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Date:   Wed, 29 Jan 2020 12:12:57 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Vasily Averin <vvs@virtuozzo.com>
Cc:     linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        NeilBrown <neilb@suse.com>, Waiman Long <longman@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Oberparleiter <oberpar@linux.ibm.com>
Subject: Re: [PATCH 3/7] t_next should increase position index
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On Fri, 24 Jan 2020 10:02:51 +0300
Vasily Averin <vvs@virtuozzo.com> wrote:

> if seq_file .next fuction does not change position index,
> read after some lseek can generate unexpected output.
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=206283
> Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
> ---
>  kernel/trace/ftrace.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 9bf1f2c..ca25210 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -3442,8 +3442,10 @@ static void *t_mod_start(struct seq_file *m, loff_t *pos)
>  	loff_t l = *pos; /* t_probe_start() must use original pos */
>  	void *ret;
>  
> -	if (unlikely(ftrace_disabled))
> +	if (unlikely(ftrace_disabled)) {
> +		(*pos)++;
>  		return NULL;
> +	}

This isn't needed. If ftrace_disabled is set, we shouldn't be printing
anything. This case isn't the same as the report in the bugzilla.

-- Steve

>  
>  	if (iter->flags & FTRACE_ITER_PROBE)
>  		return t_probe_next(m, pos);