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//===-- CodeGen/AsmPrinter/WinCFGuard.cpp - Control Flow Guard Impl ------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file contains support for writing the metadata for Windows Control Flow
// Guard, including address-taken functions and valid longjmp targets.
#include "WinCFGuard.h"
#include "llvm/CodeGen/AsmPrinter.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Metadata.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCObjectFileInfo.h"
#include "llvm/MC/MCStreamer.h"
#include <vector>
using namespace llvm;
WinCFGuard::WinCFGuard(AsmPrinter *A) : AsmPrinterHandler(), Asm(A) {}
WinCFGuard::~WinCFGuard() {}
void WinCFGuard::endFunction(const MachineFunction *MF) {
// Skip functions without any longjmp targets.
if (MF->getLongjmpTargets().empty())
// Copy the function's longjmp targets to a module-level list.
llvm::append_range(LongjmpTargets, MF->getLongjmpTargets());
/// Returns true if this function's address is escaped in a way that might make
/// it an indirect call target. Function::hasAddressTaken gives different
/// results when a function is called directly with a function prototype
/// mismatch, which requires a cast.
static bool isPossibleIndirectCallTarget(const Function *F) {
SmallVector<const Value *, 4> Users{F};
while (!Users.empty()) {
const Value *FnOrCast = Users.pop_back_val();
for (const Use &U : FnOrCast->uses()) {
const User *FnUser = U.getUser();
if (isa<BlockAddress>(FnUser))
if (const auto *Call = dyn_cast<CallBase>(FnUser)) {
if (!Call->isCallee(&U))
return true;
} else if (isa<Instruction>(FnUser)) {
// Consider any other instruction to be an escape. This has some weird
// consequences like no-op intrinsics being an escape or a store *to* a
// function address being an escape.
return true;
} else if (const auto *C = dyn_cast<Constant>(FnUser)) {
// If this is a constant pointer cast of the function, don't consider
// this escape. Analyze the uses of the cast as well. This ensures that
// direct calls with mismatched prototypes don't end up in the CFG
// table. Consider other constants, such as vtable initializers, to
// escape the function.
if (C->stripPointerCasts() == F)
return true;
return false;
MCSymbol *WinCFGuard::lookupImpSymbol(const MCSymbol *Sym) {
if (Sym->getName().startswith("__imp_"))
return nullptr;
return Asm->OutContext.lookupSymbol(Twine("__imp_") + Sym->getName());
void WinCFGuard::endModule() {
const Module *M = Asm->MMI->getModule();
std::vector<const MCSymbol *> GFIDsEntries;
std::vector<const MCSymbol *> GIATsEntries;
for (const Function &F : *M) {
if (isPossibleIndirectCallTarget(&F)) {
// If F is a dllimport and has an "__imp_" symbol already defined, add the
// "__imp_" symbol to the .giats section.
if (F.hasDLLImportStorageClass()) {
if (MCSymbol *impSym = lookupImpSymbol(Asm->getSymbol(&F))) {
// Add the function's symbol to the .gfids section.
// Note: For dllimport functions, MSVC sometimes does not add this symbol
// to the .gfids section, but only adds the corresponding "__imp_" symbol
// to the .giats section. Here we always add the symbol to the .gfids
// section, since this does not introduce security risks.
if (GFIDsEntries.empty() && GIATsEntries.empty() && LongjmpTargets.empty())
// Emit the symbol index of each GFIDs entry to form the .gfids section.
auto &OS = *Asm->OutStreamer;
for (const MCSymbol *S : GFIDsEntries)
// Emit the symbol index of each GIATs entry to form the .giats section.
for (const MCSymbol *S : GIATsEntries) {
// Emit the symbol index of each longjmp target to form the .gljmp section.
for (const MCSymbol *S : LongjmpTargets) {