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/MC/MCInstrDesc.h"
38#include "llvm/Support/Debug.h"
39#include "llvm/TargetParser/Triple.h"
40
41using namespace llvm;
42
43#define PASS_KEY "x86-return-thunks"
44#define DEBUG_TYPE PASS_KEY
45
46namespace {
47struct X86ReturnThunks final : public MachineFunctionPass {
48 static char ID;
49 X86ReturnThunks() : MachineFunctionPass(ID) {}
50 StringRef getPassName() const override { return "X86 Return Thunks"; }
51 bool runOnMachineFunction(MachineFunction &MF) override;
52};
53} // namespace
54
55char X86ReturnThunks::ID = 0;
56
57bool X86ReturnThunks::runOnMachineFunction(MachineFunction &MF) {
58 LLVM_DEBUG(dbgs() << getPassName() << "\n");
59
60 bool Modified = false;
61
62 if (!MF.getFunction().hasFnAttribute(llvm::Attribute::FnRetThunkExtern))
63 return Modified;
64
65 StringRef ThunkName = "__x86_return_thunk";
66 if (MF.getFunction().getName() == ThunkName)
67 return Modified;
68
69 const auto &ST = MF.getSubtarget<X86Subtarget>();
70 const bool Is64Bit = ST.getTargetTriple().getArch() == Triple::x86_64;
71 const unsigned RetOpc = Is64Bit ? X86::RET64 : X86::RET32;
72 SmallVector<MachineInstr *, 16> Rets;
73
74 for (MachineBasicBlock &MBB : MF)
75 for (MachineInstr &Term : MBB.terminators())
76 if (Term.getOpcode() == RetOpc)
77 Rets.push_back(Elt: &Term);
78
79 bool IndCS =
80 MF.getMMI().getModule()->getModuleFlag(Key: "indirect_branch_cs_prefix");
81 const MCInstrDesc &CS = ST.getInstrInfo()->get(X86::CS_PREFIX);
82 const MCInstrDesc &JMP = ST.getInstrInfo()->get(X86::TAILJMPd);
83
84 for (MachineInstr *Ret : Rets) {
85 if (IndCS)
86 BuildMI(BB: Ret->getParent(), MIMD: Ret->getDebugLoc(), MCID: CS);
87 BuildMI(BB: Ret->getParent(), MIMD: Ret->getDebugLoc(), MCID: JMP)
88 .addExternalSymbol(FnName: ThunkName.data());
89 Ret->eraseFromParent();
90 Modified = true;
91 }
92
93 return Modified;
94}
95
96INITIALIZE_PASS(X86ReturnThunks, PASS_KEY, "X86 Return Thunks", false, false)
97
98FunctionPass *llvm::createX86ReturnThunksPass() {
99 return new X86ReturnThunks();
100}
101

source code of llvm/lib/Target/X86/X86ReturnThunks.cpp