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 | |
41 | using namespace llvm; |
42 | |
43 | #define PASS_KEY "x86-return-thunks" |
44 | #define DEBUG_TYPE PASS_KEY |
45 | |
46 | namespace { |
47 | struct 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 | |
55 | char X86ReturnThunks::ID = 0; |
56 | |
57 | bool 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 | |
96 | INITIALIZE_PASS(X86ReturnThunks, PASS_KEY, "X86 Return Thunks" , false, false) |
97 | |
98 | FunctionPass *llvm::createX86ReturnThunksPass() { |
99 | return new X86ReturnThunks(); |
100 | } |
101 | |