linux-kernel - Re: [PATCH 1/3] comm: Introduce comm_lock seqlock to protect task->comm access

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Date:	Fri, 13 May 2011 20:13:26 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	John Stultz <john.stultz@...aro.org>
CC:	LKML <linux-kernel@...r.kernel.org>, Ted Ts'o <tytso@....edu>,
	David Rientjes <rientjes@...gle.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/3] comm: Introduce comm_lock seqlock to protect task->comm
 access

Hi

Sorry for the long delay.

>   char *get_task_comm(char *buf, struct task_struct *tsk)
>   {
> -	/* buf must be at least sizeof(tsk->comm) in size */
> -	task_lock(tsk);
> -	strncpy(buf, tsk->comm, sizeof(tsk->comm));
> -	task_unlock(tsk);
> +	unsigned long seq;
> +
> +	do {
> +		seq = read_seqbegin(&tsk->comm_lock);
> +
> +		strncpy(buf, tsk->comm, sizeof(tsk->comm));
> +
> +	} while (read_seqretry(&tsk->comm_lock, seq));
> +
>   	return buf;
>   }

Can you please explain why we should use seqlock? That said,
we didn't use seqlock for /proc items. because, plenty seqlock
write may makes readers busy wait. Then, if we don't have another
protection, we give the local DoS attack way to attackers.

task->comm is used for very fundamentally. then, I doubt we can
assume write is enough rare. Why can't we use normal spinlock?


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