linux-kernel - Re: [PATCH] Allow threads to rename siblings via /proc/pid/tasks/tid/comm

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Date:	Sat, 07 Nov 2009 23:52:20 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	john stultz <johnstul@...ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Mike Fulton <fultonm@...ibm.com>,
	Sean Foley <Sean_Foley@...ibm.com>,
	Darren Hart <dvhltc@...ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Subject: Re: [PATCH] Allow threads to rename siblings via /proc/pid/tasks/tid/comm

john stultz <johnstul@...ibm.com> writes:
> -	strlcpy(tsk->comm, buf, sizeof(tsk->comm));
> +
> +	/*
> +	 * Threads may access current->comm without holding
> +	 * the task lock, so write the string carefully
> +	 * to avoid non-terminating reads. Readers without a lock
> +	 * will get the oldname, the newname or an empty string.
> +	 */
> +	tsk->comm[0] = 0;
> +	wmb();
> +	strlcpy(tsk->comm+1, buf+1, sizeof(tsk->comm)-1);
> +	wmb();
> +	tsk->comm[0] = buf[0];

Is this really safe? 

reader                    writer


read comm[0]
                         set comm[0] to 0
                         overwrites comm[1]
read comm[1]
read comm[2]
                         writes comm[2] to 0 
read comm[3]

...
goes beyond the end
                  
Better way probably is to replace tsk->comm with a pointer
and exchange that using xchg. Drawback: 4-8 bytes more per task.

Or perhaps make comm one byte longer and make sure the last
byte is always 0, but the drawback is that a reader can
read random (but at least safe) junk then.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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