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/IR/Module.h"
  21 #include "llvm/MC/MCObjectFileInfo.h"
  22 #include "llvm/MC/MCStreamer.h"
  23
  24 #include <vector>
  25
  26 using namespace llvm;
  27
  28 WinCFGuard::WinCFGuard(AsmPrinter *A) : Asm(A) {}
  29
  30 WinCFGuard::~WinCFGuard() = default;
  31
  32 void WinCFGuard::endFunction(const MachineFunction *MF) {
  33
  34   // Skip functions without any longjmp targets.
  35   if (MF->getLongjmpTargets().empty())
  36     return;
  37
  38   // Copy the function's longjmp targets to a module-level list.
  39   llvm::append_range(LongjmpTargets, MF->getLongjmpTargets());
  40 }
  41
  42 /// Returns true if this function's address is escaped in a way that might make
  43 /// it an indirect call target. Function::hasAddressTaken gives different
  44 /// results when a function is called directly with a function prototype
  45 /// mismatch, which requires a cast.
  46 static bool isPossibleIndirectCallTarget(const Function *F) {
  47   SmallVector<const Value *, 4> Users{F};
  48   while (!Users.empty()) {
  49     const Value *FnOrCast = Users.pop_back_val();
  50     for (const Use &U : FnOrCast->uses()) {
  51       const User *FnUser = U.getUser();
  52       if (isa<BlockAddress>(FnUser))
  53         continue;
  54       if (const auto *Call = dyn_cast<CallBase>(FnUser)) {
  55         if (!Call->isCallee(&U))
  56           return true;
  57       } else if (isa<Instruction>(FnUser)) {
  58         // Consider any other instruction to be an escape. This has some weird
  59         // consequences like no-op intrinsics being an escape or a store *to* a
  60         // function address being an escape.
  61         return true;
  62       } else if (const auto *C = dyn_cast<Constant>(FnUser)) {
  63         // If this is a constant pointer cast of the function, don't consider
  64         // this escape. Analyze the uses of the cast as well. This ensures that
  65         // direct calls with mismatched prototypes don't end up in the CFG
  66         // table. Consider other constants, such as vtable initializers, to
  67         // escape the function.
  68         if (C->stripPointerCasts() == F)
  69           Users.push_back(FnUser);
  70         else
  71           return true;
  72       }
  73     }
  74   }
  75   return false;
  76 }
  77
  78 MCSymbol *WinCFGuard::lookupImpSymbol(const MCSymbol *Sym) {
  79   if (Sym->getName().starts_with("__imp_"))
  80     return nullptr;
  81   return Asm->OutContext.lookupSymbol(Twine("__imp_") + Sym->getName());
  82 }
  83
  84 void WinCFGuard::endModule() {
  85   const Module *M = Asm->MMI->getModule();
  86   std::vector<const MCSymbol *> GFIDsEntries;
  87   std::vector<const MCSymbol *> GIATsEntries;
  88   for (const Function &F : *M) {
  89     if (isPossibleIndirectCallTarget(&F)) {
  90       // If F is a dllimport and has an "__imp_" symbol already defined, add the
  91       // "__imp_" symbol to the .giats section.
  92       if (F.hasDLLImportStorageClass()) {
  93         if (MCSymbol *impSym = lookupImpSymbol(Asm->getSymbol(&F))) {
  94           GIATsEntries.push_back(impSym);
  95         }
  96       }
  97       // Add the function's symbol to the .gfids section.
  98       // Note: For dllimport functions, MSVC sometimes does not add this symbol
  99       // to the .gfids section, but only adds the corresponding "__imp_" symbol
 100       // to the .giats section. Here we always add the symbol to the .gfids
 101       // section, since this does not introduce security risks.
 102       GFIDsEntries.push_back(Asm->getSymbol(&F));
 103     }
 104   }
 105
 106   if (GFIDsEntries.empty() && GIATsEntries.empty() && LongjmpTargets.empty())
 107     return;
 108
 109   // Emit the symbol index of each GFIDs entry to form the .gfids section.
 110   auto &OS = *Asm->OutStreamer;
 111   OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGFIDsSection());
 112   for (const MCSymbol *S : GFIDsEntries)
 113     OS.emitCOFFSymbolIndex(S);
 114
 115   // Emit the symbol index of each GIATs entry to form the .giats section.
 116   OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGIATsSection());
 117   for (const MCSymbol *S : GIATsEntries) {
 118     OS.emitCOFFSymbolIndex(S);
 119   }
 120
 121   // Emit the symbol index of each longjmp target to form the .gljmp section.
 122   OS.switchSection(Asm->OutContext.getObjectFileInfo()->getGLJMPSection());
 123   for (const MCSymbol *S : LongjmpTargets) {
 124     OS.emitCOFFSymbolIndex(S);
 125   }
 126 }