[compiler-rt] [asan] Implement interception on AIX (PR #131870)

[Hubert Tong via llvm-commits]

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@@ -781,11 +781,17 @@ struct Allocator {
       u8 chunk_state = atomic_load(&m->chunk_state, memory_order_acquire);
       if (chunk_state != CHUNK_ALLOCATED)
         ReportInvalidFree(old_ptr, chunk_state, stack);
-      CHECK_NE(REAL(memcpy), nullptr);
       uptr memcpy_size = Min(new_size, m->UsedSize());
       // If realloc() races with free(), we may start copying freed memory.
       // However, we will report racy double-free later anyway.
+#if !SANITIZER_AIX
+      CHECK_NE(REAL(memcpy), nullptr);
       REAL(memcpy)(new_ptr, old_ptr, memcpy_size);
+#else
+      // AIX currently can't retrieve memcpy's address, we have to use
+      // internal_memcpy here.
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hubert-reinterpretcast wrote:

The comment as written is misleading: it implies that being unable to retrieve an address for the real `memcpy` as a member of a shared library (which, afaik, is not something that we plan to change) necessarily means that we have to use `internal_memcpy` (i.e., there are no other options).

Perhaps change this comment to reflect any future plans to use `___memcpy` on AIX? Better yet: Update this PR to make use of `___memcpy` (which is what the `memcpy` wrapper that AIX `libc` provides for static linking forwards to).

https://github.com/llvm/llvm-project/pull/131870