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/Instructions.h"
21 #include "llvm/IR/Metadata.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 
30 WinCFGuard::WinCFGuard(AsmPrinter *A) : AsmPrinterHandler(), Asm(A) {}
31 
32 WinCFGuard::~WinCFGuard() {}
33 
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.
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 
81 /// Returns true if this function should be added to the Guard Address Taken IAT
82 /// Entry Table (GIATs) instead of the Guard Function ID Table (GFIDs).
83 static bool isIATAddressTaken(const Function *F) {
84   if (F->hasDLLImportStorageClass()) {
85     return true;
86   }
87   return false;
88 }
89 
90 void WinCFGuard::endModule() {
91   const Module *M = Asm->MMI->getModule();
92   std::vector<const Function *> GFIDsEntries;
93   std::vector<const Function *> GIATsEntries;
94   for (const Function &F : *M) {
95     if (isPossibleIndirectCallTarget(&F)) {
96       if (isIATAddressTaken(&F)) {
97         // If the possible call target is reached via the IAT, add it to the
98         // GIATs table instead of the GFIDs table.
99         GIATsEntries.push_back(&F);
100       } else {
101         // Otherwise add it to the GFIDs table.
102         GFIDsEntries.push_back(&F);
103       }
104     }
105   }
106 
107   if (GFIDsEntries.empty() && GIATsEntries.empty() && LongjmpTargets.empty())
108     return;
109 
110   // Emit the symbol index of each GFIDs entry to form the GFIDs table.
111   auto &OS = *Asm->OutStreamer;
112   OS.SwitchSection(Asm->OutContext.getObjectFileInfo()->getGFIDsSection());
113   for (const Function *F : GFIDsEntries)
114     OS.EmitCOFFSymbolIndex(Asm->getSymbol(F));
115 
116   // Emit the symbol index of each GIATs entry to form the GIATs table.
117   OS.SwitchSection(Asm->OutContext.getObjectFileInfo()->getGIATsSection());
118   for (const Function *F : GIATsEntries) {
119     OS.EmitCOFFSymbolIndex(Asm->OutContext.getOrCreateSymbol(
120         Twine("__imp_") + Asm->getSymbol(F)->getName()));
121   }
122 
123   // Emit the symbol index of each longjmp target to form the GLJMP table.
124   OS.SwitchSection(Asm->OutContext.getObjectFileInfo()->getGLJMPSection());
125   for (const MCSymbol *S : LongjmpTargets) {
126     OS.EmitCOFFSymbolIndex(S);
127   }
128 }
129