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 26 using namespace llvm; 27 28 WinCFGuard::WinCFGuard(AsmPrinter *A) : Asm(A) {} 29 30 WinCFGuard::~WinCFGuard() = default; 31 32 void 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(LongjmpTargets, 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. 46 static 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>(FnUser)) 53 continue; 54 if (const auto *Call = dyn_cast<CallBase>(FnUser)) { 55 if (!Call->isCallee(&U)) 56 return true; 57 } else if (isa<Instruction>(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>(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(FnUser); 70 else 71 return true; 72 } 73 } 74 } 75 return false; 76 } 77 78 MCSymbol *WinCFGuard::lookupImpSymbol(const MCSymbol *Sym) { 79 if (Sym->getName().starts_with("__imp_")) 80 return nullptr; 81 return Asm->OutContext.lookupSymbol(Twine("__imp_") + Sym->getName()); 82 } 83 84 void 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)) { 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(Asm->getSymbol(&F))) { 94 GIATsEntries.push_back(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(Asm->getSymbol(&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(Asm->OutContext.getObjectFileInfo()->getGFIDsSection()); 112 for (const MCSymbol *S : GFIDsEntries) 113 OS.emitCOFFSymbolIndex(S); 114 115 // Emit the symbol index of each GIATs entry to form the .giats section. 116 OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGIATsSection()); 117 for (const MCSymbol *S : GIATsEntries) { 118 OS.emitCOFFSymbolIndex(S); 119 } 120 121 // Emit the symbol index of each longjmp target to form the .gljmp section. 122 OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGLJMPSection()); 123 for (const MCSymbol *S : LongjmpTargets) { 124 OS.emitCOFFSymbolIndex(S); 125 } 126 } 127