| //==- X86ReturnThunks.cpp - Replace rets with thunks or inline thunks --=// |
| // |
| // 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 |
| // |
| //===----------------------------------------------------------------------===// |
| /// \file |
| /// |
| /// Pass that replaces ret instructions with a jmp to __x86_return_thunk. |
| /// |
| /// This corresponds to -mfunction-return=thunk-extern or |
| /// __attribute__((function_return("thunk-extern"). |
| /// |
| /// This pass is a minimal implementation necessary to help mitigate |
| /// RetBleed for the Linux kernel. |
| /// |
| /// Should support for thunk or thunk-inline be necessary in the future, then |
| /// this pass should be combined with x86-retpoline-thunks which already has |
| /// machinery to emit thunks. Until then, YAGNI. |
| /// |
| /// This pass is very similar to x86-lvi-ret. |
| /// |
| //===----------------------------------------------------------------------===// |
| |
| #include "X86.h" |
| #include "X86InstrInfo.h" |
| #include "X86Subtarget.h" |
| #include "llvm/ADT/SmallVector.h" |
| #include "llvm/ADT/StringRef.h" |
| #include "llvm/CodeGen/MachineBasicBlock.h" |
| #include "llvm/CodeGen/MachineFunction.h" |
| #include "llvm/CodeGen/MachineFunctionPass.h" |
| #include "llvm/CodeGen/MachineInstr.h" |
| #include "llvm/CodeGen/MachineInstrBuilder.h" |
| #include "llvm/CodeGen/MachineModuleInfo.h" |
| #include "llvm/MC/MCInstrDesc.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/TargetParser/Triple.h" |
| |
| using namespace llvm; |
| |
| #define PASS_KEY "x86-return-thunks" |
| #define DEBUG_TYPE PASS_KEY |
| |
| namespace { |
| struct X86ReturnThunks final : public MachineFunctionPass { |
| static char ID; |
| X86ReturnThunks() : MachineFunctionPass(ID) {} |
| StringRef getPassName() const override { return "X86 Return Thunks"; } |
| bool runOnMachineFunction(MachineFunction &MF) override; |
| }; |
| } // namespace |
| |
| char X86ReturnThunks::ID = 0; |
| |
| bool X86ReturnThunks::runOnMachineFunction(MachineFunction &MF) { |
| LLVM_DEBUG(dbgs() << getPassName() << "\n"); |
| |
| bool Modified = false; |
| |
| if (!MF.getFunction().hasFnAttribute(llvm::Attribute::FnRetThunkExtern)) |
| return Modified; |
| |
| StringRef ThunkName = "__x86_return_thunk"; |
| if (MF.getFunction().getName() == ThunkName) |
| return Modified; |
| |
| const auto &ST = MF.getSubtarget<X86Subtarget>(); |
| const bool Is64Bit = ST.getTargetTriple().getArch() == Triple::x86_64; |
| const unsigned RetOpc = Is64Bit ? X86::RET64 : X86::RET32; |
| SmallVector<MachineInstr *, 16> Rets; |
| |
| for (MachineBasicBlock &MBB : MF) |
| for (MachineInstr &Term : MBB.terminators()) |
| if (Term.getOpcode() == RetOpc) |
| Rets.push_back(&Term); |
| |
| bool IndCS = |
| MF.getMMI().getModule()->getModuleFlag("indirect_branch_cs_prefix"); |
| const MCInstrDesc &CS = ST.getInstrInfo()->get(X86::CS_PREFIX); |
| const MCInstrDesc &JMP = ST.getInstrInfo()->get(X86::TAILJMPd); |
| |
| for (MachineInstr *Ret : Rets) { |
| if (IndCS) |
| BuildMI(Ret->getParent(), Ret->getDebugLoc(), CS); |
| BuildMI(Ret->getParent(), Ret->getDebugLoc(), JMP) |
| .addExternalSymbol(ThunkName.data()); |
| Ret->eraseFromParent(); |
| Modified = true; |
| } |
| |
| return Modified; |
| } |
| |
| INITIALIZE_PASS(X86ReturnThunks, PASS_KEY, "X86 Return Thunks", false, false) |
| |
| FunctionPass *llvm::createX86ReturnThunksPass() { |
| return new X86ReturnThunks(); |
| } |