Messages in this thread | | | Date | Fri, 18 Nov 2022 09:38:18 -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 09:17:26PM -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
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