[msan] Fix msan false-positive in the simulator

Bug: 344654370
Change-Id: I0d4cbc7c873ede056d0835b04a008af45c27ddab
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/5622010
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#94372}

src/execution/arm64/simulator-arm64.cc

@@ -678,6 +678,44 @@ void Simulator::DoRuntimeCall(Instruction* instr) {
   const int64_t arg19 = stack_pointer[11];
   static_assert(kMaxCParameters == 20);
 
+#ifdef V8_USE_MEMORY_SANITIZER
+  // `UnsafeGenericFunctionCall()` dispatches calls to functions with
+  // varying signatures and relies on the fact that the mismatched prototype
+  // used by the caller and the prototype used by the callee (defined using
+  // the `RUNTIME_FUNCTION*()` macros happen to line up so that things more
+  // or less work out [1].
+  //
+  // Unfortunately, this confuses MSan's uninit tracking with eager checks
+  // enabled; it's unclear if these are all false positives or if there are
+  // legitimate reports. For now, unconditionally unpoison args to
+  // unblock finding and fixing more violations with MSan eager checks.
+  //
+  // TODO(crbug.com/v8/14712): Fix the MSan violations and migrate to
+  // something like crrev.com/c/5422076 instead.
+  //
+  // [1] Yes, this is undefined behaviour. 🙈🙉🙊
+  MSAN_MEMORY_IS_INITIALIZED(&arg0, sizeof(arg0));
+  MSAN_MEMORY_IS_INITIALIZED(&arg1, sizeof(arg1));
+  MSAN_MEMORY_IS_INITIALIZED(&arg2, sizeof(arg2));
+  MSAN_MEMORY_IS_INITIALIZED(&arg3, sizeof(arg3));
+  MSAN_MEMORY_IS_INITIALIZED(&arg4, sizeof(arg4));
+  MSAN_MEMORY_IS_INITIALIZED(&arg5, sizeof(arg5));
+  MSAN_MEMORY_IS_INITIALIZED(&arg6, sizeof(arg6));
+  MSAN_MEMORY_IS_INITIALIZED(&arg7, sizeof(arg7));
+  MSAN_MEMORY_IS_INITIALIZED(&arg8, sizeof(arg8));
+  MSAN_MEMORY_IS_INITIALIZED(&arg9, sizeof(arg9));
+  MSAN_MEMORY_IS_INITIALIZED(&arg10, sizeof(arg10));
+  MSAN_MEMORY_IS_INITIALIZED(&arg11, sizeof(arg11));
+  MSAN_MEMORY_IS_INITIALIZED(&arg12, sizeof(arg12));
+  MSAN_MEMORY_IS_INITIALIZED(&arg13, sizeof(arg13));
+  MSAN_MEMORY_IS_INITIALIZED(&arg14, sizeof(arg14));
+  MSAN_MEMORY_IS_INITIALIZED(&arg15, sizeof(arg15));
+  MSAN_MEMORY_IS_INITIALIZED(&arg16, sizeof(arg16));
+  MSAN_MEMORY_IS_INITIALIZED(&arg17, sizeof(arg17));
+  MSAN_MEMORY_IS_INITIALIZED(&arg18, sizeof(arg18));
+  MSAN_MEMORY_IS_INITIALIZED(&arg19, sizeof(arg19));
+#endif  // V8_USE_MEMORY_SANITIZER
+
   switch (redirection->type()) {
     default:
       TraceSim("Type: Unknown.\n");