linux-kernel - Re: [patch] mm, oom: remove task

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Message-ID: <20150923095757.GC640@swordfish>
Date:	Wed, 23 Sep 2015 18:57:57 +0900
From:	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Vladimir Davydov <vdavydov@...allels.com>,
	Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kyle Walker <kwalker@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-mm <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Stanislav Kozina <skozina@...hat.com>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Subject: Re: [patch] mm, oom: remove task_lock protecting comm printing

On (09/23/15 18:50), 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.
> 
> right.

... no right. that should have been

me->comm[sizeof(me->comm) - 1] = 0;

to be save. no?


> hm, shouldn't set_task_comm()->__set_task_comm() do the same?

or something like this instead

---

 fs/exec.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/exec.c b/fs/exec.c
index b06623a..d7d2de0 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1072,6 +1072,7 @@ EXPORT_SYMBOL_GPL(get_task_comm);
 
 void __set_task_comm(struct task_struct *tsk, const char *buf, bool exec)
 {
+	tsk->comm[sizeof(tsk->comm) - 1] = 0;
 	task_lock(tsk);
 	trace_task_rename(tsk, buf);
 	strlcpy(tsk->comm, buf, sizeof(tsk->comm));

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