linux-kernel - Re: perf segfault in in ordered_events_

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Date:   Wed, 16 Jan 2019 13:53:55 -0800
From:   Song Liu <liu.song.a23@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     open list <linux-kernel@...r.kernel.org>,
        Jiri Olsa <jolsa@...nel.org>, acme@...nel.org
Subject: Re: perf segfault in in ordered_events__free()

Thanks Jiri!

On Wed, Jan 16, 2019 at 1:34 PM Jiri Olsa <jolsa@...hat.com> wrote:
>
> On Wed, Jan 16, 2019 at 10:57:49AM -0800, Song Liu wrote:
> > Hi,
> >
> > We are debugging a segfault of perf in ordered_events__free().
>
> hi,
> any backtrace or info on how to reproduce it?

Here is the backtrace:

#0  0x0000000000500055 in ordered_events(float, long double,...)(...) ()
#1  0x0000000000500196 in ordered_events.reinit ()
#2  0x00000000004fe413 in perf_session.process_events ()
#3  0x0000000000440431 in cmd_record ()
#4  0x00000000004a439f in run_builtin ()
#5  0x000000000042b3e5 in main ()"

>
> > Disassemble shows the segfault was caused by oe->buff == NULL
> > in the following line:
> >
> >         /*
> >          * Current buffer might not have all the events allocated
> >          * yet, we need to free only allocated ones ...
> >          */
> >         list_del(&oe->buffer->list);
> >
> > After poking around the code, I suspect it is caused by the following
> > condition in alloc_event():
> >
> >         } else if (oe->buffer) {
> >                 new = &oe->buffer->event[oe->buffer_idx];
> >                 if (++oe->buffer_idx == MAX_SAMPLE_BUFFER)
> >                         oe->buffer = NULL;
>
>
> argh.. yea, we need to check oe->buffer in ordered_events__free
>
> would attached change fix it for you?

Let me try roll a fixed version to confirm.

Thanks again!
Song

>
> thanks,
> jirka
>
>
> ---
> diff --git a/tools/perf/util/ordered-events.c b/tools/perf/util/ordered-events.c
> index 897589507d97..ea523d3b248f 100644
> --- a/tools/perf/util/ordered-events.c
> +++ b/tools/perf/util/ordered-events.c
> @@ -391,8 +391,10 @@ void ordered_events__free(struct ordered_events *oe)
>          * Current buffer might not have all the events allocated
>          * yet, we need to free only allocated ones ...
>          */
> -       list_del(&oe->buffer->list);
> -       ordered_events_buffer__free(oe->buffer, oe->buffer_idx, oe);
> +       if (oe->buffer) {
> +               list_del(&oe->buffer->list);
> +               ordered_events_buffer__free(oe->buffer, oe->buffer_idx, oe);
> +       }
>
>         /* ... and continue with the rest */
>         list_for_each_entry_safe(buffer, tmp, &oe->to_free, list) {