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/InstrTypes.h" 19 #include "llvm/IR/Module.h" 20 #include "llvm/MC/MCObjectFileInfo.h" 21 #include "llvm/MC/MCStreamer.h" 22 23 #include <vector> 24 25 using namespace llvm; 26 27 WinCFGuard::WinCFGuard(AsmPrinter *A) : Asm(A) {} 28 29 WinCFGuard::~WinCFGuard() = default; 30 31 void WinCFGuard::endFunction(const MachineFunction *MF) { 32 33 // Skip functions without any longjmp targets. 34 if (MF->getLongjmpTargets().empty()) 35 return; 36 37 // Copy the function's longjmp targets to a module-level list. 38 llvm::append_range(LongjmpTargets, MF->getLongjmpTargets()); 39 } 40 41 /// Returns true if this function's address is escaped in a way that might make 42 /// it an indirect call target. Function::hasAddressTaken gives different 43 /// results when a function is called directly with a function prototype 44 /// mismatch, which requires a cast. 45 static bool isPossibleIndirectCallTarget(const Function *F) { 46 SmallVector<const Value *, 4> Users{F}; 47 while (!Users.empty()) { 48 const Value *FnOrCast = Users.pop_back_val(); 49 for (const Use &U : FnOrCast->uses()) { 50 const User *FnUser = U.getUser(); 51 if (const auto *Call = dyn_cast<CallBase>(FnUser)) { 52 if ((!Call->isCallee(&U) || U.get() != F) && 53 !Call->getFunction()->getName().ends_with("$exit_thunk")) { 54 // Passing a function pointer to a call may lead to an indirect 55 // call. As an exception, ignore ARM64EC exit thunks. 56 return true; 57 } 58 } else if (isa<Instruction>(FnUser)) { 59 // Consider any other instruction to be an escape. This has some weird 60 // consequences like no-op intrinsics being an escape or a store *to* a 61 // function address being an escape. 62 return true; 63 } else if (const auto *G = dyn_cast<GlobalValue>(FnUser)) { 64 // Ignore llvm.arm64ec.symbolmap; it doesn't lower to an actual address. 65 if (G->getName() == "llvm.arm64ec.symbolmap") 66 continue; 67 // Globals (for example, vtables) are escapes. 68 return true; 69 } else if (isa<Constant>(FnUser)) { 70 // Constants which aren't a global are intermediate values; recursively 71 // analyze the users to see if they actually escape. 72 Users.push_back(FnUser); 73 } 74 } 75 } 76 return false; 77 } 78 79 MCSymbol *WinCFGuard::lookupImpSymbol(const MCSymbol *Sym) { 80 if (Sym->getName().starts_with("__imp_")) 81 return nullptr; 82 return Asm->OutContext.lookupSymbol(Twine("__imp_") + Sym->getName()); 83 } 84 85 void WinCFGuard::endModule() { 86 const Module *M = Asm->MMI->getModule(); 87 std::vector<const MCSymbol *> GFIDsEntries; 88 std::vector<const MCSymbol *> GIATsEntries; 89 for (const Function &F : *M) { 90 if (isPossibleIndirectCallTarget(&F)) { 91 // If F is a dllimport and has an "__imp_" symbol already defined, add the 92 // "__imp_" symbol to the .giats section. 93 if (F.hasDLLImportStorageClass()) { 94 if (MCSymbol *impSym = lookupImpSymbol(Asm->getSymbol(&F))) { 95 GIATsEntries.push_back(impSym); 96 } 97 } 98 // Add the function's symbol to the .gfids section. 99 // Note: For dllimport functions, MSVC sometimes does not add this symbol 100 // to the .gfids section, but only adds the corresponding "__imp_" symbol 101 // to the .giats section. Here we always add the symbol to the .gfids 102 // section, since this does not introduce security risks. 103 GFIDsEntries.push_back(Asm->getSymbol(&F)); 104 } 105 } 106 107 if (GFIDsEntries.empty() && GIATsEntries.empty() && LongjmpTargets.empty()) 108 return; 109 110 // Emit the symbol index of each GFIDs entry to form the .gfids section. 111 auto &OS = *Asm->OutStreamer; 112 OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGFIDsSection()); 113 for (const MCSymbol *S : GFIDsEntries) 114 OS.emitCOFFSymbolIndex(S); 115 116 // Emit the symbol index of each GIATs entry to form the .giats section. 117 OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGIATsSection()); 118 for (const MCSymbol *S : GIATsEntries) { 119 OS.emitCOFFSymbolIndex(S); 120 } 121 122 // Emit the symbol index of each longjmp target to form the .gljmp section. 123 OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGLJMPSection()); 124 for (const MCSymbol *S : LongjmpTargets) { 125 OS.emitCOFFSymbolIndex(S); 126 } 127 } 128