Re: [PATCH] proc: readdir race fix (take 3)

[Oleg Nesterov <oleg@tv-sign.ru>]

On 09/06, Jean Delvare wrote:
>
> On Wednesday 6 September 2006 11:01, Jean Delvare wrote:
> > Eric, Kame, thanks a lot for working on this. I'll be giving some good
> > testing to this patch today, and will return back to you when I'm done.
> 
> The original issue is indeed fixed, but there's a problem with the patch. 
> When stressing /proc (to verify the bug was fixed), my test machine ended 
> up crashing. Here are the 2 traces I found in the logs:
> 
> Sep  6 12:06:00 arrakis kernel: BUG: warning at 
> kernel/fork.c:113/__put_task_struct()
> Sep  6 12:06:00 arrakis kernel:  [<c0115f93>] __put_task_struct+0xf3/0x100
> Sep  6 12:06:00 arrakis kernel:  [<c019666a>] proc_pid_readdir+0x13a/0x150
> Sep  6 12:06:00 arrakis kernel:  [<c01745f0>] vfs_readdir+0x80/0xa0
> Sep  6 12:06:00 arrakis kernel:  [<c0174750>] filldir+0x0/0xd0
> Sep  6 12:06:00 arrakis kernel:  [<c017488c>] sys_getdents+0x6c/0xb0
> Sep  6 12:06:00 arrakis kernel:  [<c0174750>] filldir+0x0/0xd0
> Sep  6 12:06:00 arrakis kernel:  [<c0102fb7>] syscall_call+0x7/0xb

I think there is a bug in next_tgid(),

> -static struct task_struct *next_tgid(struct task_struct *start)
> -{
> -	struct task_struct *pos;
> +	task = NULL;
>  	rcu_read_lock();
> -	pos = start;
> -	if (pid_alive(start))
> -		pos = next_task(start);
> -	if (pid_alive(pos) &amp;&amp; (pos != &amp;init_task)) {
> -		get_task_struct(pos);
> -		goto done;
> +retry:
> +	pid = find_ge_pid(tgid);
> +	if (pid) {
> +		tgid = pid->nr + 1;
> +		task = pid_task(pid, PIDTYPE_PID);
> +		/* What we to know is if the pid we have find is the
> +		 * pid of a thread_group_leader.  Testing for task
> +		 * being a thread_group_leader is the obvious thing
> +		 * todo but there is a window when it fails, due to
> +		 * the pid transfer logic in de_thread.
> +		 *
> +		 * So we perform the straight forward test of seeing
> +		 * if the pid we have found is the pid of a thread
> +		 * group leader, and don't worry if the task we have
> +		 * found doesn't happen to be a thread group leader.
> +		 * As we don't care in the case of readdir.
> +		 */
> +		if (!task || !has_group_leader_pid(task))
> +			goto retry;
> +		get_task_struct(task);
>  	}
> -	pos = NULL;
> -done:
>  	rcu_read_unlock();
> -	put_task_struct(start);
> -	return pos;
> +	return task;
>  }

If the task found is not a group leader, we go to retry, but
the task != NULL.

Now, if find_ge_pid(tgid) returns NULL, we return that wrong
task, and it was not get_task_struct()'ed.

Oleg.