int> p : orderedSections)
1182 isd->sections.push_back(Elt: p.first);
1183 for (InputSection *isec : ArrayRef(unorderedSections).slice(N: insPt))
1184 isd->sections.push_back(Elt: isec);
1185}
1186
1187static void sortSection(OutputSection &osec,
1188 const DenseMap<const InputSectionBase *, int> &order) {
1189 StringRef name = osec.name;
1190
1191 // Never sort these.
1192 if (name == ".init" || name == ".fini")
1193 return;
1194
1195 // Sort input sections by priority using the list provided by
1196 // --symbol-ordering-file or --shuffle-sections=. This is a least significant
1197 // digit radix sort. The sections may be sorted stably again by a more
1198 // significant key.
1199 if (!order.empty())
1200 for (SectionCommand *b : osec.commands)
1201 if (auto *isd = dyn_cast<InputSectionDescription>(Val: b))
1202 sortISDBySectionOrder(isd, order, executableOutputSection: osec.flags & SHF_EXECINSTR);
1203
1204 if (script->hasSectionsCommand)
1205 return;
1206
1207 if (name == ".init_array" || name == ".fini_array") {
1208 osec.sortInitFini();
1209 } else if (name == ".ctors" || name == ".dtors") {
1210 osec.sortCtorsDtors();
1211 } else if (config->emachine == EM_PPC64 && name == ".toc") {
1212 // .toc is allocated just after .got and is accessed using GOT-relative
1213 // relocations. Object files compiled with small code model have an
1214 // addressable range of [.got, .got + 0xFFFC] for GOT-relative relocations.
1215 // To reduce the risk of relocation overflow, .toc contents are sorted so
1216 // that sections having smaller relocation offsets are at beginning of .toc
1217 assert(osec.commands.size() == 1);
1218 auto *isd = cast<InputSectionDescription>(Val: osec.commands[0]);
1219 llvm::stable_sort(Range&: isd->sections,
1220 C: [](const InputSection *a, const InputSection *b) -> bool {
1221 return a->file->ppc64SmallCodeModelTocRelocs &&
1222 !b->file->ppc64SmallCodeModelTocRelocs;
1223 });
1224 }
1225}
1226
1227// If no layout was provided by linker script, we want to apply default
1228// sorting for special input sections. This also handles --symbol-ordering-file.
1229template <class ELFT> void Writer<ELFT>::sortInputSections() {
1230 // Build the order once since it is expensive.
1231 DenseMap<const InputSectionBase *, int> order = buildSectionOrder();
1232 maybeShuffle(order);
1233 for (SectionCommand *cmd : script->sectionCommands)
1234 if (auto *osd = dyn_cast<OutputDesc>(Val: cmd))
1235 sortSection(osec&: osd->osec, order);
1236}
1237
1238template <class ELFT> void Writer<ELFT>::sortSections() {
1239 llvm::TimeTraceScope timeScope("Sort sections");
1240
1241 // Don't sort if using -r. It is not necessary and we want to preserve the
1242 // relative order for SHF_LINK_ORDER sections.
1243 if (config->relocatable) {
1244 script->adjustOutputSections();
1245 return;
1246 }
1247
1248 sortInputSections();
1249
1250 for (SectionCommand *cmd : script->sectionCommands)
1251 if (auto *osd = dyn_cast<OutputDesc>(Val: cmd))
1252 osd->osec.sortRank = getSectionRank(osec&: osd->osec);
1253 if (!script->hasSectionsCommand) {
1254 // OutputDescs are mostly contiguous, but may be interleaved with
1255 // SymbolAssignments in the presence of INSERT commands.
1256 auto mid = std::stable_partition(
1257 script->sectionCommands.begin(), script->sectionCommands.end(),
1258 [](SectionCommand *cmd) { return isa<OutputDesc>(Val: cmd); });
1259 std::stable_sort(script->sectionCommands.begin(), mid, compareSections);
1260 }
1261
1262 // Process INSERT commands and update output section attributes. From this
1263 // point onwards the order of script->sectionCommands is fixed.
1264 script->processInsertCommands();
1265 script->adjustOutputSections();
1266
1267 if (script->hasSectionsCommand)
1268 sortOrphanSections();
1269
1270 script->adjustSectionsAfterSorting();
1271}
1272
1273template <class ELFT> void Writer<ELFT>::sortOrphanSections() {
1274 // Orphan sections are sections present in the input files which are
1275 // not explicitly placed into the output file by the linker script.
1276 //
1277 // The sections in the linker script are already in the correct
1278 // order. We have to figuere out where to insert the orphan
1279 // sections.
1280 //
1281 // The order of the sections in the script is arbitrary and may not agree with
1282 // compareSections. This means that we cannot easily define a strict weak
1283 // ordering. To see why, consider a comparison of a section in the script and
1284 // one not in the script. We have a two simple options:
1285 // * Make them equivalent (a is not less than b, and b is not less than a).
1286 // The problem is then that equivalence has to be transitive and we can
1287 // have sections a, b and c with only b in a script and a less than c
1288 // which breaks this property.
1289 // * Use compareSectionsNonScript. Given that the script order doesn't have
1290 // to match, we can end up with sections a, b, c, d where b and c are in the
1291 // script and c is compareSectionsNonScript less than b. In which case d
1292 // can be equivalent to c, a to b and d < a. As a concrete example:
1293 // .a (rx) # not in script
1294 // .b (rx) # in script
1295 // .c (ro) # in script
1296 // .d (ro) # not in script
1297 //
1298 // The way we define an order then is:
1299 // * Sort only the orphan sections. They are in the end right now.
1300 // * Move each orphan section to its preferred position. We try
1301 // to put each section in the last position where it can share
1302 // a PT_LOAD.
1303 //
1304 // There is some ambiguity as to where exactly a new entry should be
1305 // inserted, because Commands contains not only output section
1306 // commands but also other types of commands such as symbol assignment
1307 // expressions. There's no correct answer here due to the lack of the
1308 // formal specification of the linker script. We use heuristics to
1309 // determine whether a new output command should be added before or
1310 // after another commands. For the details, look at shouldSkip
1311 // function.
1312
1313 auto i = script->sectionCommands.begin();
1314 auto e = script->sectionCommands.end();
1315 auto nonScriptI = std::find_if(i, e, [](SectionCommand *cmd) {
1316 if (auto *osd = dyn_cast<OutputDesc>(Val: cmd))
1317 return osd->osec.sectionIndex == UINT32_MAX;
1318 return false;
1319 });
1320
1321 // Sort the orphan sections.
1322 std::stable_sort(nonScriptI, e, compareSections);
1323
1324 // As a horrible special case, skip the first . assignment if it is before any
1325 // section. We do this because it is common to set a load address by starting
1326 // the script with ". = 0xabcd" and the expectation is that every section is
1327 // after that.
1328 auto firstSectionOrDotAssignment =
1329 std::find_if(i, e, [](SectionCommand *cmd) { return !shouldSkip(cmd); });
1330 if (firstSectionOrDotAssignment != e &&
1331 isa<SymbolAssignment>(**firstSectionOrDotAssignment))
1332 ++firstSectionOrDotAssignment;
1333 i = firstSectionOrDotAssignment;
1334
1335 while (nonScriptI != e) {
1336 auto pos = findOrphanPos(i, nonScriptI);
1337 OutputSection *orphan = &cast<OutputDesc>(*nonScriptI)->osec;
1338
1339 // As an optimization, find all sections with the same sort rank
1340 // and insert them with one rotate.
1341 unsigned rank = orphan->sortRank;
1342 auto end = std::find_if(nonScriptI + 1, e, [=](SectionCommand *cmd) {
1343 return cast<OutputDesc>(Val: cmd)->osec.sortRank != rank;
1344 });
1345 std::rotate(pos, nonScriptI, end);
1346 nonScriptI = end;
1347 }
1348}
1349
1350static bool compareByFilePosition(InputSection *a, InputSection *b) {
1351 InputSection *la = a->flags & SHF_LINK_ORDER ? a->getLinkOrderDep() : nullptr;
1352 InputSection *lb = b->flags & SHF_LINK_ORDER ? b->getLinkOrderDep() : nullptr;
1353 // SHF_LINK_ORDER sections with non-zero sh_link are ordered before
1354 // non-SHF_LINK_ORDER sections and SHF_LINK_ORDER sections with zero sh_link.
1355 if (!la || !lb)
1356 return la && !lb;
1357 OutputSection *aOut = la->getParent();
1358 OutputSection *bOut = lb->getParent();
1359
1360 if (aOut == bOut)
1361 return la->outSecOff < lb->outSecOff;
1362 if (aOut->addr == bOut->addr)
1363 return aOut->sectionIndex < bOut->sectionIndex;
1364 return aOut->addr < bOut->addr;
1365}
1366
1367template <class ELFT> void Writer<ELFT>::resolveShfLinkOrder() {
1368 llvm::TimeTraceScope timeScope("Resolve SHF_LINK_ORDER");
1369 for (OutputSection *sec : outputSections) {
1370 if (!(sec->flags & SHF_LINK_ORDER))
1371 continue;
1372
1373 // The ARM.exidx section use SHF_LINK_ORDER, but we have consolidated
1374 // this processing inside the ARMExidxsyntheticsection::finalizeContents().
1375 if (!config->relocatable && config->emachine == EM_ARM &&
1376 sec->type == SHT_ARM_EXIDX)
1377 continue;
1378
1379 // Link order may be distributed across several InputSectionDescriptions.
1380 // Sorting is performed separately.
1381 SmallVector<InputSection **, 0> scriptSections;
1382 SmallVector<InputSection *, 0> sections;
1383 for (SectionCommand *cmd : sec->commands) {
1384 auto *isd = dyn_cast<InputSectionDescription>(Val: cmd);
1385 if (!isd)
1386 continue;
1387 bool hasLinkOrder = false;
1388 scriptSections.clear();
1389 sections.clear();
1390 for (InputSection *&isec : isd->sections) {
1391 if (isec->flags & SHF_LINK_ORDER) {
1392 InputSection *link = isec->getLinkOrderDep();
1393 if (link && !link->getParent())
1394 error(msg: toString(isec) + ": sh_link points to discarded section " +
1395 toString(link));
1396 hasLinkOrder = true;
1397 }
1398 scriptSections.push_back(Elt: &isec);
1399 sections.push_back(Elt: isec);
1400 }
1401 if (hasLinkOrder && errorCount() == 0) {
1402 llvm::stable_sort(Range&: sections, C: compareByFilePosition);
1403 for (int i = 0, n = sections.size(); i != n; ++i)
1404 *scriptSections[i] = sections[i];
1405 }
1406 }
1407 }
1408}
1409
1410static void finalizeSynthetic(SyntheticSection *sec) {
1411 if (sec && sec->isNeeded() && sec->getParent()) {
1412 llvm::TimeTraceScope timeScope("Finalize synthetic sections", sec->name);
1413 sec->finalizeContents();
1414 }
1415}
1416
1417// We need to generate and finalize the content that depends on the address of
1418// InputSections. As the generation of the content may also alter InputSection
1419// addresses we must converge to a fixed point. We do that here. See the comment
1420// in Writer<ELFT>::finalizeSections().
1421template <class ELFT> void Writer<ELFT>::finalizeAddressDependentContent() {
1422 llvm::TimeTraceScope timeScope("Finalize address dependent content");
1423 ThunkCreator tc;
1424 AArch64Err843419Patcher a64p;
1425 ARMErr657417Patcher a32p;
1426 script->assignAddresses();
1427
1428 // .ARM.exidx and SHF_LINK_ORDER do not require precise addresses, but they
1429 // do require the relative addresses of OutputSections because linker scripts
1430 // can assign Virtual Addresses to OutputSections that are not monotonically
1431 // increasing. Anything here must be repeatable, since spilling may change
1432 // section order.
1433 const auto finalizeOrderDependentContent = [this] {
1434 for (Partition &part : partitions)
1435 finalizeSynthetic(sec: part.armExidx.get());
1436 resolveShfLinkOrder();
1437 };
1438 finalizeOrderDependentContent();
1439
1440 // Converts call x@GDPLT to call __tls_get_addr
1441 if (config->emachine == EM_HEXAGON)
1442 hexagonTLSSymbolUpdate(outputSections);
1443
1444 uint32_t pass = 0, assignPasses = 0;
1445 for (;;) {
1446 bool changed = target->needsThunks ? tc.createThunks(pass, outputSections)
1447 : target->relaxOnce(pass);
1448 bool spilled = script->spillSections();
1449 changed |= spilled;
1450 ++pass;
1451
1452 // With Thunk Size much smaller than branch range we expect to
1453 // converge quickly; if we get to 30 something has gone wrong.
1454 if (changed && pass >= 30) {
1455 error(msg: target->needsThunks ? "thunk creation not converged"
1456 : "relaxation not converged");
1457 break;
1458 }
1459
1460 if (config->fixCortexA53Errata843419) {
1461 if (changed)
1462 script->assignAddresses();
1463 changed |= a64p.createFixes();
1464 }
1465 if (config->fixCortexA8) {
1466 if (changed)
1467 script->assignAddresses();
1468 changed |= a32p.createFixes();
1469 }
1470
1471 finalizeSynthetic(sec: in.got.get());
1472 if (in.mipsGot)
1473 in.mipsGot->updateAllocSize();
1474
1475 for (Partition &part : partitions) {
1476 // The R_AARCH64_AUTH_RELATIVE has a smaller addend field as bits [63:32]
1477 // encode the signing schema. We've put relocations in .relr.auth.dyn
1478 // during RelocationScanner::processAux, but the target VA for some of
1479 // them might be wider than 32 bits. We can only know the final VA at this
1480 // point, so move relocations with large values from .relr.auth.dyn to
1481 // .rela.dyn. See also AArch64::relocate.
1482 if (part.relrAuthDyn) {
1483 auto it = llvm::remove_if(
1484 part.relrAuthDyn->relocs, [&part](const RelativeReloc &elem) {
1485 const Relocation &reloc = elem.inputSec->relocs()[elem.relocIdx];
1486 if (isInt<32>(x: reloc.sym->getVA(addend: reloc.addend)))
1487 return false;
1488 part.relaDyn->addReloc(reloc: {R_AARCH64_AUTH_RELATIVE, elem.inputSec,
1489 reloc.offset,
1490 DynamicReloc::AddendOnlyWithTargetVA,
1491 *reloc.sym, reloc.addend, R_ABS});
1492 return true;
1493 });
1494 changed |= (it != part.relrAuthDyn->relocs.end());
1495 part.relrAuthDyn->relocs.erase(it, part.relrAuthDyn->relocs.end());
1496 }
1497 if (part.relaDyn)
1498 changed |= part.relaDyn->updateAllocSize();
1499 if (part.relrDyn)
1500 changed |= part.relrDyn->updateAllocSize();
1501 if (part.relrAuthDyn)
1502 changed |= part.relrAuthDyn->updateAllocSize();
1503 if (part.memtagGlobalDescriptors)
1504 changed |= part.memtagGlobalDescriptors->updateAllocSize();
1505 }
1506
1507 std::pair<const OutputSection *, const Defined *> changes =
1508 script->assignAddresses();
1509 if (!changed) {
1510 // Some symbols may be dependent on section addresses. When we break the
1511 // loop, the symbol values are finalized because a previous
1512 // assignAddresses() finalized section addresses.
1513 if (!changes.first && !changes.second)
1514 break;
1515 if (++assignPasses == 5) {
1516 if (changes.first)
1517 errorOrWarn(msg: "address (0x" + Twine::utohexstr(Val: changes.first->addr) +
1518 ") of section '" + changes.first->name +
1519 "' does not converge");
1520 if (changes.second)
1521 errorOrWarn(msg: "assignment to symbol " + toString(*changes.second) +
1522 " does not converge");
1523 break;
1524 }
1525 } else if (spilled) {
1526 // Spilling can change relative section order.
1527 finalizeOrderDependentContent();
1528 }
1529 }
1530 if (!config->relocatable)
1531 target->finalizeRelax(passes: pass);
1532
1533 if (config->relocatable)
1534 for (OutputSection *sec : outputSections)
1535 sec->addr = 0;
1536
1537 // If addrExpr is set, the address may not be a multiple of the alignment.
1538 // Warn because this is error-prone.
1539 for (SectionCommand *cmd : script->sectionCommands)
1540 if (auto *osd = dyn_cast<OutputDesc>(Val: cmd)) {
1541 OutputSection *osec = &osd->osec;
1542 if (osec->addr % osec->addralign != 0)
1543 warn(msg: "address (0x" + Twine::utohexstr(Val: osec->addr) + ") of section " +
1544 osec->name + " is not a multiple of alignment (" +
1545 Twine(osec->addralign) + ")");
1546 }
1547
1548 // Sizes are no longer allowed to grow, so all allowable spills have been
1549 // taken. Remove any leftover potential spills.
1550 script->erasePotentialSpillSections();
1551}
1552
1553// If Input Sections have been shrunk (basic block sections) then
1554// update symbol values and sizes associated with these sections. With basic
1555// block sections, input sections can shrink when the jump instructions at
1556// the end of the section are relaxed.
1557static void fixSymbolsAfterShrinking() {
1558 for (InputFile *File : ctx.objectFiles) {
1559 parallelForEach(R: File->getSymbols(), Fn: [&](Symbol *Sym) {
1560 auto *def = dyn_cast<Defined>(Val: Sym);
1561 if (!def)
1562 return;
1563
1564 const SectionBase *sec = def->section;
1565 if (!sec)
1566 return;
1567
1568 const InputSectionBase *inputSec = dyn_cast<InputSectionBase>(Val: sec);
1569 if (!inputSec || !inputSec->bytesDropped)
1570 return;
1571
1572 const size_t OldSize = inputSec->content().size();
1573 const size_t NewSize = OldSize - inputSec->bytesDropped;
1574
1575 if (def->value > NewSize && def->value <= OldSize) {
1576 LLVM_DEBUG(llvm::dbgs()
1577 << "Moving symbol " << Sym->getName() << " from "
1578 << def->value << " to "
1579 << def->value - inputSec->bytesDropped << " bytes\n");
1580 def->value -= inputSec->bytesDropped;
1581 return;
1582 }
1583
1584 if (def->value + def->size > NewSize && def->value <= OldSize &&
1585 def->value + def->size <= OldSize) {
1586 LLVM_DEBUG(llvm::dbgs()
1587 << "Shrinking symbol " << Sym->getName() << " from "
1588 << def->size << " to " << def->size - inputSec->bytesDropped
1589 << " bytes\n");
1590 def->size -= inputSec->bytesDropped;
1591 }
1592 });
1593 }
1594}
1595
1596// If basic block sections exist, there are opportunities to delete fall thru
1597// jumps and shrink jump instructions after basic block reordering. This
1598// relaxation pass does that. It is only enabled when --optimize-bb-jumps
1599// option is used.
1600template <class ELFT> void Writer<ELFT>::optimizeBasicBlockJumps() {
1601 assert(config->optimizeBBJumps);
1602 SmallVector<InputSection *, 0> storage;
1603
1604 script->assignAddresses();
1605 // For every output section that has executable input sections, this
1606 // does the following:
1607 // 1. Deletes all direct jump instructions in input sections that
1608 // jump to the following section as it is not required.
1609 // 2. If there are two consecutive jump instructions, it checks
1610 // if they can be flipped and one can be deleted.
1611 for (OutputSection *osec : outputSections) {
1612 if (!(osec->flags & SHF_EXECINSTR))
1613 continue;
1614 ArrayRef<InputSection *> sections = getInputSections(os: *osec, storage);
1615 size_t numDeleted = 0;
1616 // Delete all fall through jump instructions. Also, check if two
1617 // consecutive jump instructions can be flipped so that a fall
1618 // through jmp instruction can be deleted.
1619 for (size_t i = 0, e = sections.size(); i != e; ++i) {
1620 InputSection *next = i + 1 < sections.size() ? sections[i + 1] : nullptr;
1621 InputSection &sec = *sections[i];
1622 numDeleted += target->deleteFallThruJmpInsn(is&: sec, file: sec.file, nextIS: next);
1623 }
1624 if (numDeleted > 0) {
1625 script->assignAddresses();
1626 LLVM_DEBUG(llvm::dbgs()
1627 << "Removing " << numDeleted << " fall through jumps\n");
1628 }
1629 }
1630
1631 fixSymbolsAfterShrinking();
1632
1633 for (OutputSection *osec : outputSections)
1634 for (InputSection *is : getInputSections(os: *osec, storage))
1635 is->trim();
1636}
1637
1638// In order to allow users to manipulate linker-synthesized sections,
1639// we had to add synthetic sections to the input section list early,
1640// even before we make decisions whether they are needed. This allows
1641// users to write scripts like this: ".mygot : { .got }".
1642//
1643// Doing it has an unintended side effects. If it turns out that we
1644// don't need a .got (for example) at all because there's no
1645// relocation that needs a .got, we don't want to emit .got.
1646//
1647// To deal with the above problem, this function is called after
1648// scanRelocations is called to remove synthetic sections that turn
1649// out to be empty.
1650static void removeUnusedSyntheticSections() {
1651 // All input synthetic sections that can be empty are placed after
1652 // all regular ones. Reverse iterate to find the first synthetic section
1653 // after a non-synthetic one which will be our starting point.
1654 auto start =
1655 llvm::find_if(Range: llvm::reverse(C&: ctx.inputSections), P: [](InputSectionBase *s) {
1656 return !isa<SyntheticSection>(Val: s);
1657 }).base();
1658
1659 // Remove unused synthetic sections from ctx.inputSections;
1660 DenseSet<InputSectionBase *> unused;
1661 auto end =
1662 std::remove_if(first: start, last: ctx.inputSections.end(), pred: [&](InputSectionBase *s) {
1663 auto *sec = cast<SyntheticSection>(Val: s);
1664 if (sec->getParent() && sec->isNeeded())
1665 return false;
1666 // .relr.auth.dyn relocations may be moved to .rela.dyn in
1667 // finalizeAddressDependentContent, making .rela.dyn no longer empty.
1668 // Conservatively keep .rela.dyn. .relr.auth.dyn can be made empty, but
1669 // we would fail to remove it here.
1670 if (config->emachine == EM_AARCH64 && config->relrPackDynRelocs)
1671 if (auto *relSec = dyn_cast<RelocationBaseSection>(Val: sec))
1672 if (relSec == mainPart->relaDyn.get())
1673 return false;
1674 unused.insert(V: sec);
1675 return true;
1676 });
1677 ctx.inputSections.erase(CS: end, CE: ctx.inputSections.end());
1678
1679 // Remove unused synthetic sections from the corresponding input section
1680 // description and orphanSections.
1681 for (auto *sec : unused)
1682 if (OutputSection *osec = cast<SyntheticSection>(Val: sec)->getParent())
1683 for (SectionCommand *cmd : osec->commands)
1684 if (auto *isd = dyn_cast<InputSectionDescription>(Val: cmd))
1685 llvm::erase_if(C&: isd->sections, P: [&](InputSection *isec) {
1686 return unused.count(V: isec);
1687 });
1688 llvm::erase_if(C&: script->orphanSections, P: [&](const InputSectionBase *sec) {
1689 return unused.count(V: sec);
1690 });
1691}
1692
1693// Create output section objects and add them to OutputSections.
1694template <class ELFT> void Writer<ELFT>::finalizeSections() {
1695 if (!config->relocatable) {
1696 Out::preinitArray = findSection(name: ".preinit_array");
1697 Out::initArray = findSection(name: ".init_array");
1698 Out::finiArray = findSection(name: ".fini_array");
1699
1700 // The linker needs to define SECNAME_start, SECNAME_end and SECNAME_stop
1701 // symbols for sections, so that the runtime can get the start and end
1702 // addresses of each section by section name. Add such symbols.
1703 addStartEndSymbols();
1704 for (SectionCommand *cmd : script->sectionCommands)
1705 if (auto *osd = dyn_cast<OutputDesc>(Val: cmd))
1706 addStartStopSymbols(osec&: osd->osec);
1707
1708 // Add _DYNAMIC symbol. Unlike GNU gold, our _DYNAMIC symbol has no type.
1709 // It should be okay as no one seems to care about the type.
1710 // Even the author of gold doesn't remember why gold behaves that way.
1711 // https://sourceware.org/ml/binutils/2002-03/msg00360.html
1712 if (mainPart->dynamic->parent) {
1713 Symbol *s = symtab.addSymbol(newSym: Defined{
1714 ctx.internalFile, "_DYNAMIC", STB_WEAK, STV_HIDDEN, STT_NOTYPE,
1715 /*value=*/0, /*size=*/0, mainPart->dynamic.get()});
1716 s->isUsedInRegularObj = true;
1717 }
1718
1719 // Define __rel[a]_iplt_{start,end} symbols if needed.
1720 addRelIpltSymbols();
1721
1722 // RISC-V's gp can address +/- 2 KiB, set it to .sdata + 0x800. This symbol
1723 // should only be defined in an executable. If .sdata does not exist, its
1724 // value/section does not matter but it has to be relative, so set its
1725 // st_shndx arbitrarily to 1 (Out::elfHeader).
1726 if (config->emachine == EM_RISCV) {
1727 ElfSym::riscvGlobalPointer = nullptr;
1728 if (!config->shared) {
1729 OutputSection *sec = findSection(name: ".sdata");
1730 addOptionalRegular(
1731 name: "__global_pointer$", sec: sec ? sec : Out::elfHeader, val: 0x800, stOther: STV_DEFAULT);
1732 // Set riscvGlobalPointer to be used by the optional global pointer
1733 // relaxation.
1734 if (config->relaxGP) {
1735 Symbol *s = symtab.find(name: "__global_pointer$");
1736 if (s && s->isDefined())
1737 ElfSym::riscvGlobalPointer = cast<Defined>(Val: s);
1738 }
1739 }
1740 }
1741
1742 if (config->emachine == EM_386 || config->emachine == EM_X86_64) {
1743 // On targets that support TLSDESC, _TLS_MODULE_BASE_ is defined in such a
1744 // way that:
1745 //
1746 // 1) Without relaxation: it produces a dynamic TLSDESC relocation that
1747 // computes 0.
1748 // 2) With LD->LE relaxation: _TLS_MODULE_BASE_@tpoff = 0 (lowest address
1749 // in the TLS block).
1750 //
1751 // 2) is special cased in @tpoff computation. To satisfy 1), we define it
1752 // as an absolute symbol of zero. This is different from GNU linkers which
1753 // define _TLS_MODULE_BASE_ relative to the first TLS section.
1754 Symbol *s = symtab.find(name: "_TLS_MODULE_BASE_");
1755 if (s && s->isUndefined()) {
1756 s->resolve(other: Defined{ctx.internalFile, StringRef(), STB_GLOBAL,
1757 STV_HIDDEN, STT_TLS, /*value=*/0, 0,
1758 /*section=*/nullptr});
1759 ElfSym::tlsModuleBase = cast<Defined>(Val: s);
1760 }
1761 }
1762
1763 // This responsible for splitting up .eh_frame section into
1764 // pieces. The relocation scan uses those pieces, so this has to be
1765 // earlier.
1766 {
1767 llvm::TimeTraceScope timeScope("Finalize .eh_frame");
1768 for (Partition &part : partitions)
1769 finalizeSynthetic(sec: part.ehFrame.get());
1770 }
1771 }
1772
1773 demoteSymbolsAndComputeIsPreemptible();
1774
1775 if (config->copyRelocs && config->discard != DiscardPolicy::None)
1776 markUsedLocalSymbols<ELFT>();
1777 demoteAndCopyLocalSymbols();
1778
1779 if (config->copyRelocs)
1780 addSectionSymbols();
1781
1782 // Change values of linker-script-defined symbols from placeholders (assigned
1783 // by declareSymbols) to actual definitions.
1784 script->processSymbolAssignments();
1785
1786 if (!config->relocatable) {
1787 llvm::TimeTraceScope timeScope("Scan relocations");
1788 // Scan relocations. This must be done after every symbol is declared so
1789 // that we can correctly decide if a dynamic relocation is needed. This is
1790 // called after processSymbolAssignments() because it needs to know whether
1791 // a linker-script-defined symbol is absolute.
1792 ppc64noTocRelax.clear();
1793 scanRelocations<ELFT>();
1794 reportUndefinedSymbols();
1795 postScanRelocations();
1796
1797 if (in.plt && in.plt->isNeeded())
1798 in.plt->addSymbols();
1799 if (in.iplt && in.iplt->isNeeded())
1800 in.iplt->addSymbols();
1801
1802 if (config->unresolvedSymbolsInShlib != UnresolvedPolicy::Ignore) {
1803 auto diagnose =
1804 config->unresolvedSymbolsInShlib == UnresolvedPolicy::ReportError
1805 ? errorOrWarn
1806 : warn;
1807 // Error on undefined symbols in a shared object, if all of its DT_NEEDED
1808 // entries are seen. These cases would otherwise lead to runtime errors
1809 // reported by the dynamic linker.
1810 //
1811 // ld.bfd traces all DT_NEEDED to emulate the logic of the dynamic linker
1812 // to catch more cases. That is too much for us. Our approach resembles
1813 // the one used in ld.gold, achieves a good balance to be useful but not
1814 // too smart.
1815 //
1816 // If a DSO reference is resolved by a SharedSymbol, but the SharedSymbol
1817 // is overridden by a hidden visibility Defined (which is later discarded
1818 // due to GC), don't report the diagnostic. However, this may indicate an
1819 // unintended SharedSymbol.
1820 for (SharedFile *file : ctx.sharedFiles) {
1821 bool allNeededIsKnown =
1822 llvm::all_of(file->dtNeeded, [&](StringRef needed) {
1823 return symtab.soNames.count(Val: CachedHashStringRef(needed));
1824 });
1825 if (!allNeededIsKnown)
1826 continue;
1827 for (Symbol *sym : file->requiredSymbols) {
1828 if (sym->dsoDefined)
1829 continue;
1830 if (sym->isUndefined() && !sym->isWeak()) {
1831 diagnose("undefined reference: " + toString(*sym) +
1832 "\n>>> referenced by " + toString(f: file) +
1833 " (disallowed by --no-allow-shlib-undefined)");
1834 } else if (sym->isDefined() && sym->computeBinding() == STB_LOCAL) {
1835 diagnose("non-exported symbol '" + toString(*sym) + "' in '" +
1836 toString(f: sym->file) + "' is referenced by DSO '" +
1837 toString(f: file) + "'");
1838 }
1839 }
1840 }
1841 }
1842 }
1843
1844 {
1845 llvm::TimeTraceScope timeScope("Add symbols to symtabs");
1846 // Now that we have defined all possible global symbols including linker-
1847 // synthesized ones. Visit all symbols to give the finishing touches.
1848 for (Symbol *sym : symtab.getSymbols()) {
1849 if (!sym->isUsedInRegularObj || !includeInSymtab(b: *sym))
1850 continue;
1851 if (!config->relocatable)
1852 sym->binding = sym->computeBinding();
1853 if (in.symTab)
1854 in.symTab->addSymbol(sym);
1855
1856 if (sym->includeInDynsym()) {
1857 partitions[sym->partition - 1].dynSymTab->addSymbol(sym);
1858 if (auto *file = dyn_cast_or_null<SharedFile>(Val: sym->file))
1859 if (file->isNeeded && !sym->isUndefined())
1860 addVerneed(ss: sym);
1861 }
1862 }
1863
1864 // We also need to scan the dynamic relocation tables of the other
1865 // partitions and add any referenced symbols to the partition's dynsym.
1866 for (Partition &part : MutableArrayRef<Partition>(partitions).slice(N: 1)) {
1867 DenseSet<Symbol *> syms;
1868 for (const SymbolTableEntry &e : part.dynSymTab->getSymbols())
1869 syms.insert(V: e.sym);
1870 for (DynamicReloc &reloc : part.relaDyn->relocs)
1871 if (reloc.sym && reloc.needsDynSymIndex() &&
1872 syms.insert(V: reloc.sym).second)
1873 part.dynSymTab->addSymbol(sym: reloc.sym);
1874 }
1875 }
1876
1877 if (in.mipsGot)
1878 in.mipsGot->build();
1879
1880 removeUnusedSyntheticSections();
1881 script->diagnoseOrphanHandling();
1882 script->diagnoseMissingSGSectionAddress();
1883
1884 sortSections();
1885
1886 // Create a list of OutputSections, assign sectionIndex, and populate
1887 // in.shStrTab.
1888 for (SectionCommand *cmd : script->sectionCommands)
1889 if (auto *osd = dyn_cast<OutputDesc>(Val: cmd)) {
1890 OutputSection *osec = &osd->osec;
1891 outputSections.push_back(Elt: osec);
1892 osec->sectionIndex = outputSections.size();
1893 osec->shName = in.shStrTab->addString(s: osec->name);
1894 }
1895
1896 // Prefer command line supplied address over other constraints.
1897 for (OutputSection *sec : outputSections) {
1898 auto i = config->sectionStartMap.find(Key: sec->name);
1899 if (i != config->sectionStartMap.end())
1900 sec->addrExpr = [=] { return i->second; };
1901 }
1902
1903 // With the outputSections available check for GDPLT relocations
1904 // and add __tls_get_addr symbol if needed.
1905 if (config->emachine == EM_HEXAGON && hexagonNeedsTLSSymbol(outputSections)) {
1906 Symbol *sym =
1907 symtab.addSymbol(newSym: Undefined{ctx.internalFile, "__tls_get_addr",
1908 STB_GLOBAL, STV_DEFAULT, STT_NOTYPE});
1909 sym->isPreemptible = true;
1910 partitions[0].dynSymTab->addSymbol(sym);
1911 }
1912
1913 // This is a bit of a hack. A value of 0 means undef, so we set it
1914 // to 1 to make __ehdr_start defined. The section number is not
1915 // particularly relevant.
1916 Out::elfHeader->sectionIndex = 1;
1917 Out::elfHeader->size = sizeof(typename ELFT::Ehdr);
1918
1919 // Binary and relocatable output does not have PHDRS.
1920 // The headers have to be created before finalize as that can influence the
1921 // image base and the dynamic section on mips includes the image base.
1922 if (!config->relocatable && !config->oFormatBinary) {
1923 for (Partition &part : partitions) {
1924 part.phdrs = script->hasPhdrsCommands() ? script->createPhdrs()
1925 : createPhdrs(part);
1926 if (config->emachine == EM_ARM) {
1927 // PT_ARM_EXIDX is the ARM EHABI equivalent of PT_GNU_EH_FRAME
1928 addPhdrForSection(part, shType: SHT_ARM_EXIDX, pType: PT_ARM_EXIDX, pFlags: PF_R);
1929 }
1930 if (config->emachine == EM_MIPS) {
1931 // Add separate segments for MIPS-specific sections.
1932 addPhdrForSection(part, shType: SHT_MIPS_REGINFO, pType: PT_MIPS_REGINFO, pFlags: PF_R);
1933 addPhdrForSection(part, shType: SHT_MIPS_OPTIONS, pType: PT_MIPS_OPTIONS, pFlags: PF_R);
1934 addPhdrForSection(part, shType: SHT_MIPS_ABIFLAGS, pType: PT_MIPS_ABIFLAGS, pFlags: PF_R);
1935 }
1936 if (config->emachine == EM_RISCV)
1937 addPhdrForSection(part, shType: SHT_RISCV_ATTRIBUTES, pType: PT_RISCV_ATTRIBUTES,
1938 pFlags: PF_R);
1939 }
1940 Out::programHeaders->size = sizeof(Elf_Phdr) * mainPart->phdrs.size();
1941
1942 // Find the TLS segment. This happens before the section layout loop so that
1943 // Android relocation packing can look up TLS symbol addresses. We only need
1944 // to care about the main partition here because all TLS symbols were moved
1945 // to the main partition (see MarkLive.cpp).
1946 for (PhdrEntry *p : mainPart->phdrs)
1947 if (p->p_type == PT_TLS)
1948 Out::tlsPhdr = p;
1949 }
1950
1951 // Some symbols are defined in term of program headers. Now that we
1952 // have the headers, we can find out which sections they point to.
1953 setReservedSymbolSections();
1954
1955 if (script->noCrossRefs.size()) {
1956 llvm::TimeTraceScope timeScope("Check NOCROSSREFS");
1957 checkNoCrossRefs<ELFT>();
1958 }
1959
1960 {
1961 llvm::TimeTraceScope timeScope("Finalize synthetic sections");
1962
1963 finalizeSynthetic(sec: in.bss.get());
1964 finalizeSynthetic(sec: in.bssRelRo.get());
1965 finalizeSynthetic(sec: in.symTabShndx.get());
1966 finalizeSynthetic(sec: in.shStrTab.get());
1967 finalizeSynthetic(sec: in.strTab.get());
1968 finalizeSynthetic(sec: in.got.get());
1969 finalizeSynthetic(sec: in.mipsGot.get());
1970 finalizeSynthetic(sec: in.igotPlt.get());
1971 finalizeSynthetic(sec: in.gotPlt.get());
1972 finalizeSynthetic(sec: in.relaPlt.get());
1973 finalizeSynthetic(sec: in.plt.get());
1974 finalizeSynthetic(sec: in.iplt.get());
1975 finalizeSynthetic(sec: in.ppc32Got2.get());
1976 finalizeSynthetic(sec: in.partIndex.get());
1977
1978 // Dynamic section must be the last one in this list and dynamic
1979 // symbol table section (dynSymTab) must be the first one.
1980 for (Partition &part : partitions) {
1981 if (part.relaDyn) {
1982 part.relaDyn->mergeRels();
1983 // Compute DT_RELACOUNT to be used by part.dynamic.
1984 part.relaDyn->partitionRels();
1985 finalizeSynthetic(sec: part.relaDyn.get());
1986 }
1987 if (part.relrDyn) {
1988 part.relrDyn->mergeRels();
1989 finalizeSynthetic(sec: part.relrDyn.get());
1990 }
1991 if (part.relrAuthDyn) {
1992 part.relrAuthDyn->mergeRels();
1993 finalizeSynthetic(sec: part.relrAuthDyn.get());
1994 }
1995
1996 finalizeSynthetic(sec: part.dynSymTab.get());
1997 finalizeSynthetic(sec: part.gnuHashTab.get());
1998 finalizeSynthetic(sec: part.hashTab.get());
1999 finalizeSynthetic(sec: part.verDef.get());
2000 finalizeSynthetic(sec: part.ehFrameHdr.get());
2001 finalizeSynthetic(sec: part.verSym.get());
2002 finalizeSynthetic(sec: part.verNeed.get());
2003 finalizeSynthetic(sec: part.dynamic.get());
2004 }
2005 }
2006
2007 if (!script->hasSectionsCommand && !config->relocatable)
2008 fixSectionAlignments();
2009
2010 // This is used to:
2011 // 1) Create "thunks":
2012 // Jump instructions in many ISAs have small displacements, and therefore
2013 // they cannot jump to arbitrary addresses in memory. For example, RISC-V
2014 // JAL instruction can target only +-1 MiB from PC. It is a linker's
2015 // responsibility to create and insert small pieces of code between
2016 // sections to extend the ranges if jump targets are out of range. Such
2017 // code pieces are called "thunks".
2018 //
2019 // We add thunks at this stage. We couldn't do this before this point
2020 // because this is the earliest point where we know sizes of sections and
2021 // their layouts (that are needed to determine if jump targets are in
2022 // range).
2023 //
2024 // 2) Update the sections. We need to generate content that depends on the
2025 // address of InputSections. For example, MIPS GOT section content or
2026 // android packed relocations sections content.
2027 //
2028 // 3) Assign the final values for the linker script symbols. Linker scripts
2029 // sometimes using forward symbol declarations. We want to set the correct
2030 // values. They also might change after adding the thunks.
2031 finalizeAddressDependentContent();
2032
2033 // All information needed for OutputSection part of Map file is available.
2034 if (errorCount())
2035 return;
2036
2037 {
2038 llvm::TimeTraceScope timeScope("Finalize synthetic sections");
2039 // finalizeAddressDependentContent may have added local symbols to the
2040 // static symbol table.
2041 finalizeSynthetic(sec: in.symTab.get());
2042 finalizeSynthetic(sec: in.debugNames.get());
2043 finalizeSynthetic(sec: in.ppc64LongBranchTarget.get());
2044 finalizeSynthetic(sec: in.armCmseSGSection.get());
2045 }
2046
2047 // Relaxation to delete inter-basic block jumps created by basic block
2048 // sections. Run after in.symTab is finalized as optimizeBasicBlockJumps
2049 // can relax jump instructions based on symbol offset.
2050 if (config->optimizeBBJumps)
2051 optimizeBasicBlockJumps();
2052
2053 // Fill other section headers. The dynamic table is finalized
2054 // at the end because some tags like RELSZ depend on result
2055 // of finalizing other sections.
2056 for (OutputSection *sec : outputSections)
2057 sec->finalize();
2058
2059 script->checkFinalScriptConditions();
2060
2061 if (config->emachine == EM_ARM && !config->isLE && config->armBe8) {
2062 addArmInputSectionMappingSymbols();
2063 sortArmMappingSymbols();
2064 }
2065}
2066
2067// Ensure data sections are not mixed with executable sections when
2068// --execute-only is used. --execute-only make pages executable but not
2069// readable.
2070template <class ELFT> void Writer<ELFT>::checkExecuteOnly() {
2071 if (!config->executeOnly)
2072 return;
2073
2074 SmallVector<InputSection *, 0> storage;
2075 for (OutputSection *osec : outputSections)
2076 if (osec->flags & SHF_EXECINSTR)
2077 for (InputSection *isec : getInputSections(os: *osec, storage))
2078 if (!(isec->flags & SHF_EXECINSTR))
2079 error(msg: "cannot place " + toString(isec) + " into " +
2080 toString(s: osec->name) +
2081 ": --execute-only does not support intermingling data and code");
2082}
2083
2084// The linker is expected to define SECNAME_start and SECNAME_end
2085// symbols for a few sections. This function defines them.
2086template <class ELFT> void Writer<ELFT>::addStartEndSymbols() {
2087 // If the associated output section does not exist, there is ambiguity as to
2088 // how we define _start and _end symbols for an init/fini section. Users
2089 // expect no "undefined symbol" linker errors and loaders expect equal
2090 // st_value but do not particularly care whether the symbols are defined or
2091 // not. We retain the output section so that the section indexes will be
2092 // correct.
2093 auto define = [=](StringRef start, StringRef end, OutputSection *os) {
2094 if (os) {
2095 Defined *startSym = addOptionalRegular(name: start, sec: os, val: 0);
2096 Defined *stopSym = addOptionalRegular(name: end, sec: os, val: -1);
2097 if (startSym || stopSym)
2098 os->usedInExpression = true;
2099 } else {
2100 addOptionalRegular(name: start, sec: Out::elfHeader, val: 0);
2101 addOptionalRegular(name: end, sec: Out::elfHeader, val: 0);
2102 }
2103 };
2104
2105 define("__preinit_array_start", "__preinit_array_end", Out::preinitArray);
2106 define("__init_array_start", "__init_array_end", Out::initArray);
2107 define("__fini_array_start", "__fini_array_end", Out::finiArray);
2108
2109 // As a special case, don't unnecessarily retain .ARM.exidx, which would
2110 // create an empty PT_ARM_EXIDX.
2111 if (OutputSection *sec = findSection(name: ".ARM.exidx"))
2112 define("__exidx_start", "__exidx_end", sec);
2113}
2114
2115// If a section name is valid as a C identifier (which is rare because of
2116// the leading '.'), linkers are expected to define __start_<secname> and
2117// __stop_<secname> symbols. They are at beginning and end of the section,
2118// respectively. This is not requested by the ELF standard, but GNU ld and
2119// gold provide the feature, and used by many programs.
2120template <class ELFT>
2121void Writer<ELFT>::addStartStopSymbols(OutputSection &osec) {
2122 StringRef s = osec.name;
2123 if (!isValidCIdentifier(s))
2124 return;
2125 Defined *startSym = addOptionalRegular(name: saver().save(S: "__start_" + s), sec: &osec, val: 0,
2126 stOther: config->zStartStopVisibility);
2127 Defined *stopSym = addOptionalRegular(name: saver().save(S: "__stop_" + s), sec: &osec, val: -1,
2128 stOther: config->zStartStopVisibility);
2129 if (startSym || stopSym)
2130 osec.usedInExpression = true;
2131}
2132
2133static bool needsPtLoad(OutputSection *sec) {
2134 if (!(sec->flags & SHF_ALLOC))
2135 return false;
2136
2137 // Don't allocate VA space for TLS NOBITS sections. The PT_TLS PHDR is
2138 // responsible for allocating space for them, not the PT_LOAD that
2139 // contains the TLS initialization image.
2140 if ((sec->flags & SHF_TLS) && sec->type == SHT_NOBITS)
2141 return false;
2142 return true;
2143}
2144
2145// Adjust phdr flags according to certain options.
2146static uint64_t computeFlags(uint64_t flags) {
2147 if (config->omagic)
2148 return PF_R | PF_W | PF_X;
2149 if (config->executeOnly && (flags & PF_X))
2150 return flags & ~PF_R;
2151 return flags;
2152}
2153
2154// Decide which program headers to create and which sections to include in each
2155// one.
2156template <class ELFT>
2157SmallVector<PhdrEntry *, 0> Writer<ELFT>::createPhdrs(Partition &part) {
2158 SmallVector<PhdrEntry *, 0> ret;
2159 auto addHdr = [&](unsigned type, unsigned flags) -> PhdrEntry * {
2160 ret.push_back(Elt: make<PhdrEntry>(args&: type, args&: flags));
2161 return ret.back();
2162 };
2163
2164 unsigned partNo = part.getNumber();
2165 bool isMain = partNo == 1;
2166
2167 // Add the first PT_LOAD segment for regular output sections.
2168 uint64_t flags = computeFlags(flags: PF_R);
2169 PhdrEntry *load = nullptr;
2170
2171 // nmagic or omagic output does not have PT_PHDR, PT_INTERP, or the readonly
2172 // PT_LOAD.
2173 if (!config->nmagic && !config->omagic) {
2174 // The first phdr entry is PT_PHDR which describes the program header
2175 // itself.
2176 if (isMain)
2177 addHdr(PT_PHDR, PF_R)->add(Out::programHeaders);
2178 else
2179 addHdr(PT_PHDR, PF_R)->add(part.programHeaders->getParent());
2180
2181 // PT_INTERP must be the second entry if exists.
2182 if (OutputSection *cmd = findSection(name: ".interp", partition: partNo))
2183 addHdr(PT_INTERP, cmd->getPhdrFlags())->add(cmd);
2184
2185 // Add the headers. We will remove them if they don't fit.
2186 // In the other partitions the headers are ordinary sections, so they don't
2187 // need to be added here.
2188 if (isMain) {
2189 load = addHdr(PT_LOAD, flags);
2190 load->add(sec: Out::elfHeader);
2191 load->add(sec: Out::programHeaders);
2192 }
2193 }
2194
2195 // PT_GNU_RELRO includes all sections that should be marked as
2196 // read-only by dynamic linker after processing relocations.
2197 // Current dynamic loaders only support one PT_GNU_RELRO PHDR, give
2198 // an error message if more than one PT_GNU_RELRO PHDR is required.
2199 PhdrEntry *relRo = make<PhdrEntry>(args: PT_GNU_RELRO, args: PF_R);
2200 bool inRelroPhdr = false;
2201 OutputSection *relroEnd = nullptr;
2202 for (OutputSection *sec : outputSections) {
2203 if (sec->partition != partNo || !needsPtLoad(sec))
2204 continue;
2205 if (isRelroSection(sec)) {
2206 inRelroPhdr = true;
2207 if (!relroEnd)
2208 relRo->add(sec);
2209 else
2210 error(msg: "section: " + sec->name + " is not contiguous with other relro" +
2211 " sections");
2212 } else if (inRelroPhdr) {
2213 inRelroPhdr = false;
2214 relroEnd = sec;
2215 }
2216 }
2217 relRo->p_align = 1;
2218
2219 for (OutputSection *sec : outputSections) {
2220 if (!needsPtLoad(sec))
2221 continue;
2222
2223 // Normally, sections in partitions other than the current partition are
2224 // ignored. But partition number 255 is a special case: it contains the
2225 // partition end marker (.part.end). It needs to be added to the main
2226 // partition so that a segment is created for it in the main partition,
2227 // which will cause the dynamic loader to reserve space for the other
2228 // partitions.
2229 if (sec->partition != partNo) {
2230 if (isMain && sec->partition == 255)
2231 addHdr(PT_LOAD, computeFlags(flags: sec->getPhdrFlags()))->add(sec);
2232 continue;
2233 }
2234
2235 // Segments are contiguous memory regions that has the same attributes
2236 // (e.g. executable or writable). There is one phdr for each segment.
2237 // Therefore, we need to create a new phdr when the next section has
2238 // incompatible flags or is loaded at a discontiguous address or memory
2239 // region using AT or AT> linker script command, respectively.
2240 //
2241 // As an exception, we don't create a separate load segment for the ELF
2242 // headers, even if the first "real" output has an AT or AT> attribute.
2243 //
2244 // In addition, NOBITS sections should only be placed at the end of a LOAD
2245 // segment (since it's represented as p_filesz < p_memsz). If we have a
2246 // not-NOBITS section after a NOBITS, we create a new LOAD for the latter
2247 // even if flags match, so as not to require actually writing the
2248 // supposed-to-be-NOBITS section to the output file. (However, we cannot do
2249 // so when hasSectionsCommand, since we cannot introduce the extra alignment
2250 // needed to create a new LOAD)
2251 uint64_t newFlags = computeFlags(flags: sec->getPhdrFlags());
2252 // When --no-rosegment is specified, RO and RX sections are compatible.
2253 uint32_t incompatible = flags ^ newFlags;
2254 if (config->singleRoRx && !(newFlags & PF_W))
2255 incompatible &= ~PF_X;
2256 if (incompatible)
2257 load = nullptr;
2258
2259 bool sameLMARegion =
2260 load && !sec->lmaExpr && sec->lmaRegion == load->firstSec->lmaRegion;
2261 if (load && sec != relroEnd &&
2262 sec->memRegion == load->firstSec->memRegion &&
2263 (sameLMARegion || load->lastSec == Out::programHeaders) &&
2264 (script->hasSectionsCommand || sec->type == SHT_NOBITS ||
2265 load->lastSec->type != SHT_NOBITS)) {
2266 load->p_flags |= newFlags;
2267 } else {
2268 load = addHdr(PT_LOAD, newFlags);
2269 flags = newFlags;
2270 }
2271
2272 load->add(sec);
2273 }
2274
2275 // Add a TLS segment if any.
2276 PhdrEntry *tlsHdr = make<PhdrEntry>(args: PT_TLS, args: PF_R);
2277 for (OutputSection *sec : outputSections)
2278 if (sec->partition == partNo && sec->flags & SHF_TLS)
2279 tlsHdr->add(sec);
2280 if (tlsHdr->firstSec)
2281 ret.push_back(Elt: tlsHdr);
2282
2283 // Add an entry for .dynamic.
2284 if (OutputSection *sec = part.dynamic->getParent())
2285 addHdr(PT_DYNAMIC, sec->getPhdrFlags())->add(sec);
2286
2287 if (relRo->firstSec)
2288 ret.push_back(Elt: relRo);
2289
2290 // PT_GNU_EH_FRAME is a special section pointing on .eh_frame_hdr.
2291 if (part.ehFrame->isNeeded() && part.ehFrameHdr &&
2292 part.ehFrame->getParent() && part.ehFrameHdr->getParent())
2293 addHdr(PT_GNU_EH_FRAME, part.ehFrameHdr->getParent()->getPhdrFlags())
2294 ->add(part.ehFrameHdr->getParent());
2295
2296 if (config->osabi == ELFOSABI_OPENBSD) {
2297 // PT_OPENBSD_MUTABLE makes the dynamic linker fill the segment with
2298 // zero data, like bss, but it can be treated differently.
2299 if (OutputSection *cmd = findSection(name: ".openbsd.mutable", partition: partNo))
2300 addHdr(PT_OPENBSD_MUTABLE, cmd->getPhdrFlags())->add(cmd);
2301
2302 // PT_OPENBSD_RANDOMIZE makes the dynamic linker fill the segment
2303 // with random data.
2304 if (OutputSection *cmd = findSection(name: ".openbsd.randomdata", partition: partNo))
2305 addHdr(PT_OPENBSD_RANDOMIZE, cmd->getPhdrFlags())->add(cmd);
2306
2307 // PT_OPENBSD_SYSCALLS makes the kernel and dynamic linker register
2308 // system call sites.
2309 if (OutputSection *cmd = findSection(name: ".openbsd.syscalls", partition: partNo))
2310 addHdr(PT_OPENBSD_SYSCALLS, cmd->getPhdrFlags())->add(cmd);
2311 }
2312
2313 if (config->zGnustack != GnuStackKind::None) {
2314 // PT_GNU_STACK is a special section to tell the loader to make the
2315 // pages for the stack non-executable. If you really want an executable
2316 // stack, you can pass -z execstack, but that's not recommended for
2317 // security reasons.
2318 unsigned perm = PF_R | PF_W;
2319 if (config->zGnustack == GnuStackKind::Exec)
2320 perm |= PF_X;
2321 addHdr(PT_GNU_STACK, perm)->p_memsz = config->zStackSize;
2322 }
2323
2324 // PT_OPENBSD_WXNEEDED is a OpenBSD-specific header to mark the executable
2325 // is expected to perform W^X violations, such as calling mprotect(2) or
2326 // mmap(2) with PROT_WRITE | PROT_EXEC, which is prohibited by default on
2327 // OpenBSD.
2328 if (config->zWxneeded)
2329 addHdr(PT_OPENBSD_WXNEEDED, PF_X);
2330
2331 if (OutputSection *cmd = findSection(name: ".note.gnu.property", partition: partNo))
2332 addHdr(PT_GNU_PROPERTY, PF_R)->add(cmd);
2333
2334 // Create one PT_NOTE per a group of contiguous SHT_NOTE sections with the
2335 // same alignment.
2336 PhdrEntry *note = nullptr;
2337 for (OutputSection *sec : outputSections) {
2338 if (sec->partition != partNo)
2339 continue;
2340 if (sec->type == SHT_NOTE && (sec->flags & SHF_ALLOC)) {
2341 if (!note || sec->lmaExpr || note->lastSec->addralign != sec->addralign)
2342 note = addHdr(PT_NOTE, PF_R);
2343 note->add(sec);
2344 } else {
2345 note = nullptr;
2346 }
2347 }
2348 return ret;
2349}
2350
2351template <class ELFT>
2352void Writer<ELFT>::addPhdrForSection(Partition &part, unsigned shType,
2353 unsigned pType, unsigned pFlags) {
2354 unsigned partNo = part.getNumber();
2355 auto i = llvm::find_if(outputSections, [=](OutputSection *cmd) {
2356 return cmd->partition == partNo && cmd->type == shType;
2357 });
2358 if (i == outputSections.end())
2359 return;
2360
2361 PhdrEntry *entry = make<PhdrEntry>(args&: pType, args&: pFlags);
2362 entry->add(sec: *i);
2363 part.phdrs.push_back(Elt: entry);
2364}
2365
2366// Place the first section of each PT_LOAD to a different page (of maxPageSize).
2367// This is achieved by assigning an alignment expression to addrExpr of each
2368// such section.
2369template <class ELFT> void Writer<ELFT>::fixSectionAlignments() {
2370 const PhdrEntry *prev;
2371 auto pageAlign = [&](const PhdrEntry *p) {
2372 OutputSection *cmd = p->firstSec;
2373 if (!cmd)
2374 return;
2375 cmd->alignExpr = [align = cmd->addralign]() { return align; };
2376 if (!cmd->addrExpr) {
2377 // Prefer advancing to align(dot, maxPageSize) + dot%maxPageSize to avoid
2378 // padding in the file contents.
2379 //
2380 // When -z separate-code is used we must not have any overlap in pages
2381 // between an executable segment and a non-executable segment. We align to
2382 // the next maximum page size boundary on transitions between executable
2383 // and non-executable segments.
2384 //
2385 // SHT_LLVM_PART_EHDR marks the start of a partition. The partition
2386 // sections will be extracted to a separate file. Align to the next
2387 // maximum page size boundary so that we can find the ELF header at the
2388 // start. We cannot benefit from overlapping p_offset ranges with the
2389 // previous segment anyway.
2390 if (config->zSeparate == SeparateSegmentKind::Loadable ||
2391 (config->zSeparate == SeparateSegmentKind::Code && prev &&
2392 (prev->p_flags & PF_X) != (p->p_flags & PF_X)) ||
2393 cmd->type == SHT_LLVM_PART_EHDR)
2394 cmd->addrExpr = [] {
2395 return alignToPowerOf2(Value: script->getDot(), Align: config->maxPageSize);
2396 };
2397 // PT_TLS is at the start of the first RW PT_LOAD. If `p` includes PT_TLS,
2398 // it must be the RW. Align to p_align(PT_TLS) to make sure
2399 // p_vaddr(PT_LOAD)%p_align(PT_LOAD) = 0. Otherwise, if
2400 // sh_addralign(.tdata) < sh_addralign(.tbss), we will set p_align(PT_TLS)
2401 // to sh_addralign(.tbss), while p_vaddr(PT_TLS)=p_vaddr(PT_LOAD) may not
2402 // be congruent to 0 modulo p_align(PT_TLS).
2403 //
2404 // Technically this is not required, but as of 2019, some dynamic loaders
2405 // don't handle p_vaddr%p_align != 0 correctly, e.g. glibc (i386 and
2406 // x86-64) doesn't make runtime address congruent to p_vaddr modulo
2407 // p_align for dynamic TLS blocks (PR/24606), FreeBSD rtld has the same
2408 // bug, musl (TLS Variant 1 architectures) before 1.1.23 handled TLS
2409 // blocks correctly. We need to keep the workaround for a while.
2410 else if (Out::tlsPhdr && Out::tlsPhdr->firstSec == p->firstSec)
2411 cmd->addrExpr = [] {
2412 return alignToPowerOf2(Value: script->getDot(), Align: config->maxPageSize) +
2413 alignToPowerOf2(Value: script->getDot() % config->maxPageSize,
2414 Align: Out::tlsPhdr->p_align);
2415 };
2416 else
2417 cmd->addrExpr = [] {
2418 return alignToPowerOf2(Value: script->getDot(), Align: config->maxPageSize) +
2419 script->getDot() % config->maxPageSize;
2420 };
2421 }
2422 };
2423
2424 for (Partition &part : partitions) {
2425 prev = nullptr;
2426 for (const PhdrEntry *p : part.phdrs)
2427 if (p->p_type == PT_LOAD && p->firstSec) {
2428 pageAlign(p);
2429 prev = p;
2430 }
2431 }
2432}
2433
2434// Compute an in-file position for a given section. The file offset must be the
2435// same with its virtual address modulo the page size, so that the loader can
2436// load executables without any address adjustment.
2437static uint64_t computeFileOffset(OutputSection *os, uint64_t off) {
2438 // The first section in a PT_LOAD has to have congruent offset and address
2439 // modulo the maximum page size.
2440 if (os->ptLoad && os->ptLoad->firstSec == os)
2441 return alignTo(Value: off, Align: os->ptLoad->p_align, Skew: os->addr);
2442
2443 // File offsets are not significant for .bss sections other than the first one
2444 // in a PT_LOAD/PT_TLS. By convention, we keep section offsets monotonically
2445 // increasing rather than setting to zero.
2446 if (os->type == SHT_NOBITS &&
2447 (!Out::tlsPhdr || Out::tlsPhdr->firstSec != os))
2448 return off;
2449
2450 // If the section is not in a PT_LOAD, we just have to align it.
2451 if (!os->ptLoad)
2452 return alignToPowerOf2(Value: off, Align: os->addralign);
2453
2454 // If two sections share the same PT_LOAD the file offset is calculated
2455 // using this formula: Off2 = Off1 + (VA2 - VA1).
2456 OutputSection *first = os->ptLoad->firstSec;
2457 return first->offset + os->addr - first->addr;
2458}
2459
2460template <class ELFT> void Writer<ELFT>::assignFileOffsetsBinary() {
2461 // Compute the minimum LMA of all non-empty non-NOBITS sections as minAddr.
2462 auto needsOffset = [](OutputSection &sec) {
2463 return sec.type != SHT_NOBITS && (sec.flags & SHF_ALLOC) && sec.size > 0;
2464 };
2465 uint64_t minAddr = UINT64_MAX;
2466 for (OutputSection *sec : outputSections)
2467 if (needsOffset(*sec)) {
2468 sec->offset = sec->getLMA();
2469 minAddr = std::min(a: minAddr, b: sec->offset);
2470 }
2471
2472 // Sections are laid out at LMA minus minAddr.
2473 fileSize = 0;
2474 for (OutputSection *sec : outputSections)
2475 if (needsOffset(*sec)) {
2476 sec->offset -= minAddr;
2477 fileSize = std::max(a: fileSize, b: sec->offset + sec->size);
2478 }
2479}
2480
2481static std::string rangeToString(uint64_t addr, uint64_t len) {
2482 return "[0x" + utohexstr(X: addr) + ", 0x" + utohexstr(X: addr + len - 1) + "]";
2483}
2484
2485// Assign file offsets to output sections.
2486template <class ELFT> void Writer<ELFT>::assignFileOffsets() {
2487 Out::programHeaders->offset = Out::elfHeader->size;
2488 uint64_t off = Out::elfHeader->size + Out::programHeaders->size;
2489
2490 PhdrEntry *lastRX = nullptr;
2491 for (Partition &part : partitions)
2492 for (PhdrEntry *p : part.phdrs)
2493 if (p->p_type == PT_LOAD && (p->p_flags & PF_X))
2494 lastRX = p;
2495
2496 // Layout SHF_ALLOC sections before non-SHF_ALLOC sections. A non-SHF_ALLOC
2497 // will not occupy file offsets contained by a PT_LOAD.
2498 for (OutputSection *sec : outputSections) {
2499 if (!(sec->flags & SHF_ALLOC))
2500 continue;
2501 off = computeFileOffset(os: sec, off);
2502 sec->offset = off;
2503 if (sec->type != SHT_NOBITS)
2504 off += sec->size;
2505
2506 // If this is a last section of the last executable segment and that
2507 // segment is the last loadable segment, align the offset of the
2508 // following section to avoid loading non-segments parts of the file.
2509 if (config->zSeparate != SeparateSegmentKind::None && lastRX &&
2510 lastRX->lastSec == sec)
2511 off = alignToPowerOf2(Value: off, Align: config->maxPageSize);
2512 }
2513 for (OutputSection *osec : outputSections) {
2514 if (osec->flags & SHF_ALLOC)
2515 continue;
2516 osec->offset = alignToPowerOf2(Value: off, Align: osec->addralign);
2517 off = osec->offset + osec->size;
2518 }
2519
2520 sectionHeaderOff = alignToPowerOf2(Value: off, Align: config->wordsize);
2521 fileSize = sectionHeaderOff + (outputSections.size() + 1) * sizeof(Elf_Shdr);
2522
2523 // Our logic assumes that sections have rising VA within the same segment.
2524 // With use of linker scripts it is possible to violate this rule and get file
2525 // offset overlaps or overflows. That should never happen with a valid script
2526 // which does not move the location counter backwards and usually scripts do
2527 // not do that. Unfortunately, there are apps in the wild, for example, Linux
2528 // kernel, which control segment distribution explicitly and move the counter
2529 // backwards, so we have to allow doing that to support linking them. We
2530 // perform non-critical checks for overlaps in checkSectionOverlap(), but here
2531 // we want to prevent file size overflows because it would crash the linker.
2532 for (OutputSection *sec : outputSections) {
2533 if (sec->type == SHT_NOBITS)
2534 continue;
2535 if ((sec->offset > fileSize) || (sec->offset + sec->size > fileSize))
2536 error(msg: "unable to place section " + sec->name + " at file offset " +
2537 rangeToString(addr: sec->offset, len: sec->size) +
2538 "; check your linker script for overflows");
2539 }
2540}
2541
2542// Finalize the program headers. We call this function after we assign
2543// file offsets and VAs to all sections.
2544template <class ELFT> void Writer<ELFT>::setPhdrs(Partition &part) {
2545 for (PhdrEntry *p : part.phdrs) {
2546 OutputSection *first = p->firstSec;
2547 OutputSection *last = p->lastSec;
2548
2549 // .ARM.exidx sections may not be within a single .ARM.exidx
2550 // output section. We always want to describe just the
2551 // SyntheticSection.
2552 if (part.armExidx && p->p_type == PT_ARM_EXIDX) {
2553 p->p_filesz = part.armExidx->getSize();
2554 p->p_memsz = part.armExidx->getSize();
2555 p->p_offset = first->offset + part.armExidx->outSecOff;
2556 p->p_vaddr = first->addr + part.armExidx->outSecOff;
2557 p->p_align = part.armExidx->addralign;
2558 if (part.elfHeader)
2559 p->p_offset -= part.elfHeader->getParent()->offset;
2560
2561 if (!p->hasLMA)
2562 p->p_paddr = first->getLMA() + part.armExidx->outSecOff;
2563 return;
2564 }
2565
2566 if (first) {
2567 p->p_filesz = last->offset - first->offset;
2568 if (last->type != SHT_NOBITS)
2569 p->p_filesz += last->size;
2570
2571 p->p_memsz = last->addr + last->size - first->addr;
2572 p->p_offset = first->offset;
2573 p->p_vaddr = first->addr;
2574
2575 // File offsets in partitions other than the main partition are relative
2576 // to the offset of the ELF headers. Perform that adjustment now.
2577 if (part.elfHeader)
2578 p->p_offset -= part.elfHeader->getParent()->offset;
2579
2580 if (!p->hasLMA)
2581 p->p_paddr = first->getLMA();
2582 }
2583 }
2584}
2585
2586// A helper struct for checkSectionOverlap.
2587namespace {
2588struct SectionOffset {
2589 OutputSection *sec;
2590 uint64_t offset;
2591};
2592} // namespace
2593
2594// Check whether sections overlap for a specific address range (file offsets,
2595// load and virtual addresses).
2596static void checkOverlap(StringRef name, std::vector<SectionOffset> &sections,
2597 bool isVirtualAddr) {
2598 llvm::sort(C&: sections, Comp: [=](const SectionOffset &a, const SectionOffset &b) {
2599 return a.offset < b.offset;
2600 });
2601
2602 // Finding overlap is easy given a vector is sorted by start position.
2603 // If an element starts before the end of the previous element, they overlap.
2604 for (size_t i = 1, end = sections.size(); i < end; ++i) {
2605 SectionOffset a = sections[i - 1];
2606 SectionOffset b = sections[i];
2607 if (b.offset >= a.offset + a.sec->size)
2608 continue;
2609
2610 // If both sections are in OVERLAY we allow the overlapping of virtual
2611 // addresses, because it is what OVERLAY was designed for.
2612 if (isVirtualAddr && a.sec->inOverlay && b.sec->inOverlay)
2613 continue;
2614
2615 errorOrWarn(msg: "section " + a.sec->name + " " + name +
2616 " range overlaps with " + b.sec->name + "\n>>> " + a.sec->name +
2617 " range is " + rangeToString(addr: a.offset, len: a.sec->size) + "\n>>> " +
2618 b.sec->name + " range is " +
2619 rangeToString(addr: b.offset, len: b.sec->size));
2620 }
2621}
2622
2623// Check for overlapping sections and address overflows.
2624//
2625// In this function we check that none of the output sections have overlapping
2626// file offsets. For SHF_ALLOC sections we also check that the load address
2627// ranges and the virtual address ranges don't overlap
2628template <class ELFT> void Writer<ELFT>::checkSections() {
2629 // First, check that section's VAs fit in available address space for target.
2630 for (OutputSection *os : outputSections)
2631 if ((os->addr + os->size < os->addr) ||
2632 (!ELFT::Is64Bits && os->addr + os->size > uint64_t(UINT32_MAX) + 1))
2633 errorOrWarn(msg: "section " + os->name + " at 0x" + utohexstr(X: os->addr) +
2634 " of size 0x" + utohexstr(X: os->size) +
2635 " exceeds available address space");
2636
2637 // Check for overlapping file offsets. In this case we need to skip any
2638 // section marked as SHT_NOBITS. These sections don't actually occupy space in
2639 // the file so Sec->Offset + Sec->Size can overlap with others. If --oformat
2640 // binary is specified only add SHF_ALLOC sections are added to the output
2641 // file so we skip any non-allocated sections in that case.
2642 std::vector<SectionOffset> fileOffs;
2643 for (OutputSection *sec : outputSections)
2644 if (sec->size > 0 && sec->type != SHT_NOBITS &&
2645 (!config->oFormatBinary || (sec->flags & SHF_ALLOC)))
2646 fileOffs.push_back(x: {.sec: sec, .offset: sec->offset});
2647 checkOverlap(name: "file", sections&: fileOffs, isVirtualAddr: false);
2648
2649 // When linking with -r there is no need to check for overlapping virtual/load
2650 // addresses since those addresses will only be assigned when the final
2651 // executable/shared object is created.
2652 if (config->relocatable)
2653 return;
2654
2655 // Checking for overlapping virtual and load addresses only needs to take
2656 // into account SHF_ALLOC sections since others will not be loaded.
2657 // Furthermore, we also need to skip SHF_TLS sections since these will be
2658 // mapped to other addresses at runtime and can therefore have overlapping
2659 // ranges in the file.
2660 std::vector<SectionOffset> vmas;
2661 for (OutputSection *sec : outputSections)
2662 if (sec->size > 0 && (sec->flags & SHF_ALLOC) && !(sec->flags & SHF_TLS))
2663 vmas.push_back(x: {.sec: sec, .offset: sec->addr});
2664 checkOverlap(name: "virtual address", sections&: vmas, isVirtualAddr: true);
2665
2666 // Finally, check that the load addresses don't overlap. This will usually be
2667 // the same as the virtual addresses but can be different when using a linker
2668 // script with AT().
2669 std::vector<SectionOffset> lmas;
2670 for (OutputSection *sec : outputSections)
2671 if (sec->size > 0 && (sec->flags & SHF_ALLOC) && !(sec->flags & SHF_TLS))
2672 lmas.push_back(x: {.sec: sec, .offset: sec->getLMA()});
2673 checkOverlap(name: "load address", sections&: lmas, isVirtualAddr: false);
2674}
2675
2676// The entry point address is chosen in the following ways.
2677//
2678// 1. the '-e' entry command-line option;
2679// 2. the ENTRY(symbol) command in a linker control script;
2680// 3. the value of the symbol _start, if present;
2681// 4. the number represented by the entry symbol, if it is a number;
2682// 5. the address 0.
2683static uint64_t getEntryAddr() {
2684 // Case 1, 2 or 3
2685 if (Symbol *b = symtab.find(name: config->entry))
2686 return b->getVA();
2687
2688 // Case 4
2689 uint64_t addr;
2690 if (to_integer(S: config->entry, Num&: addr))
2691 return addr;
2692
2693 // Case 5
2694 if (config->warnMissingEntry)
2695 warn(msg: "cannot find entry symbol " + config->entry +
2696 "; not setting start address");
2697 return 0;
2698}
2699
2700static uint16_t getELFType() {
2701 if (config->isPic)
2702 return ET_DYN;
2703 if (config->relocatable)
2704 return ET_REL;
2705 return ET_EXEC;
2706}
2707
2708template <class ELFT> void Writer<ELFT>::writeHeader() {
2709 writeEhdr<ELFT>(Out::bufferStart, *mainPart);
2710 writePhdrs<ELFT>(Out::bufferStart + sizeof(Elf_Ehdr), *mainPart);
2711
2712 auto *eHdr = reinterpret_cast<Elf_Ehdr *>(Out::bufferStart);
2713 eHdr->e_type = getELFType();
2714 eHdr->e_entry = getEntryAddr();
2715 eHdr->e_shoff = sectionHeaderOff;
2716
2717 // Write the section header table.
2718 //
2719 // The ELF header can only store numbers up to SHN_LORESERVE in the e_shnum
2720 // and e_shstrndx fields. When the value of one of these fields exceeds
2721 // SHN_LORESERVE ELF requires us to put sentinel values in the ELF header and
2722 // use fields in the section header at index 0 to store
2723 // the value. The sentinel values and fields are:
2724 // e_shnum = 0, SHdrs[0].sh_size = number of sections.
2725 // e_shstrndx = SHN_XINDEX, SHdrs[0].sh_link = .shstrtab section index.
2726 auto *sHdrs = reinterpret_cast<Elf_Shdr *>(Out::bufferStart + eHdr->e_shoff);
2727 size_t num = outputSections.size() + 1;
2728 if (num >= SHN_LORESERVE)
2729 sHdrs->sh_size = num;
2730 else
2731 eHdr->e_shnum = num;
2732
2733 uint32_t strTabIndex = in.shStrTab->getParent()->sectionIndex;
2734 if (strTabIndex >= SHN_LORESERVE) {
2735 sHdrs->sh_link = strTabIndex;
2736 eHdr->e_shstrndx = SHN_XINDEX;
2737 } else {
2738 eHdr->e_shstrndx = strTabIndex;
2739 }
2740
2741 for (OutputSection *sec : outputSections)
2742 sec->writeHeaderTo<ELFT>(++sHdrs);
2743}
2744
2745// Open a result file.
2746template <class ELFT> void Writer<ELFT>::openFile() {
2747 uint64_t maxSize = config->is64 ? INT64_MAX : UINT32_MAX;
2748 if (fileSize != size_t(fileSize) || maxSize < fileSize) {
2749 std::string msg;
2750 raw_string_ostream s(msg);
2751 s << "output file too large: " << Twine(fileSize) << " bytes\n"
2752 << "section sizes:\n";
2753 for (OutputSection *os : outputSections)
2754 s << os->name << ' ' << os->size << "\n";
2755 error(msg: s.str());
2756 return;
2757 }
2758
2759 unlinkAsync(path: config->outputFile);
2760 unsigned flags = 0;
2761 if (!config->relocatable)
2762 flags |= FileOutputBuffer::F_executable;
2763 if (!config->mmapOutputFile)
2764 flags |= FileOutputBuffer::F_no_mmap;
2765 Expected<std::unique_ptr<FileOutputBuffer>> bufferOrErr =
2766 FileOutputBuffer::create(FilePath: config->outputFile, Size: fileSize, Flags: flags);
2767
2768 if (!bufferOrErr) {
2769 error(msg: "failed to open " + config->outputFile + ": " +
2770 llvm::toString(E: bufferOrErr.takeError()));
2771 return;
2772 }
2773 buffer = std::move(*bufferOrErr);
2774 Out::bufferStart = buffer->getBufferStart();
2775}
2776
2777template <class ELFT> void Writer<ELFT>::writeSectionsBinary() {
2778 parallel::TaskGroup tg;
2779 for (OutputSection *sec : outputSections)
2780 if (sec->flags & SHF_ALLOC)
2781 sec->writeTo<ELFT>(Out::bufferStart + sec->offset, tg);
2782}
2783
2784static void fillTrap(uint8_t *i, uint8_t *end) {
2785 for (; i + 4 <= end; i += 4)
2786 memcpy(dest: i, src: &target->trapInstr, n: 4);
2787}
2788
2789// Fill the last page of executable segments with trap instructions
2790// instead of leaving them as zero. Even though it is not required by any
2791// standard, it is in general a good thing to do for security reasons.
2792//
2793// We'll leave other pages in segments as-is because the rest will be
2794// overwritten by output sections.
2795template <class ELFT> void Writer<ELFT>::writeTrapInstr() {
2796 for (Partition &part : partitions) {
2797 // Fill the last page.
2798 for (PhdrEntry *p : part.phdrs)
2799 if (p->p_type == PT_LOAD && (p->p_flags & PF_X))
2800 fillTrap(i: Out::bufferStart +
2801 alignDown(Value: p->firstSec->offset + p->p_filesz, Align: 4),
2802 end: Out::bufferStart +
2803 alignToPowerOf2(Value: p->firstSec->offset + p->p_filesz,
2804 Align: config->maxPageSize));
2805
2806 // Round up the file size of the last segment to the page boundary iff it is
2807 // an executable segment to ensure that other tools don't accidentally
2808 // trim the instruction padding (e.g. when stripping the file).
2809 PhdrEntry *last = nullptr;
2810 for (PhdrEntry *p : part.phdrs)
2811 if (p->p_type == PT_LOAD)
2812 last = p;
2813
2814 if (last && (last->p_flags & PF_X))
2815 last->p_memsz = last->p_filesz =
2816 alignToPowerOf2(Value: last->p_filesz, Align: config->maxPageSize);
2817 }
2818}
2819
2820// Write section contents to a mmap'ed file.
2821template <class ELFT> void Writer<ELFT>::writeSections() {
2822 llvm::TimeTraceScope timeScope("Write sections");
2823
2824 {
2825 // In -r or --emit-relocs mode, write the relocation sections first as in
2826 // ELf_Rel targets we might find out that we need to modify the relocated
2827 // section while doing it.
2828 parallel::TaskGroup tg;
2829 for (OutputSection *sec : outputSections)
2830 if (isStaticRelSecType(type: sec->type))
2831 sec->writeTo<ELFT>(Out::bufferStart + sec->offset, tg);
2832 }
2833 {
2834 parallel::TaskGroup tg;
2835 for (OutputSection *sec : outputSections)
2836 if (!isStaticRelSecType(type: sec->type))
2837 sec->writeTo<ELFT>(Out::bufferStart + sec->offset, tg);
2838 }
2839
2840 // Finally, check that all dynamic relocation addends were written correctly.
2841 if (config->checkDynamicRelocs && config->writeAddends) {
2842 for (OutputSection *sec : outputSections)
2843 if (isStaticRelSecType(type: sec->type))
2844 sec->checkDynRelAddends(bufStart: Out::bufferStart);
2845 }
2846}
2847
2848// Computes a hash value of Data using a given hash function.
2849// In order to utilize multiple cores, we first split data into 1MB
2850// chunks, compute a hash for each chunk, and then compute a hash value
2851// of the hash values.
2852static void
2853computeHash(llvm::MutableArrayRef<uint8_t> hashBuf,
2854 llvm::ArrayRef<uint8_t> data,
2855 std::function<void(uint8_t *dest, ArrayRef<uint8_t> arr)> hashFn) {
2856 std::vector<ArrayRef<uint8_t>> chunks = split(arr: data, chunkSize: 1024 * 1024);
2857 const size_t hashesSize = chunks.size() * hashBuf.size();
2858 std::unique_ptr<uint8_t[]> hashes(new uint8_t[hashesSize]);
2859
2860 // Compute hash values.
2861 parallelFor(Begin: 0, End: chunks.size(), Fn: [&](size_t i) {
2862 hashFn(hashes.get() + i * hashBuf.size(), chunks[i]);
2863 });
2864
2865 // Write to the final output buffer.
2866 hashFn(hashBuf.data(), ArrayRef(hashes.get(), hashesSize));
2867}
2868
2869template <class ELFT> void Writer<ELFT>::writeBuildId() {
2870 if (!mainPart->buildId || !mainPart->buildId->getParent())
2871 return;
2872
2873 if (config->buildId == BuildIdKind::Hexstring) {
2874 for (Partition &part : partitions)
2875 part.buildId->writeBuildId(buf: config->buildIdVector);
2876 return;
2877 }
2878
2879 // Compute a hash of all sections of the output file.
2880 size_t hashSize = mainPart->buildId->hashSize;
2881 std::unique_ptr<uint8_t[]> buildId(new uint8_t[hashSize]);
2882 MutableArrayRef<uint8_t> output(buildId.get(), hashSize);
2883 llvm::ArrayRef<uint8_t> input{Out::bufferStart, size_t(fileSize)};
2884
2885 // Fedora introduced build ID as "approximation of true uniqueness across all
2886 // binaries that might be used by overlapping sets of people". It does not
2887 // need some security goals that some hash algorithms strive to provide, e.g.
2888 // (second-)preimage and collision resistance. In practice people use 'md5'
2889 // and 'sha1' just for different lengths. Implement them with the more
2890 // efficient BLAKE3.
2891 switch (config->buildId) {
2892 case BuildIdKind::Fast:
2893 computeHash(output, input, [](uint8_t *dest, ArrayRef<uint8_t> arr) {
2894 write64le(P: dest, V: xxh3_64bits(data: arr));
2895 });
2896 break;
2897 case BuildIdKind::Md5:
2898 computeHash(output, input, [&](uint8_t *dest, ArrayRef<uint8_t> arr) {
2899 memcpy(dest: dest, src: BLAKE3::hash<16>(Data: arr).data(), n: hashSize);
2900 });
2901 break;
2902 case BuildIdKind::Sha1:
2903 computeHash(output, input, [&](uint8_t *dest, ArrayRef<uint8_t> arr) {
2904 memcpy(dest: dest, src: BLAKE3::hash<20>(Data: arr).data(), n: hashSize);
2905 });
2906 break;
2907 case BuildIdKind::Uuid:
2908 if (auto ec = llvm::getRandomBytes(Buffer: buildId.get(), Size: hashSize))
2909 error(msg: "entropy source failure: " + ec.message());
2910 break;
2911 default:
2912 llvm_unreachable("unknown BuildIdKind");
2913 }
2914 for (Partition &part : partitions)
2915 part.buildId->writeBuildId(buf: output);
2916}
2917
2918template void elf::writeResult<ELF32LE>();
2919template void elf::writeResult<ELF32BE>();
2920template void elf::writeResult<ELF64LE>();
2921template void elf::writeResult<ELF64BE>();
2922