[compiler-rt] 7184114 - Fix `asan/TestCases/Darwin/scrible.cpp` to work on platforms where `long` is not 64-bits.

[Dan Liew via llvm-commits]

Author: Dan Liew
Date: 2021-09-08T09:53:09-07:00
New Revision: 71841146b6222ef7eea06b9facd94d0e0c29c62b

URL: https://github.com/llvm/llvm-project/commit/71841146b6222ef7eea06b9facd94d0e0c29c62b
DIFF: https://github.com/llvm/llvm-project/commit/71841146b6222ef7eea06b9facd94d0e0c29c62b.diff

LOG: Fix `asan/TestCases/Darwin/scrible.cpp` to work on platforms where `long` is not 64-bits.

Previously the test was failing on platforms where `long` was less than
64-bits wide (e.g. older WatchOS simulators and arm64_32) because the
`padding` field was too small.

The test currently relies on the `my_object->isa` being scribbled or
left unmodified after `my_object` is freed. However, this was not the
case because the `isa` pointer intersected with
`ChunkHeader::free_context_id`.  `free_context_id` starts at the
beginning of user memory but it only initialized once the memory is
freed. This caused the `isa` pointer to change after it was freed
leading to the test crashing.

To fix this the `padding` field has been made explicitly 64-bits wide
(same size as `ChunkHeader::free_context_id`).

rdar://75806757

Differential Revision: https://reviews.llvm.org/D109409

Added: 
    

Modified: 
    compiler-rt/test/asan/TestCases/Darwin/scribble.cpp

Removed: 
    


################################################################################
diff  --git a/compiler-rt/test/asan/TestCases/Darwin/scribble.cpp b/compiler-rt/test/asan/TestCases/Darwin/scribble.cpp
index 8303cf316a9c2..60ed40200abf6 100644
--- a/compiler-rt/test/asan/TestCases/Darwin/scribble.cpp
+++ b/compiler-rt/test/asan/TestCases/Darwin/scribble.cpp
@@ -13,7 +13,14 @@ struct Isa {
 };
 
 struct MyClass {
-  long padding;
+  // User memory and `ChunkHeader` overlap. In particular the `free_context_id`
+  // is stored at the beginning of user memory when it is freed. That part of
+  // user memory is not scribbled and is changed when the memory is freed. This
+  // test relies on `isa` being scribbled or unmodified after memory is freed.
+  // In order for this to work the start of `isa` must come after whatever is in
+  // `ChunkHeader` (currently the 64-bit `free_context_id`). The padding here is
+  // to ensure this is the case.
+  uint64_t padding;
   Isa *isa;
   long data;