[compiler-rt] Revert "[msan] Detect dereferencing zero-alloc as use-of-uninitialized-value" (PR #156148)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

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

Author: Thurston Dang (thurstond)

<details>
<summary>Changes</summary>

Reverts llvm/llvm-project#<!-- -->155944

Per post-commit discussion in https://github.com/llvm/llvm-project/pull/155944#discussion_r2311822441, this is detecting OOB access, which is not in scope for MSan. To be logically consistent with how MSan does not try to handle OOB in other cases, this patch reverts the MSan change. Dereferencing zero-alloc can be detected with ASan instead (as of https://github.com/llvm/llvm-project/pull/155943).


---
Full diff: https://github.com/llvm/llvm-project/pull/156148.diff


2 Files Affected:

- (modified) compiler-rt/lib/msan/msan_allocator.cpp (-6) 
- (modified) compiler-rt/test/msan/zero_alloc.cpp (+4) 


``````````diff
diff --git a/compiler-rt/lib/msan/msan_allocator.cpp b/compiler-rt/lib/msan/msan_allocator.cpp
index 64df863839c06..2b543db49d36e 100644
--- a/compiler-rt/lib/msan/msan_allocator.cpp
+++ b/compiler-rt/lib/msan/msan_allocator.cpp
@@ -230,12 +230,6 @@ static void *MsanAllocate(BufferedStackTrace *stack, uptr size, uptr alignment,
       __msan_set_origin(allocated, size, o.raw_id());
     }
   }
-
-  uptr actually_allocated_size = allocator.GetActuallyAllocatedSize(allocated);
-  // For compatibility, the allocator converted 0-sized allocations into 1 byte
-  if (size == 0 && actually_allocated_size > 0 && flags()->poison_in_malloc)
-    __msan_poison(allocated, 1);
-
   UnpoisonParam(2);
   RunMallocHooks(allocated, size);
   return allocated;
diff --git a/compiler-rt/test/msan/zero_alloc.cpp b/compiler-rt/test/msan/zero_alloc.cpp
index 6e38ce4c0a8f8..e60051872eba2 100644
--- a/compiler-rt/test/msan/zero_alloc.cpp
+++ b/compiler-rt/test/msan/zero_alloc.cpp
@@ -1,5 +1,9 @@
 // RUN: %clang_msan -Wno-alloc-size -fsanitize-recover=memory %s -o %t && not %run %t 2>&1 | FileCheck %s
 
+// MSan doesn't catch this because internally it translates 0-byte allocations
+// into 1-byte
+// XFAIL: *
+
 #include <malloc.h>
 #include <stdio.h>
 

``````````

</details>


https://github.com/llvm/llvm-project/pull/156148