linux-kernel - Re: [PATCH v2 4/4] kernel/kthread: show a warning if kthread's comm is truncated

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Date:   Fri, 8 Oct 2021 20:14:21 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Petr Mladek <pmladek@...e.com>, Kees Cook <keescook@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        qiang.zhang@...driver.com, robdclark@...omium.org,
        christian@...uner.io, Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] kernel/kthread: show a warning if kthread's comm
 is truncated

On Fri, Oct 8, 2021 at 1:41 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Thu,  7 Oct 2021 12:07:52 +0000
> Yafang Shao <laoar.shao@...il.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