[compiler-rt][asan] Fix for flaky asan check

[PiJoules]

This fixes #87324.

We haven't been able to come up with a minimal reproducer but we can reliabely avoid this failure with the following fix. Prior to the GetGlobalLowLevelAllocator change, the old LowLevelAllocator aquired a lock associated with it preventing that specific allocator from being accessed at the same time by many threads. With the GetGlobalLowLevelAllocator change, I had accidentally replaced it but not taken into account the lock, so we can have a data race if the allocator is used at any point while a thread is being created. The global allocator can be used for flag parsing or registering asan globals.

[llvmbot]

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: None (PiJoules)

Changes

This fixes #87324.

We haven't been able to come up with a minimal reproducer but we can reliabely avoid this failure with the following fix. Prior to the GetGlobalLowLevelAllocator change, the old LowLevelAllocator aquired a lock associated with it preventing that specific allocator from being accessed at the same time by many threads. With the GetGlobalLowLevelAllocator change, I had accidentally replaced it but not taken into account the lock, so we can have a data race if the allocator is used at any point while a thread is being created. The global allocator can be used for flag parsing or registering asan globals.

---

Full diff: https://github.com/llvm/llvm-project/pull/88177.diff

1 Files Affected:

• (modified) compiler-rt/lib/asan/asan_thread.cpp (+2-1)

diff --git a/compiler-rt/lib/asan/asan_thread.cpp b/compiler-rt/lib/asan/asan_thread.cpp
index 8798968947e82e..95aeccd070b551 100644
--- a/compiler-rt/lib/asan/asan_thread.cpp
+++ b/compiler-rt/lib/asan/asan_thread.cpp
@@ -44,10 +44,11 @@ static ThreadRegistry *asan_thread_registry;
 static ThreadArgRetval *thread_data;
 
 static Mutex mu_for_thread_context;
+static LowLevelAllocator allocator_for_thread_context;
 
 static ThreadContextBase *GetAsanThreadContext(u32 tid) {
   Lock lock(&mu_for_thread_context);
-  return new (GetGlobalLowLevelAllocator()) AsanThreadContext(tid);
+  return new (allocator_for_thread_context) AsanThreadContext(tid);
 }
 
 static void InitThreads() {

[PiJoules]

cc @landell-xperi

[vitalybuka]

On https://reviews.llvm.org/D158786 I didn't realized that it's not thread safe, it should be easy to make LowLevelAllocator thread safe.

Feel free just to add TODO, or we can do thread safe in this patch?

This is approx idea how this could be done
https://github.com/llvm/llvm-project/blob/6bf71be9f920417a772b123fd42ab855496ad149/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.cpp

[PiJoules]

On https://reviews.llvm.org/D158786 I didn't realized that it's not thread safe, it should be easy to make LowLevelAllocator thread safe.

Feel free just to add TODO, or we can do thread safe in this patch?

This is approx idea how this could be done https://github.com/llvm/llvm-project/blob/6bf71be9f920417a772b123fd42ab855496ad149/compiler-rt/lib/sanitizer_common/sanitizer_stack_store.cpp

Added just the TODO for now since we know this quick fix works but I'll followup with the threadsafe changes afterwards. Just want to make sure the libc tests this might affect for us downstream absorb this change well.