1//===-- CodeGen/AsmPrinter/WinCFGuard.cpp - Control Flow Guard Impl ------===//
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// This file contains support for writing the metadata for Windows Control Flow
10// Guard, including address-taken functions and valid longjmp targets.
11//
12//===----------------------------------------------------------------------===//
13
14#include "WinCFGuard.h"
15#include "llvm/CodeGen/AsmPrinter.h"
16#include "llvm/CodeGen/MachineFunction.h"
17#include "llvm/CodeGen/MachineModuleInfo.h"
18#include "llvm/IR/Constants.h"
19#include "llvm/IR/InstrTypes.h"
20#include "llvm/IR/Module.h"
21#include "llvm/MC/MCObjectFileInfo.h"
22#include "llvm/MC/MCStreamer.h"
23
24#include <vector>
25
26using namespace llvm;
27
28WinCFGuard::WinCFGuard(AsmPrinter *A) : Asm(A) {}
29
30WinCFGuard::~WinCFGuard() = default;
31
32void WinCFGuard::endFunction(const MachineFunction *MF) {
33
34 // Skip functions without any longjmp targets.
35 if (MF->getLongjmpTargets().empty())
36 return;
37
38 // Copy the function's longjmp targets to a module-level list.
39 llvm::append_range(C&: LongjmpTargets, R: MF->getLongjmpTargets());
40}
41
42/// Returns true if this function's address is escaped in a way that might make
43/// it an indirect call target. Function::hasAddressTaken gives different
44/// results when a function is called directly with a function prototype
45/// mismatch, which requires a cast.
46static bool isPossibleIndirectCallTarget(const Function *F) {
47 SmallVector<const Value *, 4> Users{F};
48 while (!Users.empty()) {
49 const Value *FnOrCast = Users.pop_back_val();
50 for (const Use &U : FnOrCast->uses()) {
51 const User *FnUser = U.getUser();
52 if (isa<BlockAddress>(Val: FnUser))
53 continue;
54 if (const auto *Call = dyn_cast<CallBase>(Val: FnUser)) {
55 if (!Call->isCallee(U: &U))
56 return true;
57 } else if (isa<Instruction>(Val: FnUser)) {
58 // Consider any other instruction to be an escape. This has some weird
59 // consequences like no-op intrinsics being an escape or a store *to* a
60 // function address being an escape.
61 return true;
62 } else if (const auto *C = dyn_cast<Constant>(Val: FnUser)) {
63 // If this is a constant pointer cast of the function, don't consider
64 // this escape. Analyze the uses of the cast as well. This ensures that
65 // direct calls with mismatched prototypes don't end up in the CFG
66 // table. Consider other constants, such as vtable initializers, to
67 // escape the function.
68 if (C->stripPointerCasts() == F)
69 Users.push_back(Elt: FnUser);
70 else
71 return true;
72 }
73 }
74 }
75 return false;
76}
77
78MCSymbol *WinCFGuard::lookupImpSymbol(const MCSymbol *Sym) {
79 if (Sym->getName().starts_with(Prefix: "__imp_"))
80 return nullptr;
81 return Asm->OutContext.lookupSymbol(Name: Twine("__imp_") + Sym->getName());
82}
83
84void WinCFGuard::endModule() {
85 const Module *M = Asm->MMI->getModule();
86 std::vector<const MCSymbol *> GFIDsEntries;
87 std::vector<const MCSymbol *> GIATsEntries;
88 for (const Function &F : *M) {
89 if (isPossibleIndirectCallTarget(F: &F)) {
90 // If F is a dllimport and has an "__imp_" symbol already defined, add the
91 // "__imp_" symbol to the .giats section.
92 if (F.hasDLLImportStorageClass()) {
93 if (MCSymbol *impSym = lookupImpSymbol(Sym: Asm->getSymbol(GV: &F))) {
94 GIATsEntries.push_back(x: impSym);
95 }
96 }
97 // Add the function's symbol to the .gfids section.
98 // Note: For dllimport functions, MSVC sometimes does not add this symbol
99 // to the .gfids section, but only adds the corresponding "__imp_" symbol
100 // to the .giats section. Here we always add the symbol to the .gfids
101 // section, since this does not introduce security risks.
102 GFIDsEntries.push_back(x: Asm->getSymbol(GV: &F));
103 }
104 }
105
106 if (GFIDsEntries.empty() && GIATsEntries.empty() && LongjmpTargets.empty())
107 return;
108
109 // Emit the symbol index of each GFIDs entry to form the .gfids section.
110 auto &OS = *Asm->OutStreamer;
111 OS.switchSection(Section: Asm->OutContext.getObjectFileInfo()->getGFIDsSection());
112 for (const MCSymbol *S : GFIDsEntries)
113 OS.emitCOFFSymbolIndex(Symbol: S);
114
115 // Emit the symbol index of each GIATs entry to form the .giats section.
116 OS.switchSection(Section: Asm->OutContext.getObjectFileInfo()->getGIATsSection());
117 for (const MCSymbol *S : GIATsEntries) {
118 OS.emitCOFFSymbolIndex(Symbol: S);
119 }
120
121 // Emit the symbol index of each longjmp target to form the .gljmp section.
122 OS.switchSection(Section: Asm->OutContext.getObjectFileInfo()->getGLJMPSection());
123 for (const MCSymbol *S : LongjmpTargets) {
124 OS.emitCOFFSymbolIndex(Symbol: S);
125 }
126}
127