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

[Steven Rostedt <rostedt@kernel.org>]

On Tue, 23 Sep 2025 11:19:35 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> On Mon, Sep 08, 2025 at 01:14:14PM -0400, Steven Rostedt wrote:
> 
> > +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:
> 
> > +
> > +	/*
> > +	 * 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..
> 
> > +
> > +	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