Date:	Tue, 10 Mar 2009 00:19:40 +0530
From:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
To:	Kay Sievers <kay.sievers@...y.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: sched: delayed cleanup of user_struct

On Mon, Mar 09, 2009 at 07:37:17PM +0100, Kay Sievers wrote:
> From: Kay Sievers <kay.sievers@...y.org>
> Subject: sched: delayed cleanup of user_struct
> 
> During bootup performance tracing we see repeated occurrences of
> /sys/kernel/uid/* events for the same uid, leading to a,
> in this case, rather pointless userspace processing for the
> same uid over and over.
> 
> This is usally caused by tools which change their uid to "nobody",
> to run without privileges to read data supplied by untrusted users.
> 
> This change delays the execution of the (already existing) scheduled
> work, to cleanup the uid after 0.5 seconds, so the allocated and announced
> uid can possibly be re-used by another process.
> 
> This is the current behavior, where almost every invocation of a
> binary, which changes the uid, creates two events:
>   $ read START < /sys/kernel/uevent_seqnum; \
>   for i in `seq 100`; do su --shell=/bin/true bin; done; \
>   read END < /sys/kernel/uevent_seqnum; \
>   echo $(($END - $START))
>   178
> 
> With the delayed cleanup, we get only two events, and userspace finishes
> a bit faster too:
>   $ read START < /sys/kernel/uevent_seqnum; \
>   for i in `seq 100`; do su --shell=/bin/true bin; done; \
>   read END < /sys/kernel/uevent_seqnum; \
>   echo $(($END - $START))
>   1
> 

makes sense. I do have a patch though which changes some of the cleanup
code (fixing a memory leak) in -mm. These two patches will conflict.

> Cc: Dhaval Giani <dhaval@...ux.vnet.ibm.com>
> Cc: Greg Kroah-Hartman <gregkh@...e.de>
> Signed-off-by: Kay Sievers <kay.sievers@...y.org>
> ---
>  include/linux/sched.h |    2 +-
>  kernel/user.c         |   28 +++++++++++++---------------
>  2 files changed, 14 insertions(+), 16 deletions(-)
> 
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -670,7 +670,7 @@ struct user_struct {
>  	struct task_group *tg;
>  #ifdef CONFIG_SYSFS
>  	struct kobject kobj;
> -	struct work_struct work;
> +	struct delayed_work work;
>  #endif
>  #endif
>  };
> --- a/kernel/user.c
> +++ b/kernel/user.c
> @@ -75,6 +75,7 @@ static void uid_hash_remove(struct user_
>  	put_user_ns(up->user_ns);
>  }
> 
> +/* uidhash_lock must be held */
>  static struct user_struct *uid_hash_find(uid_t uid, struct hlist_head *hashent)
>  {
>  	struct user_struct *user;
> @@ -82,7 +83,9 @@ static struct user_struct *uid_hash_find
> 
>  	hlist_for_each_entry(user, h, hashent, uidhash_node) {
>  		if (user->uid == uid) {
> -			atomic_inc(&user->__count);
> +			/* possibly resurrect an "almost deleted" object */
> +			if (atomic_inc_return(&user->__count) == 1)
> +				cancel_delayed_work(&user->work);
>  			return user;
>  		}
>  	}
> @@ -283,12 +286,12 @@ int __init uids_sysfs_init(void)
>  	return uids_user_create(&root_user);
>  }
> 
> -/* work function to remove sysfs directory for a user and free up
> +/* delayed work function to remove the user and free up
>   * corresponding structures.
>   */
> -static void remove_user_sysfs_dir(struct work_struct *w)
> +static void remove_user_delayed(struct work_struct *w)
>  {
> -	struct user_struct *up = container_of(w, struct user_struct, work);
> +	struct user_struct *up = container_of(w, struct user_struct, work.work);
>  	unsigned long flags;
>  	int remove_user = 0;
> 
> @@ -299,15 +302,12 @@ static void remove_user_sysfs_dir(struct
>  	 */
>  	uids_mutex_lock();
> 
> -	local_irq_save(flags);
> -
> -	if (atomic_dec_and_lock(&up->__count, &uidhash_lock)) {
> +	spin_lock_irqsave(&uidhash_lock, flags);
> +	if (atomic_read(&up->__count) == 0) {
>  		uid_hash_remove(up);
>  		remove_user = 1;
> -		spin_unlock_irqrestore(&uidhash_lock, flags);
> -	} else {
> -		local_irq_restore(flags);
>  	}
> +	spin_unlock_irqrestore(&uidhash_lock, flags);
> 
>  	if (!remove_user)
>  		goto done;
> @@ -331,12 +331,8 @@ done:
>   */
>  static void free_user(struct user_struct *up, unsigned long flags)
>  {
> -	/* restore back the count */
> -	atomic_inc(&up->__count);
>  	spin_unlock_irqrestore(&uidhash_lock, flags);
> -
> -	INIT_WORK(&up->work, remove_user_sysfs_dir);
> -	schedule_work(&up->work);
> +	schedule_delayed_work(&up->work, msecs_to_jiffies(500));
>  }
> 
>  #else	/* CONFIG_USER_SCHED && CONFIG_SYSFS */
> @@ -442,6 +438,8 @@ struct user_struct *alloc_uid(struct use
>  		if (uids_user_create(new))
>  			goto out_destoy_sched;
> 
> +		INIT_DELAYED_WORK(&new->work, remove_user_delayed);
> +
>  		/*
>  		 * Before adding this, check whether we raced
>  		 * on adding the same user already..
> 

-- 
regards,
Dhaval
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