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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Fri, 8 Oct 2021 20:14:21 +0800
Message-ID: <CALOAHbDW4KnHndEXFVs-teQy6EQWuSi7thjkp7Ye8kboUGgvKg@mail.gmail.com>
Subject: Re: [PATCH v2 4/4] kernel/kthread: show a warning if kthread's comm
 is truncated
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Petr Mladek <pmladek@suse.com>, Kees Cook <keescook@chromium.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Valentin Schneider <valentin.schneider@arm.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        qiang.zhang@windriver.com, robdclark@chromium.org,
        christian@brauner.io, Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Benjamin Segall <bsegall@google.com>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Fri, Oct 8, 2021 at 1:41 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Thu,  7 Oct 2021 12:07:52 +0000
> Yafang Shao <laoar.shao@gmail.com> wrote:
>
> > -             vsnprintf(name, sizeof(name), namefmt, args);
> > +             len = vsnprintf(name, sizeof(name), namefmt, args);
> > +             if (len >= TASK_COMM_LEN) {
> > +                     pr_warn("truncated kthread comm:%s, pid:%d by %d characters\n",
> > +                             name, task->pid, len - TASK_COMM_LEN + 1);
>
> Instead of saying how many characters it is truncated to, what about just
> showing what it was truncated to?
>
>                         pr_warn("truncated kthread comm from:%s to:%.*s for pid:%d\n",
>                                 name, TASK_COMM_LEN - 1, name, task->pid);
>
> ?
>

The 'name' is the truncated one. So it will be printed like,
[    0.222126] truncated kthread comm from:rcu_tasks_kthre
to:rcu_tasks_kthre for pid:10

If we want to show the full name, we have to use the namefmt,  which
is not suggested to use by Petr.
See also https://lore.kernel.org/lkml/YVXVBXSZ1m4ScvbX@alley/

Or we can do it as follows,

-               char name[TASK_COMM_LEN];
+                /* To show the full name if it will be truncated. */
+               char name[TASK_COMM_LEN + 8];

Then the full name will be printed:
[    0.222587] truncated kthread comm from:rcu_tasks_kthread
to:rcu_tasks_kthre for pid:10

But that seems a little overkill ?

-- 
Thanks
Yafang