Re: [RESEND][PATCH v15 2/4] perf: Support deferred user callchains

On Tue, 23 Sep 2025 11:19:35 +0200
Peter Zijlstra [off-list ref] wrote:

On Mon, Sep 08, 2025 at 01:14:14PM -0400, Steven Rostedt wrote:

quoted

+static void perf_event_callchain_deferred(struct callback_head *work)
+{
+	struct perf_event *event = container_of(work, struct perf_event, pending_unwind_work);
+	struct perf_callchain_deferred_event deferred_event;
+	u64 callchain_context = PERF_CONTEXT_USER;
+	struct unwind_stacktrace trace;
+	struct perf_output_handle handle;
+	struct perf_sample_data data;
+	u64 nr;
+
+	if (!event->pending_unwind_callback)
+		return;
+
+	if (unwind_user_faultable(&trace) < 0)
+		goto out;  

This is broken. Because:

quoted

+
+	/*
+	 * All accesses to the event must belong to the same implicit RCU
+	 * read-side critical section as the ->pending_unwind_callback reset.
+	 * See comment in perf_pending_unwind_sync().
+	 */
+	guard(rcu)();  

Here you start a guard, that lasts until close of function..

quoted

+
+	if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
+		goto out;
+
+	nr = trace.nr + 1 ; /* '+1' == callchain_context */
+
+	deferred_event.header.type = PERF_RECORD_CALLCHAIN_DEFERRED;
+	deferred_event.header.misc = PERF_RECORD_MISC_USER;
+	deferred_event.header.size = sizeof(deferred_event) + (nr * sizeof(u64));
+
+	deferred_event.nr = nr;
+	deferred_event.cookie = unwind_user_get_cookie();
+
+	perf_event_header__init_id(&deferred_event.header, &data, event);
+
+	if (perf_output_begin(&handle, &data, event, deferred_event.header.size))
+		goto out;
+
+	perf_output_put(&handle, deferred_event);
+	perf_output_put(&handle, callchain_context);
+	/* trace.entries[] are not guaranteed to be 64bit */
+	for (int i = 0; i < trace.nr; i++) {
+		u64 entry = trace.entries[i];
+		perf_output_put(&handle, entry);
+	}
+	perf_event__output_id_sample(event, &handle, &data);
+
+	perf_output_end(&handle);
+
+out:  

Which very much includes here, so your goto jumps into a scope, which is
not permitted.

Nice catch.

GCC can fail to warn on this, but clang will consistently fail to
compile this. Surely the robot would've told you by now -- even if
you're not using clang yourself.

Unfortunately it hasn't :-(

I need to start building with clang more often.

I even pushed this to a git tree. Not sure why it didn't get flagged.

-- Steve