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"
15 #include "llvm/CodeGen/AsmPrinter.h"
16 #include "llvm/CodeGen/MachineFunction.h"
17 #include "llvm/CodeGen/MachineModuleInfo.h"
18 #include "llvm/CodeGen/MachineOperand.h"
19 #include "llvm/IR/Constants.h"
20 #include "llvm/IR/Metadata.h"
21 #include "llvm/IR/Instructions.h"
22 #include "llvm/MC/MCAsmInfo.h"
23 #include "llvm/MC/MCObjectFileInfo.h"
24 #include "llvm/MC/MCStreamer.h"
25
26 #include <vector>
27
28 using namespace llvm;
29
WinCFGuard(AsmPrinter * A)30 WinCFGuard::WinCFGuard(AsmPrinter *A) : AsmPrinterHandler(), Asm(A) {}
31
~WinCFGuard()32 WinCFGuard::~WinCFGuard() {}
33
endFunction(const MachineFunction * MF)34 void WinCFGuard::endFunction(const MachineFunction *MF) {
35
36 // Skip functions without any longjmp targets.
37 if (MF->getLongjmpTargets().empty())
38 return;
39
40 // Copy the function's longjmp targets to a module-level list.
41 LongjmpTargets.insert(LongjmpTargets.end(), MF->getLongjmpTargets().begin(),
42 MF->getLongjmpTargets().end());
43 }
44
45 /// Returns true if this function's address is escaped in a way that might make
46 /// it an indirect call target. Function::hasAddressTaken gives different
47 /// results when a function is called directly with a function prototype
48 /// mismatch, which requires a cast.
isPossibleIndirectCallTarget(const Function * F)49 static bool isPossibleIndirectCallTarget(const Function *F) {
50 SmallVector<const Value *, 4> Users{F};
51 while (!Users.empty()) {
52 const Value *FnOrCast = Users.pop_back_val();
53 for (const Use &U : FnOrCast->uses()) {
54 const User *FnUser = U.getUser();
55 if (isa<BlockAddress>(FnUser))
56 continue;
57 if (const auto *Call = dyn_cast<CallBase>(FnUser)) {
58 if (!Call->isCallee(&U))
59 return true;
60 } else if (isa<Instruction>(FnUser)) {
61 // Consider any other instruction to be an escape. This has some weird
62 // consequences like no-op intrinsics being an escape or a store *to* a
63 // function address being an escape.
64 return true;
65 } else if (const auto *C = dyn_cast<Constant>(FnUser)) {
66 // If this is a constant pointer cast of the function, don't consider
67 // this escape. Analyze the uses of the cast as well. This ensures that
68 // direct calls with mismatched prototypes don't end up in the CFG
69 // table. Consider other constants, such as vtable initializers, to
70 // escape the function.
71 if (C->stripPointerCasts() == F)
72 Users.push_back(FnUser);
73 else
74 return true;
75 }
76 }
77 }
78 return false;
79 }
80
endModule()81 void WinCFGuard::endModule() {
82 const Module *M = Asm->MMI->getModule();
83 std::vector<const Function *> Functions;
84 for (const Function &F : *M)
85 if (isPossibleIndirectCallTarget(&F))
86 Functions.push_back(&F);
87 if (Functions.empty() && LongjmpTargets.empty())
88 return;
89 auto &OS = *Asm->OutStreamer;
90 OS.SwitchSection(Asm->OutContext.getObjectFileInfo()->getGFIDsSection());
91 for (const Function *F : Functions)
92 OS.EmitCOFFSymbolIndex(Asm->getSymbol(F));
93
94 // Emit the symbol index of each longjmp target.
95 OS.SwitchSection(Asm->OutContext.getObjectFileInfo()->getGLJMPSection());
96 for (const MCSymbol *S : LongjmpTargets) {
97 OS.EmitCOFFSymbolIndex(S);
98 }
99 }
100