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