linux-kernel - Re: c/r: broken locking when executing map

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Date:	Sat, 5 May 2012 23:32:01 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Vasiliy Kulikov <segoon@...nwall.com>
Cc:	Sasha Levin <levinsasha928@...il.com>, khlebnikov@...nvz.org,
	xemul@...allels.com, Dave Jones <davej@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: c/r: broken locking when executing map_files

On Sat, May 05, 2012 at 10:53:06PM +0400, Cyrill Gorcunov wrote:
> On Sat, May 05, 2012 at 10:20:51PM +0400, Vasiliy Kulikov wrote:
> ...
> > > ---
> > >  fs/proc/base.c |    8 +++-----
> > >  1 file changed, 3 insertions(+), 5 deletions(-)
> > > 
> > > Index: linux-2.6.git/fs/proc/base.c
> > > ===================================================================
> > > --- linux-2.6.git.orig/fs/proc/base.c
> > > +++ linux-2.6.git/fs/proc/base.c
> > > @@ -2226,16 +2226,16 @@ static struct dentry *proc_map_files_loo
> > >  		goto out;
> > >  
> > >  	result = ERR_PTR(-EACCES);
> > > -	if (lock_trace(task))
> > > +	if (!ptrace_may_access(task, PTRACE_MODE_READ))
> > 
> > Probably it will be better to change mutex_lock_killable() to
> > mutex_lock_killable_nested() inside of lock_trace() instead of this change?
> > It would keep the race-free check.
> 
> Yup, if I'm not missing something SINGLE_DEPTH_NESTING should do the trick
> for us. I'll test and report.

Hmm, this doesn't work well, the mutex remanins killable so when one does

 | [root@...tune ~]# /proc/self/map_files/400000-419000

it sleeps forever until killed, which is not good I think. Vasiliy, could
you remind me what exactly is problem if we use unlocked ptrace_may_access
here?

	Cyrill
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