Re: [PATCH] tracing/eprobe: Update cond flag before enabling trigger

[Rafael Mendonca <rafaelmendsr@gmail.com>]

On Thu, Nov 17, 2022 at 09:17:26PM -0500, Steven Rostedt wrote:
> On Wed, 16 Nov 2022 16:25:51 -0300
> Rafael Mendonca <rafaelmendsr@gmail.com> wrote:
> 
> > That happens because enable_eprobe() will eventually trigger the
> > kmem/mm_page_alloc trace event:
> > 
> > - enable_eprobe [trace_eprobe.c]
> >   - trace_event_trigger_enable_disable [trace_events_trigger.c]
> >     - trace_event_enable_disable [trace_events.c]
> >       - __ftrace_event_enable_disable [trace_events.c]
> >         - trace_buffered_event_enable [trace.c]
> >           - alloc_pages_node [gfp.h]
> > 	   ...
> >             - __alloc_pages [page_alloc.c]
> >               - trace_mm_page_alloc // eprobe event file without TRIGGER_COND bit set
> > 
> > By the time kmem/mm_page_alloc trace event is hit, the eprobe event file
> > does not have the TRIGGER_COND flag set yet, which causes the eprobe's
> > trigger to be invoked (through the trace_trigger_soft_disabled() path)
> > without a trace record, causing a NULL pointer dereference when fetching
> > the event fields.
> > 
> > Fix this by setting the cond flag beforehand when enabling the eprobe's
> > trigger.
> > 
> > Fixes: 7491e2c44278 ("tracing: Add a probe that attaches to trace events")
> > Signed-off-by: Rafael Mendonca <rafaelmendsr@gmail.com>
> > ---
> 
> Thanks for the report, but I'm worried that this isn't enough because of
> how memory ordering can happen on different architectures. That is, just
> because you switch the order of updates, doesn't mean that the architecture
> will honor it.
> 
> I don't want to add memory barriers in the fast path, but instead we can
> simply check if rec is NULL in the handler.

Ok. That was my first approach but then looking for the same pattern in
trace_events_trigger.c and trace_events_hist.c I saw commit 4e4a4d75700d
("tracing: Update cond flag when enabling or disabling a trigger") [1]
and I thought the same would apply here without much thought.

[1] https://lore.kernel.org/lkml/20151126022048.111221642@goodmis.org/

> 
> So basically:
> 
> 
> static void eprobe_trigger_func(struct event_trigger_data *data,
> 				struct trace_buffer *buffer, void *rec,
> 				struct ring_buffer_event *rbe)
> {
> 	struct eprobe_data *edata = data->private_data;
> 
> 	if (!rec)
> 		return;
> 
> 	__eprobe_trace_func(edata, rec);
> }
> 
> And this should be documented.
> 
> -- Steve