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From:   Namhyung Kim <namhyung@kernel.org>
Date:   Thu, 18 Jun 2020 23:48:45 +0900
Message-ID: <CAM9d7ciTTJ5e0JjrK0K7y5034kTQ08tcfvJ7wry6yY8O0BGDxg@mail.gmail.com>
Subject: Re: [PATCH] [perf] Fix null pointer deference in nest_epollfd
To:     Gaurav Singh <gaurav1086@gmail.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Tommi Rantala <tommi.t.rantala@nokia.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-kernel@vger.kernel.org>
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Hello,

On Thu, Jun 18, 2020 at 9:39 AM Gaurav Singh <gaurav1086@gmail.com> wrote:
>
> Add a NULL check for worker before dereferencing.

Did you actually see a segfault due to this?
It seems it's called with NULL only if multiq is false
so there should not be a NULL dereference.

>
> Signed-off-by: Gaurav Singh <gaurav1086@gmail.com>
> ---
>  tools/perf/bench/epoll-wait.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/tools/perf/bench/epoll-wait.c b/tools/perf/bench/epoll-wait.c
> index 75dca9773186..42983eb7f82e 100644
> --- a/tools/perf/bench/epoll-wait.c
> +++ b/tools/perf/bench/epoll-wait.c
> @@ -239,6 +239,9 @@ static void *workerfn(void *arg)
>
>  static void nest_epollfd(struct worker *w)
>  {
> +       if (!w)
> +               return;
> +
>         unsigned int i;
>         struct epoll_event ev;
>         int efd = multiq ? w->epollfd : epollfd;

Maybe it's more intuitive to check w instead of multiq here.

Thanks
Namhyung


> --
> 2.17.1
>