[compiler-rt] [msan] Mark allocator padding as uninitialized, with new origin tag (PR #157187)

[Vitaly Buka via llvm-commits]

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@@ -217,25 +217,44 @@ static void *MsanAllocate(BufferedStackTrace *stack, uptr size, uptr alignment,
   }
   auto *meta = reinterpret_cast<Metadata *>(allocator.GetMetaData(allocated));
   meta->requested_size = size;
+  uptr actually_allocated_size = allocator.GetActuallyAllocatedSize(allocated);
+  void* padding_start =
+      reinterpret_cast<void*>(reinterpret_cast<uptr>(allocated) + size);
+  uptr padding_size = actually_allocated_size - size;
+
+  // Origins have 4-byte granularity. Set the TAG_ALLOC_PADDING origin first,
+  // so the TAG_ALLOC origin will take precedence if necessary e.g.,
+  // - if we have malloc(7) that actually takes up 16 bytes:
+  //     bytes 0-7:  uninitialized, origin TAG_ALLOC
+  //     bytes 8-15: uninitialized, origin TAG_ALLOC_PADDING
+  // - with calloc(7,1):
+  //     bytes 0-6:  initialized,   origin not set (and irrelevant)
+  //     byte  7:    uninitialized, origin TAG_ALLOC_PADDING (unlike malloc)
+  //     bytes 8-15: uninitialized, origin TAG_ALLOC_PADDING
+  if (__msan_get_track_origins() && flags()->poison_in_malloc) {
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vitalybuka wrote:

I would recommend to move it next to corresponding __msan_poison
Now reader needs to guess whare it coming from

also you probably will save on on if __msan_get_track_origins, which probably negligle
but Origin::CreateHeapOrigin(stack); is expensive

https://github.com/llvm/llvm-project/pull/157187