[all-commits] [llvm/llvm-project] 781719: [ASan][Windows] Fix false positive for zero sized ...

[Zack Johnson via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 781719778003ca298ca57e486ab629b7f384844b
      https://github.com/llvm/llvm-project/commit/781719778003ca298ca57e486ab629b7f384844b
  Author: Zack Johnson <zacklj89 at gmail.com>
  Date:   2026-02-13 (Fri, 13 Feb 2026)

  Changed paths:
    M compiler-rt/lib/asan/asan_allocator.cpp
    M compiler-rt/lib/asan/asan_malloc_win.cpp
    M compiler-rt/test/asan/TestCases/Windows/heaprealloc_alloc_zero.cpp
    A compiler-rt/test/asan/TestCases/Windows/rtlsizeheap_zero.cpp

  Log Message:
  -----------
  [ASan][Windows] Fix false positive for zero sized rtl allocations (#181015)

This is a follow up to #155943

On Windows, ASan's allocator internally upgrades zero-size allocation
requests to size 1 (since malloc(0) must return a unique non-NULL
pointer). However, when the user queries the allocation size through
Windows heap APIs (RtlSizeHeap, HeapSize, \_msize, GlobalSize,
LocalSize), ASan reports the internal size (1) instead of the originally
requested size (0).

This causes false positive heap-buffer-overflow errors in a common
pattern:

```c++
void *buf = HeapAlloc(GetProcessHeap(), 0, 0);
SIZE_T size = HeapSize(GetProcessHeap(), 0, buf);  // Returns 1, should be 0
if(size > 0) // could remove this and still be correct
    memset(buf, 0, size);  // ASan reports heap-buffer-overflow
```

The change adds a `from_zero_alloc` bit to `ChunkHeader` that tracks
whether an allocation was originally zero-size. This bit fits in the
existing spare capacity of the header's bitfield byte, so the 16-byte
ChunkHeader size is unchanged, but it also isn't the most elegant.

The 1-byte user region of a zero-size allocation is still poisoned, so
any actual access to it is correctly reported as an overflow.



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