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;
X86ReturnThunks__anon9bfa515e0111::X86ReturnThunks50 X86ReturnThunks() : MachineFunctionPass(ID) {}
getPassName__anon9bfa515e0111::X86ReturnThunks51 StringRef getPassName() const override { return "X86 Return Thunks"; }
52 bool runOnMachineFunction(MachineFunction &MF) override;
53 };
54 } // namespace
55
56 char X86ReturnThunks::ID = 0;
57
runOnMachineFunction(MachineFunction & MF)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
createX86ReturnThunksPass()99 FunctionPass *llvm::createX86ReturnThunksPass() {
100 return new X86ReturnThunks();
101 }
102