1//===- DWARF.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// The --gdb-index option instructs the linker to emit a .gdb_index section.
10// The section contains information to make gdb startup faster.
11// The format of the section is described at
12// https://sourceware.org/gdb/onlinedocs/gdb/Index-Section-Format.html.
13//
14//===----------------------------------------------------------------------===//
15
16#include "DWARF.h"
17#include "InputSection.h"
18#include "Symbols.h"
19#include "lld/Common/Memory.h"
20#include "llvm/DebugInfo/DWARF/DWARFDebugPubTable.h"
21#include "llvm/Object/ELFObjectFile.h"
22
23using namespace llvm;
24using namespace llvm::object;
25using namespace lld;
26using namespace lld::elf;
27
28template <class ELFT> LLDDwarfObj<ELFT>::LLDDwarfObj(ObjFile<ELFT> *obj) {
29 // Get the ELF sections to retrieve sh_flags. See the SHF_GROUP comment below.
30 ArrayRef<typename ELFT::Shdr> objSections = obj->template getELFShdrs<ELFT>();
31 assert(objSections.size() == obj->getSections().size());
32 for (auto [i, sec] : llvm::enumerate(obj->getSections())) {
33 if (!sec)
34 continue;
35
36 if (LLDDWARFSection *m =
37 StringSwitch<LLDDWARFSection *>(sec->name)
38 .Case(S: ".debug_addr", Value: &addrSection)
39 .Case(S: ".debug_gnu_pubnames", Value: &gnuPubnamesSection)
40 .Case(S: ".debug_gnu_pubtypes", Value: &gnuPubtypesSection)
41 .Case(S: ".debug_line", Value: &lineSection)
42 .Case(S: ".debug_loclists", Value: &loclistsSection)
43 .Case(S: ".debug_names", Value: &namesSection)
44 .Case(S: ".debug_ranges", Value: &rangesSection)
45 .Case(S: ".debug_rnglists", Value: &rnglistsSection)
46 .Case(S: ".debug_str_offsets", Value: &strOffsetsSection)
47 .Default(Value: nullptr)) {
48 m->Data = toStringRef(sec->contentMaybeDecompress());
49 m->sec = sec;
50 continue;
51 }
52
53 if (sec->name == ".debug_abbrev")
54 abbrevSection = toStringRef(sec->contentMaybeDecompress());
55 else if (sec->name == ".debug_str")
56 strSection = toStringRef(sec->contentMaybeDecompress());
57 else if (sec->name == ".debug_line_str")
58 lineStrSection = toStringRef(sec->contentMaybeDecompress());
59 else if (sec->name == ".debug_info" &&
60 !(objSections[i].sh_flags & ELF::SHF_GROUP)) {
61 // In DWARF v5, -fdebug-types-section places type units in .debug_info
62 // sections in COMDAT groups. They are not compile units and thus should
63 // be ignored for .gdb_index/diagnostics purposes.
64 //
65 // We use a simple heuristic: the compile unit does not have the SHF_GROUP
66 // flag. If we place compile units in COMDAT groups in the future, we may
67 // need to perform a lightweight parsing. We drop the SHF_GROUP flag when
68 // the InputSection was created, so we need to retrieve sh_flags from the
69 // associated ELF section header.
70 infoSection.Data = toStringRef(sec->contentMaybeDecompress());
71 infoSection.sec = sec;
72 }
73 }
74}
75
76namespace {
77template <class RelTy> struct LLDRelocationResolver {
78 // In the ELF ABIs, S sepresents the value of the symbol in the relocation
79 // entry. For Rela, the addend is stored as part of the relocation entry and
80 // is provided by the `findAux` method.
81 // In resolve() methods, the `type` and `offset` arguments would always be 0,
82 // because we don't set an owning object for the `RelocationRef` instance that
83 // we create in `findAux()`.
84 static uint64_t resolve(uint64_t /*type*/, uint64_t /*offset*/, uint64_t s,
85 uint64_t /*locData*/, int64_t addend) {
86 return s + addend;
87 }
88};
89
90template <class ELFT> struct LLDRelocationResolver<Elf_Rel_Impl<ELFT, false>> {
91 // For Rel, the addend is extracted from the relocated location and is
92 // supplied by the caller.
93 static uint64_t resolve(uint64_t /*type*/, uint64_t /*offset*/, uint64_t s,
94 uint64_t locData, int64_t /*addend*/) {
95 return s + locData;
96 }
97};
98} // namespace
99
100// Find if there is a relocation at Pos in Sec. The code is a bit
101// more complicated than usual because we need to pass a section index
102// to llvm since it has no idea about InputSection.
103template <class ELFT>
104template <class RelTy>
105std::optional<RelocAddrEntry>
106LLDDwarfObj<ELFT>::findAux(const InputSectionBase &sec, uint64_t pos,
107 ArrayRef<RelTy> rels) const {
108 auto it =
109 partition_point(rels, [=](const RelTy &a) { return a.r_offset < pos; });
110 if (it == rels.end() || it->r_offset != pos)
111 return std::nullopt;
112 const RelTy &rel = *it;
113
114 const ObjFile<ELFT> *file = sec.getFile<ELFT>();
115 uint32_t symIndex = rel.getSymbol(config->isMips64EL);
116 const typename ELFT::Sym &sym = file->template getELFSyms<ELFT>()[symIndex];
117 uint32_t secIndex = file->getSectionIndex(sym);
118
119 // An undefined symbol may be a symbol defined in a discarded section. We
120 // shall still resolve it. This is important for --gdb-index: the end address
121 // offset of an entry in .debug_ranges is relocated. If it is not resolved,
122 // its zero value will terminate the decoding of .debug_ranges prematurely.
123 Symbol &s = file->getRelocTargetSym(rel);
124 uint64_t val = 0;
125 if (auto *dr = dyn_cast<Defined>(Val: &s))
126 val = dr->value;
127
128 DataRefImpl d;
129 d.p = getAddend<ELFT>(rel);
130 return RelocAddrEntry{secIndex, RelocationRef(d, nullptr),
131 val, std::optional<object::RelocationRef>(),
132 0, LLDRelocationResolver<RelTy>::resolve};
133}
134
135template <class ELFT>
136std::optional<RelocAddrEntry>
137LLDDwarfObj<ELFT>::find(const llvm::DWARFSection &s, uint64_t pos) const {
138 auto &sec = static_cast<const LLDDWARFSection &>(s);
139 const RelsOrRelas<ELFT> rels =
140 sec.sec->template relsOrRelas<ELFT>(/*supportsCrel=*/false);
141 if (rels.areRelocsRel())
142 return findAux(*sec.sec, pos, rels.rels);
143 return findAux(*sec.sec, pos, rels.relas);
144}
145
146template class elf::LLDDwarfObj<ELF32LE>;
147template class elf::LLDDwarfObj<ELF32BE>;
148template class elf::LLDDwarfObj<ELF64LE>;
149template class elf::LLDDwarfObj<ELF64BE>;
150