llvm/lib/Target/X86/X86ReturnThunks.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-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 /// \file
   9 ///
  10 /// Pass that replaces ret instructions with a jmp to __x86_return_thunk.
  11 ///
  12 /// This corresponds to -mfunction-return=thunk-extern or
  13 /// __attribute__((function_return("thunk-extern").
  14 ///
  15 /// This pass is a minimal implementation necessary to help mitigate
  16 /// RetBleed for the Linux kernel.
  17 ///
  18 /// Should support for thunk or thunk-inline be necessary in the future, then
  19 /// this pass should be combined with x86-retpoline-thunks which already has
  20 /// 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
  42 using namespace llvm;
  43
  44 #define PASS_KEY "x86-return-thunks"
  45 #define DEBUG_TYPE PASS_KEY
  46
  47 namespace {
  48 struct 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 } // namespace
  55
  56 char X86ReturnThunks::ID = 0;
  57
  58 bool 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().getArch() == Triple::x86_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
  97 INITIALIZE_PASS(X86ReturnThunks, PASS_KEY, "X86 Return Thunks", false, false)
  98
  99 FunctionPass *llvm::createX86ReturnThunksPass() {
 100   return new X86ReturnThunks();
 101 }