linux-kernel - Re: [PATCH 2/3] printk: Add %ptc to safely print a task's comm

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Message-ID: <1305076561.2939.72.camel@work-vm>
Date:	Tue, 10 May 2011 18:16:01 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Joe Perches <joe@...ches.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, "Ted Ts'o" <tytso@....edu>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	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 2/3] printk: Add %ptc to safely print a task's comm

On Tue, 2011-05-10 at 18:10 -0700, John Stultz wrote:
> On Tue, 2011-05-10 at 17:51 -0700, Joe Perches wrote:
> > Could misuse of %ptc (not using current) cause system lockup?
> 
> It very well could. Although I don't see other %p options tring to
> handle invalid pointers. Any suggestions on how to best handle this?

And just to clarify on this point, I'm responding to if a invalid
pointer was provided, causing the dereference to go awry.

If a valid non-current task was provided, the locking should be ok as we
disable irqs while the write_seqlock is held in set_task_comm().

The only places this could cause a problem was if you tried to printk
with a %ptc while holding the task->comm_lock. However, the lock is only
shortly held in task_comm_string, and get_task_comm and set_task_comm.
So it is fairly easy to audit for correctness.

If there is some other situation you had in mind, please let me know.

thanks
-john

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