LLVM 19.0.0git
X86ReturnThunks.cpp
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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"
30#include "llvm/ADT/StringRef.h"
37#include "llvm/MC/MCInstrDesc.h"
38#include "llvm/Support/Debug.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;
73
74 for (MachineBasicBlock &MBB : MF)
75 for (MachineInstr &Term : MBB.terminators())
76 if (Term.getOpcode() == RetOpc)
77 Rets.push_back(&Term);
78
79 bool IndCS =
80 MF.getMMI().getModule()->getModuleFlag("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(Ret->getParent(), Ret->getDebugLoc(), CS);
87 BuildMI(Ret->getParent(), Ret->getDebugLoc(), JMP)
88 .addExternalSymbol(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
99 return new X86ReturnThunks();
100}
MachineBasicBlock & MBB
#define LLVM_DEBUG(X)
Definition: Debug.h:101
#define INITIALIZE_PASS(passName, arg, name, cfg, analysis)
Definition: PassSupport.h:38
This file defines the SmallVector class.
#define PASS_KEY
FunctionPass class - This class is used to implement most global optimizations.
Definition: Pass.h:311
bool hasFnAttribute(Attribute::AttrKind Kind) const
Return true if the function has the attribute.
Definition: Function.cpp:683
Describe properties that are true of each instruction in the target description file.
Definition: MCInstrDesc.h:198
iterator_range< iterator > terminators()
MachineFunctionPass - This class adapts the FunctionPass interface to allow convenient creation of pa...
virtual bool runOnMachineFunction(MachineFunction &MF)=0
runOnMachineFunction - This method must be overloaded to perform the desired machine code transformat...
const TargetSubtargetInfo & getSubtarget() const
getSubtarget - Return the subtarget for which this machine code is being compiled.
Function & getFunction()
Return the LLVM function that this machine code represents.
const MachineInstrBuilder & addExternalSymbol(const char *FnName, unsigned TargetFlags=0) const
Representation of each machine instruction.
Definition: MachineInstr.h:69
virtual StringRef getPassName() const
getPassName - Return a nice clean name for a pass.
Definition: Pass.cpp:81
void push_back(const T &Elt)
Definition: SmallVector.h:426
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1209
StringRef - Represent a constant reference to a string, i.e.
Definition: StringRef.h:50
constexpr const char * data() const
data - Get a pointer to the start of the string (which may not be null terminated).
Definition: StringRef.h:131
StringRef getName() const
Return a constant reference to the value's name.
Definition: Value.cpp:309
unsigned ID
LLVM IR allows to use arbitrary numbers as calling convention identifiers.
Definition: CallingConv.h:24
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
MachineInstrBuilder BuildMI(MachineFunction &MF, const MIMetadata &MIMD, const MCInstrDesc &MCID)
Builder interface. Specify how to create the initial instruction itself.
raw_ostream & dbgs()
dbgs() - This returns a reference to a raw_ostream for debugging messages.
Definition: Debug.cpp:163
FunctionPass * createX86ReturnThunksPass()
This pass replaces ret instructions with jmp's to __x86_return thunk.