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Date:   Mon, 25 Oct 2021 14:35:42 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Yafang Shao <laoar.shao@gmail.com>
Cc:     akpm@linux-foundation.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, arnaldo.melo@gmail.com,
        pmladek@suse.com, peterz@infradead.org, viro@zeniv.linux.org.uk,
        valentin.schneider@arm.com, qiang.zhang@windriver.com,
        robdclark@chromium.org, christian@brauner.io,
        dietmar.eggemann@arm.com, mingo@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, davem@davemloft.net, kuba@kernel.org,
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        john.fastabend@gmail.com, kpsingh@kernel.org,
        dennis.dalessandro@cornelisnetworks.com,
        mike.marciniszyn@cornelisnetworks.com, dledford@redhat.com,
        jgg@ziepe.ca, linux-rdma@vger.kernel.org, netdev@vger.kernel.org,
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        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, oliver.sang@intel.com, lkp@intel.com,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>
Subject: Re: [PATCH v6 12/12] kernel/kthread: show a warning if kthread's
 comm is truncated
Message-ID: <202110251431.F594652F@keescook>
References: <20211025083315.4752-1-laoar.shao@gmail.com>
 <20211025083315.4752-13-laoar.shao@gmail.com>
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On Mon, Oct 25, 2021 at 08:33:15AM +0000, Yafang Shao wrote:
> Show a warning if task comm is truncated. Below is the result
> of my test case:
> 
> truncated kthread comm:I-am-a-kthread-with-lon, pid:14 by 6 characters
> 
> Suggested-by: Petr Mladek <pmladek@suse.com>
> Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>
> Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Al Viro <viro@zeniv.linux.org.uk>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Petr Mladek <pmladek@suse.com>
> ---
>  kernel/kthread.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 5b37a8567168..46b924c92078 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -399,12 +399,17 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
>  	if (!IS_ERR(task)) {
>  		static const struct sched_param param = { .sched_priority = 0 };
>  		char name[TASK_COMM_LEN];
> +		int len;
>  
>  		/*
>  		 * task is already visible to other tasks, so updating
>  		 * COMM must be protected.
>  		 */
> -		vsnprintf(name, sizeof(name), namefmt, args);
> +		len = vsnprintf(name, sizeof(name), namefmt, args);
> +		if (len >= TASK_COMM_LEN) {

And since this failure case is slow-path, we could improve the warning
as other had kind of suggested earlier with something like this instead:

			char *full_comm;

			full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
			pr_warn("truncated kthread comm '%s' to '%s' (pid:%d)\n",
				full_comm, name);

			kfree(full_comm);
		}
>  		set_task_comm(task, name);
>  		/*
>  		 * root may have changed our (kthreadd's) priority or CPU mask.
> -- 
> 2.17.1
> 

-- 
Kees Cook