[msan] Detect dereferencing zero-alloc as use-of-uninitialized-value #155944
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When a zero-byte allocation is requested, MSan actually allocates 1-byte for compatibility. This change poisons that byte, to detect dereferences. Also updates the test from llvm#155934
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@llvm/pr-subscribers-compiler-rt-sanitizer Author: Thurston Dang (thurstond) ChangesWhen a zero-byte allocation is requested, MSan actually allocates 1-byte for compatibility. This change poisons that byte, to detect dereferences. Also updates the test from #155934 Full diff: https://github.com/llvm/llvm-project/pull/155944.diff 2 Files Affected:
diff --git a/compiler-rt/lib/msan/msan_allocator.cpp b/compiler-rt/lib/msan/msan_allocator.cpp
index 2b543db49d36e..64df863839c06 100644
--- a/compiler-rt/lib/msan/msan_allocator.cpp
+++ b/compiler-rt/lib/msan/msan_allocator.cpp
@@ -230,6 +230,12 @@ 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 e60051872eba2..6e38ce4c0a8f8 100644
--- a/compiler-rt/test/msan/zero_alloc.cpp
+++ b/compiler-rt/test/msan/zero_alloc.cpp
@@ -1,9 +1,5 @@
// 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>
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fmayer
approved these changes
Aug 28, 2025
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When a zero-byte allocation is requested, MSan actually allocates 1-byte for compatibility. This change poisons that byte, to detect dereferences.
Also updates the test from #155934