Messages in this thread | | | Date | Fri, 18 Nov 2022 09:40:09 -0300 | From | Rafael Mendonca <> | Subject | Re: [PATCH] tracing/eprobe: Update cond flag before enabling trigger |
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On Thu, Nov 17, 2022 at 09:31:09PM -0500, Steven Rostedt wrote: > On Thu, 17 Nov 2022 21:17:26 -0500 > Steven Rostedt <rostedt@goodmis.org> 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. > > > > 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. > > > > In fact, does this fix it for you?
It does. I found this while enabling eprobe for all events in my setup. Doing the same test wih the proposed patch it did not trigger any issue. Thanks.
> > I'm going to take this patch and reference you as a reported-by, as I have > a lot of urgent code that needs to got upstream, and I need to start > testing it. > > Thanks! > > -- Steve > > diff --git a/kernel/trace/trace_eprobe.c b/kernel/trace/trace_eprobe.c > index 5dd0617e5df6..6b31b74954d9 100644 > --- a/kernel/trace/trace_eprobe.c > +++ b/kernel/trace/trace_eprobe.c > @@ -563,6 +563,9 @@ static void eprobe_trigger_func(struct event_trigger_data *data, > { > struct eprobe_data *edata = data->private_data; > > + if (!rec) > + return; > + > __eprobe_trace_func(edata, rec); > } > >
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