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

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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