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

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Date:	Tue, 10 May 2011 18:20:50 -0700
From:	Joe Perches <joe@...ches.com>
To:	John Stultz <john.stultz@...aro.org>
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:
> > On Tue, 2011-05-10 at 17:23 -0700, John Stultz wrote:
> > > Acessing task->comm requires proper locking. However in the past
> > > access to current->comm could be done without locking. This
> > > is no longer the case, so all comm access needs to be done
> > > while holding the comm_lock.
> > 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?

The only one I know of is ipv6 which copies a 16 byte buffer
in case the pointed to value is unaligned.  I suppose %pI6c
could be a problem or maybe %pS too, but it hasn't been in
practice.  The use of %ptc somehow seemed more error prone.

> Most users are current, so forcing the more rare
> non-current users to copy it to a buffer first and use the normal %s
> would not be of much impact.
> 
> Although I'm not sure if there's precedent for a %p value that didn't
> take a argument. Thoughts on that? Anyone else have an opinion here?

The uses of %ptc must add an argument or else gcc will complain.
I suggest you just ignore the argument value and use current.


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