blob: 2ed668712897ce7898df93a27bfba24b27cc1560 [file] [log] [blame]
//===- AArch64GlobalsTagging.cpp - Global tagging in IR -------------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
#include "AArch64.h"
#include "llvm/BinaryFormat/ELF.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/GlobalValue.h"
#include "llvm/IR/GlobalVariable.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <set>
using namespace llvm;
static const Align kTagGranuleSize = Align(16);
static bool shouldTagGlobal(GlobalVariable &G) {
if (!G.isTagged())
return false;
assert(G.hasSanitizerMetadata() &&
"Missing sanitizer metadata, but symbol is apparently tagged.");
GlobalValue::SanitizerMetadata Meta = G.getSanitizerMetadata();
// For now, don't instrument constant data, as it'll be in .rodata anyway. It
// may be worth instrumenting these in future to stop them from being used as
// gadgets.
if (G.getName().startswith("llvm.") || G.isThreadLocal() || G.isConstant()) {
Meta.Memtag = false;
return false;
return true;
// Technically, due to ELF symbol interposition semantics, we can't change the
// alignment or size of symbols. If we increase the alignment or size of a
// symbol, the compiler may make optimisations based on this new alignment or
// size. If the symbol is interposed, this optimisation could lead to
// alignment-related or OOB read/write crashes.
// This is handled in the linker. When the linker sees multiple declarations of
// a global variable, and some are tagged, and some are untagged, it resolves it
// to be an untagged definition - but preserves the tag-granule-rounded size and
// tag-granule-alignment. This should prevent these kind of crashes intra-DSO.
// For cross-DSO, it's been a reasonable contract that if you're interposing a
// sanitizer-instrumented global, then the interposer also needs to be
// sanitizer-instrumented.
// FIXME: In theory, this can be fixed by splitting the size/alignment of
// globals into two uses: an "output alignment" that's emitted to the ELF file,
// and an "optimisation alignment" that's used for optimisation. Thus, we could
// adjust the output alignment only, and still optimise based on the pessimistic
// pre-tagging size/alignment.
static void tagGlobalDefinition(Module &M, GlobalVariable *G) {
Constant *Initializer = G->getInitializer();
uint64_t SizeInBytes =
uint64_t NewSize = alignTo(SizeInBytes, kTagGranuleSize);
if (SizeInBytes != NewSize) {
// Pad the initializer out to the next multiple of 16 bytes.
llvm::SmallVector<uint8_t> Init(NewSize - SizeInBytes, 0);
Constant *Padding = ConstantDataArray::get(M.getContext(), Init);
Initializer = ConstantStruct::getAnon({Initializer, Padding});
auto *NewGV = new GlobalVariable(
M, Initializer->getType(), G->isConstant(), G->getLinkage(),
Initializer, "", G, G->getThreadLocalMode(), G->getAddressSpace());
NewGV->copyMetadata(G, 0);
G = NewGV;
G->setAlignment(std::max(G->getAlign().valueOrOne(), kTagGranuleSize));
// Ensure that tagged globals don't get merged by ICF - as they should have
// different tags at runtime.
namespace {
class AArch64GlobalsTagging : public ModulePass {
static char ID;
explicit AArch64GlobalsTagging() : ModulePass(ID) {
bool runOnModule(Module &M) override;
StringRef getPassName() const override { return "AArch64 Globals Tagging"; }
std::set<GlobalVariable *> GlobalsToTag;
} // anonymous namespace
char AArch64GlobalsTagging::ID = 0;
bool AArch64GlobalsTagging::runOnModule(Module &M) {
// No mutating the globals in-place, or iterator invalidation occurs.
std::vector<GlobalVariable *> GlobalsToTag;
for (GlobalVariable &G : M.globals()) {
if (G.isDeclaration() || !shouldTagGlobal(G))
for (GlobalVariable *G : GlobalsToTag) {
tagGlobalDefinition(M, G);
return true;
INITIALIZE_PASS_BEGIN(AArch64GlobalsTagging, "aarch64-globals-tagging",
"AArch64 Globals Tagging Pass", false, false)
INITIALIZE_PASS_END(AArch64GlobalsTagging, "aarch64-globals-tagging",
"AArch64 Globals Tagging Pass", false, false)
ModulePass *llvm::createAArch64GlobalsTaggingPass() {
return new AArch64GlobalsTagging();