linux-kernel - Re: [PATCH] proc-remove-races-from-proc_id

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Date:	Tue, 20 Nov 2007 13:45:08 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...nvz.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] proc-remove-races-from-proc_id_readdir-factor-out-tgid-increment

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

> On 11/19, Eric W. Biederman wrote:
>> 
>> So to fix the problem this patch modifies next_tgid() to return
>> both a tgid and the task struct in question.
>> 
>> A structure is introduced to return these values because it is
>> slightly cleaner and easier to optimize, and the resulting code
>> is a little shorter.
>
> Nice. Perhaps we can also factor out the incrementing of iter.tgid? Lessens
> the generated code, and (imho) cleanups the source a little bit.

Well the name doesn't imply what the exact sematic is and we
only have one caller, so we might as well.  Looking at the
code I don't see any problems.

Acked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

> (untested, 2.6.24-rc2-mm1 doesn't boot on my machine)
>
> Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
>
> --- PT/fs/proc/base.c~	2007-11-20 19:00:05.000000000 +0300
> +++ PT/fs/proc/base.c	2007-11-20 19:06:28.000000000 +0300
> @@ -2378,6 +2378,7 @@ static struct tgid_iter next_tgid(struct
>  	rcu_read_lock();
>  retry:
>  	iter.task = NULL;
> +	iter.tgid++;
>  	pid = find_ge_pid(iter.tgid, ns);
>  	if (pid) {
>  		iter.tgid = pid_nr_ns(pid, ns);
> @@ -2394,10 +2395,8 @@ retry:
>  		 * found doesn't happen to be a thread group leader.
>  		 * As we don't care in the case of readdir.
>  		 */
> -		if (!iter.task || !has_group_leader_pid(iter.task)) {
> -			iter.tgid += 1;
> +		if (!iter.task || !has_group_leader_pid(iter.task))
>  			goto retry;
> -		}
>  		get_task_struct(iter.task);
>  	}
>  	rcu_read_unlock();
> @@ -2434,10 +2433,8 @@ int proc_pid_readdir(struct file * filp,
>  
>  	ns = filp->f_dentry->d_sb->s_fs_info;
>  	iter.task = NULL;
> -	iter.tgid = filp->f_pos - TGID_OFFSET;
> -	for (iter = next_tgid(ns, iter);
> -	     iter.task;
> -	     iter.tgid += 1, iter = next_tgid(ns, iter)) {
> +	iter.tgid = filp->f_pos - TGID_OFFSET - 1;
> +	while ((iter = next_tgid(ns, iter)).task) {
>  		filp->f_pos = iter.tgid + TGID_OFFSET;
>  		if (proc_pid_fill_cache(filp, dirent, filldir, iter) < 0) {
>  			put_task_struct(iter.task);
-
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