Detecting leaked instructions with metadata and freeing the metadata to avoid use-after-free

[akokins]

Summary

Original bug

When instructions with metadata are accidentally leaked, the result is a difficult-to-find memory corruption in ~LLVMContextImpl that leads to random crashes.

This patch implements the detection and graceful handling of such issues.

[vsk]

Thanks for the patch.

I like the idea of having a cheap leak check in debug builds. However, I don’t think we should add a mitigation for improper use of the API, because it can build up to code confusion/bloat over time. It becomes less clear what the API does/doesn’t support.

Could you please:

• Just keep the code conditional on !ndebug
• Update a diff with context (git diff -U10000)
• End sentences in comments with periods

[akokins]

Made the required changes to the diff.

[akokins]

Please ignore the previous diff, I missed that the comment didn't make sense after the changes.

[vsk]

LGTM, thanks. Do you need someone to commit this for you?

[akokins]

In D48476#1142388, @vsk wrote:

LGTM, thanks. Do you need someone to commit this for you?

Yes, it would be great if someone could commit this as I don't have commit access.

[vsk]

Sorry for the delay. I've committed this in r336010.

lib/IR/LLVMContextImpl.cpp

 
 LLVMContextImpl::~LLVMContextImpl() {
   // NOTE: We need to delete the contents of OwnedModules, but Module's dtor
   // will call LLVMContextImpl::removeModule, thus invalidating iterators into
   // the container. Avoid iterators during this operation:
   while (!OwnedModules.empty())
     delete *OwnedModules.begin();
 
+#ifndef NDEBUG
+  // Check for metadata references from leaked Instructions.
+  for (auto &Pair : InstructionMetadata)
+    Pair.first->dump();
+  assert(InstructionMetadata.empty() &&
+         "Instructions with metadata have been leaked");
+#endif
+
   // Drop references for MDNodes.  Do this before Values get deleted to avoid
   // unnecessary RAUW when nodes are still unresolved.
   for (auto *I : DistinctMDNodes)
     I->dropAllReferences();
 #define HANDLE_MDNODE_LEAF_UNIQUABLE(CLASS)                                    \
   for (auto *I : CLASS##s)                                                     \
     I->dropAllReferences();
 #include "llvm/IR/Metadata.def"