MSan: handle llvm.lifetime.start intrinsic

Summary:
When a variable goes into scope several times within a single function
or when two variables from different scopes share a stack slot it may
be incorrect to poison such scoped locals at the beginning of the
function.
In the former case it may lead to false negatives (see
https://github.com/google/sanitizers/issues/590), in the latter - to
incorrect reports (because only one origin remains on the stack).

If Clang emits lifetime intrinsics for such scoped variables we insert
code poisoning them after each call to llvm.lifetime.start().
If for a certain intrinsic we fail to find a corresponding alloca, we
fall back to poisoning allocas for the whole function, as it's now
impossible to tell which alloca was missed.

The new instrumentation may slow down hot loops containing local
variables with lifetime intrinsics, so we allow disabling it with
-mllvm -msan-handle-lifetime-intrinsics=false.

Reviewers: eugenis, pcc

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60617

llvm-svn: 359536

1 files changed, 54 insertions, 8 deletions

diff --git a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
index 9e6173c5aef..85912695ab5 100644
--- a/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -143,6 +143,7 @@
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
@@ -247,6 +248,13 @@ static cl::opt<bool> ClHandleICmpExact("msan-handle-icmp-exact",
        cl::desc("exact handling of relational integer ICmp"),
        cl::Hidden, cl::init(false));
 
+static cl::opt<bool> ClHandleLifetimeIntrinsics(
+    "msan-handle-lifetime-intrinsics",
+    cl::desc(
+        "when possible, poison scoped variables at the beginning of the scope "
+        "(slower, but more precise)"),
+    cl::Hidden, cl::init(true));
+
 // When compiling the Linux kernel, we sometimes see false positives related to
 // MSan being unable to understand that inline assembly calls may initialize
 // local variables.
@@ -1023,6 +1031,9 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
       : Shadow(S), Origin(O), OrigIns(I) {}
   };
   SmallVector<ShadowOriginAndInsertPoint, 16> InstrumentationList;
+  bool InstrumentLifetimeStart = ClHandleLifetimeIntrinsics;
+  SmallSet<AllocaInst *, 16> AllocaSet;
+  SmallVector<std::pair<IntrinsicInst *, AllocaInst *>, 16> LifetimeStartList;
   SmallVector<StoreInst *, 16> StoreList;
 
   MemorySanitizerVisitor(Function &F, MemorySanitizer &MS,
@@ -1279,6 +1290,19 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
 
     VAHelper->finalizeInstrumentation();
 
+    // Poison llvm.lifetime.start intrinsics, if we haven't fallen back to
+    // instrumenting only allocas.
+    if (InstrumentLifetimeStart) {
+      for (auto Item : LifetimeStartList) {
+        instrumentAlloca(*Item.second, Item.first);
+        AllocaSet.erase(Item.second);
+      }
+    }
+    // Poison the allocas for which we didn't instrument the corresponding
+    // lifetime intrinsics.
+    for (AllocaInst *AI : AllocaSet)
+      instrumentAlloca(*AI);
+
     bool InstrumentWithCalls = ClInstrumentationWithCallThreshold >= 0 &&
                                InstrumentationList.size() + StoreList.size() >
                                    (unsigned)ClInstrumentationWithCallThreshold;
@@ -2536,6 +2560,17 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     return false;
   }
 
+  void handleLifetimeStart(IntrinsicInst &I) {
+    if (!PoisonStack)
+      return;
+    DenseMap<Value *, AllocaInst *> AllocaForValue;
+    AllocaInst *AI =
+        llvm::findAllocaForValue(I.getArgOperand(1), AllocaForValue);
+    if (!AI)
+      InstrumentLifetimeStart = false;
+    LifetimeStartList.push_back(std::make_pair(&I, AI));
+  }
+
   void handleBswap(IntrinsicInst &I) {
     IRBuilder<> IRB(&I);
     Value *Op = I.getArgOperand(0);
@@ -2951,6 +2986,9 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
 
   void visitIntrinsicInst(IntrinsicInst &I) {
     switch (I.getIntrinsicID()) {
+    case Intrinsic::lifetime_start:
+      handleLifetimeStart(I);
+      break;
     case Intrinsic::bswap:
       handleBswap(I);
       break;
@@ -3379,7 +3417,7 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
                                                 StackDescription.str());
   }
 
-  void instrumentAllocaUserspace(AllocaInst &I, IRBuilder<> &IRB, Value *Len) {
+  void poisonAllocaUserspace(AllocaInst &I, IRBuilder<> &IRB, Value *Len) {
     if (PoisonStack && ClPoisonStackWithCall) {
       IRB.CreateCall(MS.MsanPoisonStackFn,
                      {IRB.CreatePointerCast(&I, IRB.getInt8PtrTy()), Len});
@@ -3401,7 +3439,7 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     }
   }
 
-  void instrumentAllocaKmsan(AllocaInst &I, IRBuilder<> &IRB, Value *Len) {
+  void poisonAllocaKmsan(AllocaInst &I, IRBuilder<> &IRB, Value *Len) {
     Value *Descr = getLocalVarDescription(I);
     if (PoisonStack) {
       IRB.CreateCall(MS.MsanPoisonAllocaFn,
@@ -3413,10 +3451,10 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     }
   }
 
-  void visitAllocaInst(AllocaInst &I) {
-    setShadow(&I, getCleanShadow(&I));
-    setOrigin(&I, getCleanOrigin());
-    IRBuilder<> IRB(I.getNextNode());
+  void instrumentAlloca(AllocaInst &I, Instruction *InsPoint = nullptr) {
+    if (!InsPoint)
+      InsPoint = &I;
+    IRBuilder<> IRB(InsPoint->getNextNode());
     const DataLayout &DL = F.getParent()->getDataLayout();
     uint64_t TypeSize = DL.getTypeAllocSize(I.getAllocatedType());
     Value *Len = ConstantInt::get(MS.IntptrTy, TypeSize);
@@ -3424,9 +3462,17 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
       Len = IRB.CreateMul(Len, I.getArraySize());
 
     if (MS.CompileKernel)
-      instrumentAllocaKmsan(I, IRB, Len);
+      poisonAllocaKmsan(I, IRB, Len);
     else
-      instrumentAllocaUserspace(I, IRB, Len);
+      poisonAllocaUserspace(I, IRB, Len);
+  }
+
+  void visitAllocaInst(AllocaInst &I) {
+    setShadow(&I, getCleanShadow(&I));
+    setOrigin(&I, getCleanOrigin());
+    // We'll get to this alloca later unless it's poisoned at the corresponding
+    // llvm.lifetime.start.
+    AllocaSet.insert(&I);
   }
 
   void visitSelectInst(SelectInst& I) {