[compiler-rt] Revert "[msan] Detect dereferencing zero-alloc as use-of-uninitialized-value" (PR #156148)

[Thurston Dang via llvm-commits]

https://github.com/thurstond created https://github.com/llvm/llvm-project/pull/156148

Reverts llvm/llvm-project#155944

Per post-commit discussion in https://github.com/llvm/llvm-project/pull/155944#discussion_r2311822441, this is detecting OOB access, which is not in scope for MSan. To be logically consistent with how MSan does not try to handle OOB in other cases, this patch reverts the MSan change. Dereferencing zero-alloc can be detected with ASan instead (as of https://github.com/llvm/llvm-project/pull/155943).


>From 1e406814dd21aad7da6bd1c1a8e0c6f1c1cd5369 Mon Sep 17 00:00:00 2001
From: Thurston Dang <thurston.dang at gmail.com>
Date: Sat, 30 Aug 2025 00:03:56 -0700
Subject: [PATCH] =?UTF-8?q?Revert=20"[msan]=20Detect=20dereferencing=20zer?=
 =?UTF-8?q?o-alloc=20as=20use-of-uninitialized-value=20=E2=80=A6"?=
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This reverts commit 8784dcef3485a3862e5cacc9e18c646d0225483b.
---
 compiler-rt/lib/msan/msan_allocator.cpp | 6 ------
 compiler-rt/test/msan/zero_alloc.cpp    | 4 ++++
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/compiler-rt/lib/msan/msan_allocator.cpp b/compiler-rt/lib/msan/msan_allocator.cpp
index 64df863839c06..2b543db49d36e 100644
--- a/compiler-rt/lib/msan/msan_allocator.cpp
+++ b/compiler-rt/lib/msan/msan_allocator.cpp
@@ -230,12 +230,6 @@ static void *MsanAllocate(BufferedStackTrace *stack, uptr size, uptr alignment,
       __msan_set_origin(allocated, size, o.raw_id());
     }
   }
-
-  uptr actually_allocated_size = allocator.GetActuallyAllocatedSize(allocated);
-  // For compatibility, the allocator converted 0-sized allocations into 1 byte
-  if (size == 0 && actually_allocated_size > 0 && flags()->poison_in_malloc)
-    __msan_poison(allocated, 1);
-
   UnpoisonParam(2);
   RunMallocHooks(allocated, size);
   return allocated;
diff --git a/compiler-rt/test/msan/zero_alloc.cpp b/compiler-rt/test/msan/zero_alloc.cpp
index 6e38ce4c0a8f8..e60051872eba2 100644
--- a/compiler-rt/test/msan/zero_alloc.cpp
+++ b/compiler-rt/test/msan/zero_alloc.cpp
@@ -1,5 +1,9 @@
 // RUN: %clang_msan -Wno-alloc-size -fsanitize-recover=memory %s -o %t && not %run %t 2>&1 | FileCheck %s
 
+// MSan doesn't catch this because internally it translates 0-byte allocations
+// into 1-byte
+// XFAIL: *
+
 #include <malloc.h>
 #include <stdio.h>