1 //===-- CrossDSOCFI.cpp - Externalize this module's CFI checks ------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass exports all llvm.bitset's found in the module in the form of a
11 // __cfi_check function, which can be used to verify cross-DSO call targets.
12 //
13 //===----------------------------------------------------------------------===//
14 
15 #include "llvm/Transforms/IPO/CrossDSOCFI.h"
16 #include "llvm/ADT/DenseSet.h"
17 #include "llvm/ADT/EquivalenceClasses.h"
18 #include "llvm/ADT/Statistic.h"
19 #include "llvm/IR/Constant.h"
20 #include "llvm/IR/Constants.h"
21 #include "llvm/IR/Function.h"
22 #include "llvm/IR/GlobalObject.h"
23 #include "llvm/IR/GlobalVariable.h"
24 #include "llvm/IR/IRBuilder.h"
25 #include "llvm/IR/Instructions.h"
26 #include "llvm/IR/Intrinsics.h"
27 #include "llvm/IR/MDBuilder.h"
28 #include "llvm/IR/Module.h"
29 #include "llvm/IR/Operator.h"
30 #include "llvm/Pass.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/raw_ostream.h"
33 #include "llvm/Transforms/IPO.h"
34 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
35 
36 using namespace llvm;
37 
38 #define DEBUG_TYPE "cross-dso-cfi"
39 
40 STATISTIC(NumTypeIds, "Number of unique type identifiers");
41 
42 namespace {
43 
44 struct CrossDSOCFI : public ModulePass {
45   static char ID;
46   CrossDSOCFI() : ModulePass(ID) {
47     initializeCrossDSOCFIPass(*PassRegistry::getPassRegistry());
48   }
49 
50   MDNode *VeryLikelyWeights;
51 
52   ConstantInt *extractNumericTypeId(MDNode *MD);
53   void buildCFICheck(Module &M);
54   bool runOnModule(Module &M) override;
55 };
56 
57 } // anonymous namespace
58 
59 INITIALIZE_PASS_BEGIN(CrossDSOCFI, "cross-dso-cfi", "Cross-DSO CFI", false,
60                       false)
61 INITIALIZE_PASS_END(CrossDSOCFI, "cross-dso-cfi", "Cross-DSO CFI", false, false)
62 char CrossDSOCFI::ID = 0;
63 
64 ModulePass *llvm::createCrossDSOCFIPass() { return new CrossDSOCFI; }
65 
66 /// Extracts a numeric type identifier from an MDNode containing type metadata.
67 ConstantInt *CrossDSOCFI::extractNumericTypeId(MDNode *MD) {
68   // This check excludes vtables for classes inside anonymous namespaces.
69   auto TM = dyn_cast<ValueAsMetadata>(MD->getOperand(1));
70   if (!TM)
71     return nullptr;
72   auto C = dyn_cast_or_null<ConstantInt>(TM->getValue());
73   if (!C) return nullptr;
74   // We are looking for i64 constants.
75   if (C->getBitWidth() != 64) return nullptr;
76 
77   return C;
78 }
79 
80 /// buildCFICheck - emits __cfi_check for the current module.
81 void CrossDSOCFI::buildCFICheck(Module &M) {
82   // FIXME: verify that __cfi_check ends up near the end of the code section,
83   // but before the jump slots created in LowerTypeTests.
84   llvm::DenseSet<uint64_t> TypeIds;
85   SmallVector<MDNode *, 2> Types;
86   for (GlobalObject &GO : M.global_objects()) {
87     Types.clear();
88     GO.getMetadata(LLVMContext::MD_type, Types);
89     for (MDNode *Type : Types) {
90       // Sanity check. GO must not be a function declaration.
91       assert(!isa<Function>(&GO) || !cast<Function>(&GO)->isDeclaration());
92 
93       if (ConstantInt *TypeId = extractNumericTypeId(Type))
94         TypeIds.insert(TypeId->getZExtValue());
95     }
96   }
97 
98   NamedMDNode *CfiFunctionsMD = M.getNamedMetadata("cfi.functions");
99   if (CfiFunctionsMD) {
100     for (auto Func : CfiFunctionsMD->operands()) {
101       assert(Func->getNumOperands() >= 2);
102       for (unsigned I = 2; I < Func->getNumOperands(); ++I)
103         if (ConstantInt *TypeId =
104                 extractNumericTypeId(cast<MDNode>(Func->getOperand(I).get())))
105           TypeIds.insert(TypeId->getZExtValue());
106     }
107   }
108 
109   LLVMContext &Ctx = M.getContext();
110   Constant *C = M.getOrInsertFunction(
111       "__cfi_check", Type::getVoidTy(Ctx), Type::getInt64Ty(Ctx),
112       Type::getInt8PtrTy(Ctx), Type::getInt8PtrTy(Ctx));
113   Function *F = dyn_cast<Function>(C);
114   // Take over the existing function. The frontend emits a weak stub so that the
115   // linker knows about the symbol; this pass replaces the function body.
116   F->deleteBody();
117   F->setAlignment(4096);
118   auto args = F->arg_begin();
119   Value &CallSiteTypeId = *(args++);
120   CallSiteTypeId.setName("CallSiteTypeId");
121   Value &Addr = *(args++);
122   Addr.setName("Addr");
123   Value &CFICheckFailData = *(args++);
124   CFICheckFailData.setName("CFICheckFailData");
125   assert(args == F->arg_end());
126 
127   BasicBlock *BB = BasicBlock::Create(Ctx, "entry", F);
128   BasicBlock *ExitBB = BasicBlock::Create(Ctx, "exit", F);
129 
130   BasicBlock *TrapBB = BasicBlock::Create(Ctx, "fail", F);
131   IRBuilder<> IRBFail(TrapBB);
132   Constant *CFICheckFailFn = M.getOrInsertFunction(
133       "__cfi_check_fail", Type::getVoidTy(Ctx), Type::getInt8PtrTy(Ctx),
134       Type::getInt8PtrTy(Ctx));
135   IRBFail.CreateCall(CFICheckFailFn, {&CFICheckFailData, &Addr});
136   IRBFail.CreateBr(ExitBB);
137 
138   IRBuilder<> IRBExit(ExitBB);
139   IRBExit.CreateRetVoid();
140 
141   IRBuilder<> IRB(BB);
142   SwitchInst *SI = IRB.CreateSwitch(&CallSiteTypeId, TrapBB, TypeIds.size());
143   for (uint64_t TypeId : TypeIds) {
144     ConstantInt *CaseTypeId = ConstantInt::get(Type::getInt64Ty(Ctx), TypeId);
145     BasicBlock *TestBB = BasicBlock::Create(Ctx, "test", F);
146     IRBuilder<> IRBTest(TestBB);
147     Function *BitsetTestFn = Intrinsic::getDeclaration(&M, Intrinsic::type_test);
148 
149     Value *Test = IRBTest.CreateCall(
150         BitsetTestFn, {&Addr, MetadataAsValue::get(
151                                   Ctx, ConstantAsMetadata::get(CaseTypeId))});
152     BranchInst *BI = IRBTest.CreateCondBr(Test, ExitBB, TrapBB);
153     BI->setMetadata(LLVMContext::MD_prof, VeryLikelyWeights);
154 
155     SI->addCase(CaseTypeId, TestBB);
156     ++NumTypeIds;
157   }
158 }
159 
160 bool CrossDSOCFI::runOnModule(Module &M) {
161   if (skipModule(M))
162     return false;
163 
164   VeryLikelyWeights =
165     MDBuilder(M.getContext()).createBranchWeights((1U << 20) - 1, 1);
166   if (M.getModuleFlag("Cross-DSO CFI") == nullptr)
167     return false;
168   buildCFICheck(M);
169   return true;
170 }
171 
172 PreservedAnalyses CrossDSOCFIPass::run(Module &M, ModuleAnalysisManager &AM) {
173   CrossDSOCFI Impl;
174   bool Changed = Impl.runOnModule(M);
175   if (!Changed)
176     return PreservedAnalyses::all();
177   return PreservedAnalyses::none();
178 }
179