1 //===- AArch64GlobalsTagging.cpp - Global tagging in IR -------------------===// 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 11 #include "AArch64.h" 12 #include "llvm/BinaryFormat/ELF.h" 13 #include "llvm/IR/Attributes.h" 14 #include "llvm/IR/Constants.h" 15 #include "llvm/IR/GlobalValue.h" 16 #include "llvm/IR/GlobalVariable.h" 17 #include "llvm/IR/IRBuilder.h" 18 #include "llvm/IR/Module.h" 19 #include "llvm/Pass.h" 20 #include "llvm/Support/raw_ostream.h" 21 22 #include <algorithm> 23 #include <set> 24 25 using namespace llvm; 26 27 static const Align kTagGranuleSize = Align(16); 28 29 static bool shouldTagGlobal(GlobalVariable &G) { 30 if (!G.isTagged()) 31 return false; 32 33 assert(G.hasSanitizerMetadata() && 34 "Missing sanitizer metadata, but symbol is apparently tagged."); 35 GlobalValue::SanitizerMetadata Meta = G.getSanitizerMetadata(); 36 37 // For now, don't instrument constant data, as it'll be in .rodata anyway. It 38 // may be worth instrumenting these in future to stop them from being used as 39 // gadgets. 40 if (G.getName().startswith("llvm.") || G.isThreadLocal() || G.isConstant()) { 41 Meta.Memtag = false; 42 G.setSanitizerMetadata(Meta); 43 return false; 44 } 45 46 return true; 47 } 48 49 // Technically, due to ELF symbol interposition semantics, we can't change the 50 // alignment or size of symbols. If we increase the alignment or size of a 51 // symbol, the compiler may make optimisations based on this new alignment or 52 // size. If the symbol is interposed, this optimisation could lead to 53 // alignment-related or OOB read/write crashes. 54 // 55 // This is handled in the linker. When the linker sees multiple declarations of 56 // a global variable, and some are tagged, and some are untagged, it resolves it 57 // to be an untagged definition - but preserves the tag-granule-rounded size and 58 // tag-granule-alignment. This should prevent these kind of crashes intra-DSO. 59 // For cross-DSO, it's been a reasonable contract that if you're interposing a 60 // sanitizer-instrumented global, then the interposer also needs to be 61 // sanitizer-instrumented. 62 // 63 // FIXME: In theory, this can be fixed by splitting the size/alignment of 64 // globals into two uses: an "output alignment" that's emitted to the ELF file, 65 // and an "optimisation alignment" that's used for optimisation. Thus, we could 66 // adjust the output alignment only, and still optimise based on the pessimistic 67 // pre-tagging size/alignment. 68 static void tagGlobalDefinition(Module &M, GlobalVariable *G) { 69 Constant *Initializer = G->getInitializer(); 70 uint64_t SizeInBytes = 71 M.getDataLayout().getTypeAllocSize(Initializer->getType()); 72 73 uint64_t NewSize = alignTo(SizeInBytes, kTagGranuleSize); 74 if (SizeInBytes != NewSize) { 75 // Pad the initializer out to the next multiple of 16 bytes. 76 llvm::SmallVector<uint8_t> Init(NewSize - SizeInBytes, 0); 77 Constant *Padding = ConstantDataArray::get(M.getContext(), Init); 78 Initializer = ConstantStruct::getAnon({Initializer, Padding}); 79 auto *NewGV = new GlobalVariable( 80 M, Initializer->getType(), G->isConstant(), G->getLinkage(), 81 Initializer, "", G, G->getThreadLocalMode(), G->getAddressSpace()); 82 NewGV->copyAttributesFrom(G); 83 NewGV->setComdat(G->getComdat()); 84 NewGV->copyMetadata(G, 0); 85 86 NewGV->takeName(G); 87 G->replaceAllUsesWith(NewGV); 88 G->eraseFromParent(); 89 G = NewGV; 90 } 91 92 G->setAlignment(std::max(G->getAlign().valueOrOne(), kTagGranuleSize)); 93 94 // Ensure that tagged globals don't get merged by ICF - as they should have 95 // different tags at runtime. 96 G->setUnnamedAddr(GlobalValue::UnnamedAddr::None); 97 } 98 99 namespace { 100 class AArch64GlobalsTagging : public ModulePass { 101 public: 102 static char ID; 103 104 explicit AArch64GlobalsTagging() : ModulePass(ID) { 105 initializeAArch64GlobalsTaggingPass(*PassRegistry::getPassRegistry()); 106 } 107 108 bool runOnModule(Module &M) override; 109 110 StringRef getPassName() const override { return "AArch64 Globals Tagging"; } 111 112 private: 113 std::set<GlobalVariable *> GlobalsToTag; 114 }; 115 } // anonymous namespace 116 117 char AArch64GlobalsTagging::ID = 0; 118 119 bool AArch64GlobalsTagging::runOnModule(Module &M) { 120 // No mutating the globals in-place, or iterator invalidation occurs. 121 std::vector<GlobalVariable *> GlobalsToTag; 122 for (GlobalVariable &G : M.globals()) { 123 if (G.isDeclaration() || !shouldTagGlobal(G)) 124 continue; 125 GlobalsToTag.push_back(&G); 126 } 127 128 for (GlobalVariable *G : GlobalsToTag) { 129 tagGlobalDefinition(M, G); 130 } 131 132 return true; 133 } 134 135 INITIALIZE_PASS_BEGIN(AArch64GlobalsTagging, "aarch64-globals-tagging", 136 "AArch64 Globals Tagging Pass", false, false) 137 INITIALIZE_PASS_END(AArch64GlobalsTagging, "aarch64-globals-tagging", 138 "AArch64 Globals Tagging Pass", false, false) 139 140 ModulePass *llvm::createAArch64GlobalsTaggingPass() { 141 return new AArch64GlobalsTagging(); 142 } 143