102 lines · cpp
1//==- X86ReturnThunks.cpp - Replace rets with thunks or inline thunks --=//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-exception6//7//===----------------------------------------------------------------------===//8/// \file9///10/// Pass that replaces ret instructions with a jmp to __x86_return_thunk.11///12/// This corresponds to -mfunction-return=thunk-extern or13/// __attribute__((function_return("thunk-extern").14///15/// This pass is a minimal implementation necessary to help mitigate16/// RetBleed for the Linux kernel.17///18/// Should support for thunk or thunk-inline be necessary in the future, then19/// this pass should be combined with x86-retpoline-thunks which already has20/// machinery to emit thunks. Until then, YAGNI.21///22/// This pass is very similar to x86-lvi-ret.23///24//===----------------------------------------------------------------------===//25 26#include "X86.h"27#include "X86InstrInfo.h"28#include "X86Subtarget.h"29#include "llvm/ADT/SmallVector.h"30#include "llvm/ADT/StringRef.h"31#include "llvm/CodeGen/MachineBasicBlock.h"32#include "llvm/CodeGen/MachineFunction.h"33#include "llvm/CodeGen/MachineFunctionPass.h"34#include "llvm/CodeGen/MachineInstr.h"35#include "llvm/CodeGen/MachineInstrBuilder.h"36#include "llvm/CodeGen/MachineModuleInfo.h"37#include "llvm/IR/Module.h"38#include "llvm/MC/MCInstrDesc.h"39#include "llvm/Support/Debug.h"40#include "llvm/TargetParser/Triple.h"41 42using namespace llvm;43 44#define PASS_KEY "x86-return-thunks"45#define DEBUG_TYPE PASS_KEY46 47namespace {48struct X86ReturnThunks final : public MachineFunctionPass {49 static char ID;50 X86ReturnThunks() : MachineFunctionPass(ID) {}51 StringRef getPassName() const override { return "X86 Return Thunks"; }52 bool runOnMachineFunction(MachineFunction &MF) override;53};54} // namespace55 56char X86ReturnThunks::ID = 0;57 58bool X86ReturnThunks::runOnMachineFunction(MachineFunction &MF) {59 LLVM_DEBUG(dbgs() << getPassName() << "\n");60 61 bool Modified = false;62 63 if (!MF.getFunction().hasFnAttribute(llvm::Attribute::FnRetThunkExtern))64 return Modified;65 66 StringRef ThunkName = "__x86_return_thunk";67 if (MF.getFunction().getName() == ThunkName)68 return Modified;69 70 const auto &ST = MF.getSubtarget<X86Subtarget>();71 const bool Is64Bit = ST.getTargetTriple().isX86_64();72 const unsigned RetOpc = Is64Bit ? X86::RET64 : X86::RET32;73 SmallVector<MachineInstr *, 16> Rets;74 75 for (MachineBasicBlock &MBB : MF)76 for (MachineInstr &Term : MBB.terminators())77 if (Term.getOpcode() == RetOpc)78 Rets.push_back(&Term);79 80 bool IndCS =81 MF.getFunction().getParent()->getModuleFlag("indirect_branch_cs_prefix");82 const MCInstrDesc &CS = ST.getInstrInfo()->get(X86::CS_PREFIX);83 const MCInstrDesc &JMP = ST.getInstrInfo()->get(X86::TAILJMPd);84 85 for (MachineInstr *Ret : Rets) {86 if (IndCS)87 BuildMI(Ret->getParent(), Ret->getDebugLoc(), CS);88 BuildMI(Ret->getParent(), Ret->getDebugLoc(), JMP)89 .addExternalSymbol(ThunkName.data());90 Ret->eraseFromParent();91 Modified = true;92 }93 94 return Modified;95}96 97INITIALIZE_PASS(X86ReturnThunks, PASS_KEY, "X86 Return Thunks", false, false)98 99FunctionPass *llvm::createX86ReturnThunksPass() {100 return new X86ReturnThunks();101}102