[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:

>> It would be expensive to do the extra poisoning.

> I don't think so. It's about alignment tail which likely smaller than requested allocation, which we poison anyway.

It is expensive from a cost-benefit perspective. It has some cost, but the benefit is limited (detecting issues that are out of scope for MSan).

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