[strings] Teach MSAN about uninitialized SeqString padding bytes

Bug: v8:12939
Change-Id: I853a718dec39485ae1ab1263e2be8f04567447f1
No-Tree-Checks: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3722455Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Owners-Override: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81339}

src/snapshot/serializer.cc

@@ -1178,8 +1178,15 @@ void Serializer::ObjectSerializer::OutputRawData(Address up_to) {
     }
 #ifdef MEMORY_SANITIZER
     // Check that we do not serialize uninitialized memory.
+    int msan_bytes_to_output = bytes_to_output;
+    if (object_->IsSeqString()) {
+      // SeqStrings may have uninitialized padding bytes. These padding
+      // bytes are never read and serialized as 0s.
+      msan_bytes_to_output -=
+          SeqString::cast(*object_).GetDataAndPaddingSizes().padding_size;
+    }
     __msan_check_mem_is_initialized(
-        reinterpret_cast<void*>(object_start + base), bytes_to_output);
+        reinterpret_cast<void*>(object_start + base), msan_bytes_to_output);
 #endif  // MEMORY_SANITIZER
     PtrComprCageBase cage_base(isolate_);
     if (object_->IsBytecodeArray(cage_base)) {