linux-kernel - Re: [PATCH] introduce get_task_pid() to fix unsafe get

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Date:	Sun, 10 Sep 2006 21:58:25 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] introduce get_task_pid() to fix unsafe get_pid()

Oleg Nesterov <oleg@...sign.ru> writes:

> (COMPILE TESTED, needs an ack from Eric)
>
> proc_pid_make_inode:
>
> 	ei->pid = get_pid(task_pid(task));
>
> I think this is not safe. get_pid() can be preempted after checking
> "pid != NULL". Then the task exits, does detach_pid(), and RCU frees
> the pid.

Ugh.  I had forgotten that the pid of a task gets freed even if you
hold a reference to the task struct.  So the preemption case looks possible.

Your technique to handle this problem looks fine.

As for the functions can we build them in all 4 varieties.
struct pid *get_task_pid(struct task *);
struct pid *get_task_tgid(struct task *);
struct pid *get_task_pgrp(struct task *);
struct pid *get_task_session(struct task *);

Functions without a flag are less error prone to use, and clearer to read.

Either that or we can just drop in some rcu_read_lock() rcu_read_unlock()
into the call sites.

Eric
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