[compiler-rt] [msan] Detect dereferencing zero-alloc as use-of-uninitialized-value (PR #155944)

[Thurston Dang via llvm-commits]

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@@ -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);
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thurstond wrote:

> Why did you create this patch than, it falls into the same category?

Zero-sized allocations are a special case. POSIX allows that malloc(0) could even return NULL, which definitely should not be dereferenced. The new behavior (returning a non-NULL pointer, but not allowing it to be dereferenced) is a compromise between compatibility and detection.

https://github.com/llvm/llvm-project/pull/155944