linux-kernel - Re: [PATCH 4/7] pid: modify task_tgid

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Message-ID: <20140220183518.GA23993@redhat.com>
Date:	Thu, 20 Feb 2014 19:35:18 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Richard Guy Briggs <rgb@...hat.com>
Cc:	linux-audit@...hat.com, linux-kernel@...r.kernel.org,
	eparis@...hat.com, sgrubb@...hat.com, akpm@...ux-foundation.org,
	peterz@...radead.org, "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH 4/7] pid: modify task_tgid_nr to work without
	task->tgid.

On 01/23, Richard Guy Briggs wrote:
>
> task->tgid is an error prone construct and results in duplicate maintenance.
> Start it's demise by modifying task_tgid_nr to not use it.

Well, I disagree.

Yes I agree that ->tgid should probably die. But this change itself doesn't
help, it only makes task_tgid_nr() slower. We need to convert other users
first, then consider this change along with ->tgid removal.

Besides, this is not that simple and the patch doesn't look right:

>  static inline pid_t task_tgid_nr(struct task_struct *tsk)
>  {
> -	return tsk->tgid;
> +	return pid_nr(task_tgid(tsk));
>  }

And what protect task_tgid? This is racy.

The race is very unlikely, pid_nr() will likely hit pid == NULL if tsk
exits. But still it can use the freed/unmapped/reused memory.

And even if we add rcu_read_lock() the patch will add the semantics change,
task_tgid_nr() can return 0 if tsk has already exited. At least this should
be documented, but you also need to audit the users.

Oleg.

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