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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 https://llvm.org/LICENSE.txt 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/Metadata.h"
#include "llvm/IR/Instructions.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())
return;
// Copy the function's longjmp targets to a module-level list.
LongjmpTargets.insert(LongjmpTargets.end(), MF->getLongjmpTargets().begin(),
MF->getLongjmpTargets().end());
}
/// 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))
continue;
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)
Users.push_back(FnUser);
else
return true;
}
}
}
return false;
}
void WinCFGuard::endModule() {
const Module *M = Asm->MMI->getModule();
std::vector<const Function *> Functions;
for (const Function &F : *M)
if (isPossibleIndirectCallTarget(&F))
Functions.push_back(&F);
if (Functions.empty() && LongjmpTargets.empty())
return;
auto &OS = *Asm->OutStreamer;
OS.SwitchSection(Asm->OutContext.getObjectFileInfo()->getGFIDsSection());
for (const Function *F : Functions)
OS.EmitCOFFSymbolIndex(Asm->getSymbol(F));
// Emit the symbol index of each longjmp target.
OS.SwitchSection(Asm->OutContext.getObjectFileInfo()->getGLJMPSection());
for (const MCSymbol *S : LongjmpTargets) {
OS.EmitCOFFSymbolIndex(S);
}
}
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