llvm/lib/CodeGen/AsmPrinter/WinCFGuard.cpp

   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/IR/Constants.h"
  19 #include "llvm/IR/InstrTypes.h"
  20 #include "llvm/MC/MCObjectFileInfo.h"
  21 #include "llvm/MC/MCStreamer.h"
  22
  23 #include <vector>
  24
  25 using namespace llvm;
  26
  27 WinCFGuard::WinCFGuard(AsmPrinter *A) : Asm(A) {}
  28
  29 WinCFGuard::~WinCFGuard() = default;
  30
  31 void WinCFGuard::endFunction(const MachineFunction *MF) {
  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.
  45 static bool isPossibleIndirectCallTarget(const Function *F) {
  46   SmallVector<const Value *, 4> Users{F};
  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
  77 MCSymbol *WinCFGuard::lookupImpSymbol(const MCSymbol *Sym) {
  78   if (Sym->getName().starts_with("__imp_"))
  79     return nullptr;
  80   return Asm->OutContext.lookupSymbol(Twine("__imp_") + Sym->getName());
  81 }
  82
  83 void WinCFGuard::endModule() {
  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) {
  88     if (isPossibleIndirectCallTarget(&F)) {
  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 }