better error when native code tries to execute Rust function

Author: RalfJung

src/tools/miri/src/shims/native_lib/trace/parent.rs

@@ -530,7 +530,18 @@ fn handle_segfault(
     // Don't use wait_for_signal here since 1 instruction doesn't give room
     // for any uncertainty + we don't want it `cont()`ing randomly by accident
     // Also, don't let it continue with unprotected memory if something errors!
-    let _ = wait::waitid(wait::Id::Pid(pid), WAIT_FLAGS).map_err(|_| ExecEnd(None))?;
+    let stat = wait::waitid(wait::Id::Pid(pid), WAIT_FLAGS).map_err(|_| ExecEnd(None))?;
+    match stat {
+        wait::WaitStatus::Signaled(_, s, _)
+        | wait::WaitStatus::Stopped(_, s)
+        | wait::WaitStatus::PtraceEvent(_, s, _) =>
+            assert!(
+                !matches!(s, signal::SIGSEGV),
+                "native code segfaulted when re-trying memory access\n\
+                is the native code trying to call a Rust function?"
+            ),
+        _ => (),
+    }
 
     // Zero out again to be safe
     for a in (ch_stack..ch_stack.strict_add(CALLBACK_STACK_SIZE)).step_by(ARCH_WORD_SIZE) {