LLVM 17.0.0git
WinCFGuard.cpp
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1//===-- CodeGen/AsmPrinter/WinCFGuard.cpp - Control Flow Guard Impl ------===//
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//
9// This file contains support for writing the metadata for Windows Control Flow
10// Guard, including address-taken functions and valid longjmp targets.
11//
12//===----------------------------------------------------------------------===//
13
14#include "WinCFGuard.h"
18#include "llvm/IR/Constants.h"
19#include "llvm/IR/InstrTypes.h"
21#include "llvm/MC/MCStreamer.h"
22
23#include <vector>
24
25using namespace llvm;
26
28
29WinCFGuard::~WinCFGuard() = default;
30
32
33 // Skip functions without any longjmp targets.
34 if (MF->getLongjmpTargets().empty())
35 return;
36
37 // Copy the function's longjmp targets to a module-level list.
38 llvm::append_range(LongjmpTargets, MF->getLongjmpTargets());
39}
40
41/// Returns true if this function's address is escaped in a way that might make
42/// it an indirect call target. Function::hasAddressTaken gives different
43/// results when a function is called directly with a function prototype
44/// mismatch, which requires a cast.
47 while (!Users.empty()) {
48 const Value *FnOrCast = Users.pop_back_val();
49 for (const Use &U : FnOrCast->uses()) {
50 const User *FnUser = U.getUser();
51 if (isa<BlockAddress>(FnUser))
52 continue;
53 if (const auto *Call = dyn_cast<CallBase>(FnUser)) {
54 if (!Call->isCallee(&U))
55 return true;
56 } else if (isa<Instruction>(FnUser)) {
57 // Consider any other instruction to be an escape. This has some weird
58 // consequences like no-op intrinsics being an escape or a store *to* a
59 // function address being an escape.
60 return true;
61 } else if (const auto *C = dyn_cast<Constant>(FnUser)) {
62 // If this is a constant pointer cast of the function, don't consider
63 // this escape. Analyze the uses of the cast as well. This ensures that
64 // direct calls with mismatched prototypes don't end up in the CFG
65 // table. Consider other constants, such as vtable initializers, to
66 // escape the function.
67 if (C->stripPointerCasts() == F)
68 Users.push_back(FnUser);
69 else
70 return true;
71 }
72 }
73 }
74 return false;
75}
76
77MCSymbol *WinCFGuard::lookupImpSymbol(const MCSymbol *Sym) {
78 if (Sym->getName().startswith("__imp_"))
79 return nullptr;
80 return Asm->OutContext.lookupSymbol(Twine("__imp_") + Sym->getName());
81}
82
84 const Module *M = Asm->MMI->getModule();
85 std::vector<const MCSymbol *> GFIDsEntries;
86 std::vector<const MCSymbol *> GIATsEntries;
87 for (const Function &F : *M) {
89 // If F is a dllimport and has an "__imp_" symbol already defined, add the
90 // "__imp_" symbol to the .giats section.
91 if (F.hasDLLImportStorageClass()) {
92 if (MCSymbol *impSym = lookupImpSymbol(Asm->getSymbol(&F))) {
93 GIATsEntries.push_back(impSym);
94 }
95 }
96 // Add the function's symbol to the .gfids section.
97 // Note: For dllimport functions, MSVC sometimes does not add this symbol
98 // to the .gfids section, but only adds the corresponding "__imp_" symbol
99 // to the .giats section. Here we always add the symbol to the .gfids
100 // section, since this does not introduce security risks.
101 GFIDsEntries.push_back(Asm->getSymbol(&F));
102 }
103 }
104
105 if (GFIDsEntries.empty() && GIATsEntries.empty() && LongjmpTargets.empty())
106 return;
107
108 // Emit the symbol index of each GFIDs entry to form the .gfids section.
109 auto &OS = *Asm->OutStreamer;
110 OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGFIDsSection());
111 for (const MCSymbol *S : GFIDsEntries)
112 OS.emitCOFFSymbolIndex(S);
113
114 // Emit the symbol index of each GIATs entry to form the .giats section.
115 OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGIATsSection());
116 for (const MCSymbol *S : GIATsEntries) {
117 OS.emitCOFFSymbolIndex(S);
118 }
119
120 // Emit the symbol index of each longjmp target to form the .gljmp section.
121 OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGLJMPSection());
122 for (const MCSymbol *S : LongjmpTargets) {
123 OS.emitCOFFSymbolIndex(S);
124 }
125}
static GCRegistry::Add< ErlangGC > A("erlang", "erlang-compatible garbage collector")
This file contains the declarations for the subclasses of Constant, which represent the different fla...
iv Induction Variable Users
Definition: IVUsers.cpp:48
#define F(x, y, z)
Definition: MD5.cpp:55
static bool isPossibleIndirectCallTarget(const Function *F)
Returns true if this function's address is escaped in a way that might make it an indirect call targe...
Definition: WinCFGuard.cpp:45
This class is intended to be used as a driving class for all asm writers.
Definition: AsmPrinter.h:84
MCSymbol * getSymbol(const GlobalValue *GV) const
Definition: AsmPrinter.cpp:643
MachineModuleInfo * MMI
This is a pointer to the current MachineModuleInfo.
Definition: AsmPrinter.h:105
MCContext & OutContext
This is the context for the output file that we are streaming.
Definition: AsmPrinter.h:94
std::unique_ptr< MCStreamer > OutStreamer
This is the MCStreamer object for the file we are generating.
Definition: AsmPrinter.h:99
const MCObjectFileInfo * getObjectFileInfo() const
Definition: MCContext.h:450
MCSymbol * lookupSymbol(const Twine &Name) const
Get the symbol for Name, or null.
Definition: MCContext.cpp:359
MCSection * getGFIDsSection() const
MCSection * getGLJMPSection() const
MCSection * getGIATsSection() const
MCSymbol - Instances of this class represent a symbol name in the MC file, and MCSymbols are created ...
Definition: MCSymbol.h:41
StringRef getName() const
getName - Get the symbol name.
Definition: MCSymbol.h:203
const std::vector< MCSymbol * > & getLongjmpTargets() const
Returns a reference to a list of symbols immediately following calls to _setjmp in the function.
const Module * getModule() const
A Module instance is used to store all the information related to an LLVM module.
Definition: Module.h:65
This is a 'vector' (really, a variable-sized array), optimized for the case when the array is small.
Definition: SmallVector.h:1200
bool startswith(StringRef Prefix) const
Definition: StringRef.h:261
Twine - A lightweight data structure for efficiently representing the concatenation of temporary valu...
Definition: Twine.h:81
A Use represents the edge between a Value definition and its users.
Definition: Use.h:43
LLVM Value Representation.
Definition: Value.h:74
iterator_range< use_iterator > uses()
Definition: Value.h:376
WinCFGuard(AsmPrinter *A)
Definition: WinCFGuard.cpp:27
void endModule() override
Emit the Control Flow Guard function ID table.
Definition: WinCFGuard.cpp:83
void endFunction(const MachineFunction *MF) override
Gather post-function debug information.
Definition: WinCFGuard.cpp:31
~WinCFGuard() override
@ C
The default llvm calling convention, compatible with C.
Definition: CallingConv.h:34
This is an optimization pass for GlobalISel generic memory operations.
Definition: AddressRanges.h:18
void append_range(Container &C, Range &&R)
Wrapper function to append a range to a container.
Definition: STLExtras.h:2014