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Date: Wed, 23 Sep 2015 12:41:34 +0200
From: Michal Hocko <mhocko@kernel.org>
To: Vladimir Davydov <vdavydov@parallels.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        David Rientjes <rientjes@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Oleg Nesterov <oleg@redhat.com>,
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        Johannes Weiner <hannes@cmpxchg.org>, linux-mm <linux-mm@kvack.org>,
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        Stanislav Kozina <skozina@redhat.com>,
        Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Subject: Re: [patch] mm, oom: remove task_lock protecting comm printing
Message-ID: <20150923104133.GD8644@dhcp22.suse.cz>
References: <alpine.DEB.2.10.1509221629440.7794@chino.kir.corp.google.com>
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On Wed 23-09-15 13:07:40, Vladimir Davydov wrote:
> On Wed, Sep 23, 2015 at 06:50:22PM +0900, Sergey Senozhatsky wrote:
> > On (09/23/15 11:43), Michal Hocko wrote:
> > [..]
> > > > > the previous name was already null terminated,
> > > > 
> > > > Yeah, but if the old name is shorter than the new one, set_task_comm()
> > > > overwrites the terminating null of the old name before writing the new
> > > > terminating null, so there is a short time window during which tsk->comm
> > > > might be not null-terminated, no?
> > > 
> > > Not really:
> > >         case PR_SET_NAME:
> > >                 comm[sizeof(me->comm) - 1] = 0;
> > >                 if (strncpy_from_user(comm, (char __user *)arg2,
> > >                                       sizeof(me->comm) - 1) < 0)
> > >                         return -EFAULT;
> > > 
> > > So it first writes the terminating 0 and only then starts copying.
> 
> It writes 0 to a temporary buffer, not to tsk->comm, so I don't think
> it's related. However, reading tsk->comm w/o locking must be safe
> anyway, because tsk->comm[TASK_COMM_LEN-1] is always 0 (inherited from
> init_task) and it never gets overwritten, because __set_task_comm() uses
> strlcpy().

Right you are! I am blind obviously... Thought we are copying directly
to the task->comm.

-- 
Michal Hocko
SUSE Labs