linux-kernel - Re: [Patch] Convert max_user

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Date:	Tue, 5 Oct 2010 19:21:09 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Robin Holt <holt@....com>
cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Patch] Convert max_user_watches to long.

On Mon, 4 Oct 2010, Robin Holt wrote:

> On a 16TB machine, max_user_watches has an integer overflow.  Convert it
> to use a long and handle the associated fallout.
> 
> 
> Signed-off-by: Robin Holt <holt@....com>
> To: "Eric W. Biederman" <ebiederm@...ssion.com>
> To: Davide Libenzi <davidel@...ilserver.org>
> To: linux-kernel@...r.kernel.org
> To: Pekka Enberg <penberg@...helsinki.fi>
> 
> ---
> 
> Davide,  I changed the logic a bit in ep_insert.  It looked to me like
> there was a window between when the epoll_watches is checked and when it
> is incremented where multiple epoll_insert callers could be adding watches
> at the same time and allow epoll_watches to exceed max_user_watches.
> Not sure of the case where this could happen, but I assume something
> like that must be possible or we would not be using atomics.  If that
> is not to your liking, I will happily remove it.

The case can happen, but the effect is not something we should be too 
worried about.
You seem to be leaking a count in case kmem_cache_alloc() and following 
fail.
I'd rather not have that code there, and have the patch cover the 'long' 
conversion only.  Or you need a proper cleanup goto target.





> 
>  fs/eventpoll.c        |   22 +++++++++++++---------
>  include/linux/sched.h |    2 +-
>  2 files changed, 14 insertions(+), 10 deletions(-)
> 
> Index: pv1010933/fs/eventpoll.c
> ===================================================================
> --- pv1010933.orig/fs/eventpoll.c	2010-10-02 06:38:15.000000000 -0500
> +++ pv1010933/fs/eventpoll.c	2010-10-04 11:05:21.643823297 -0500
> @@ -220,7 +220,7 @@ struct ep_send_events_data {
>   * Configuration options available inside /proc/sys/fs/epoll/
>   */
>  /* Maximum number of epoll watched descriptors, per user */
> -static int max_user_watches __read_mostly;
> +static long max_user_watches __read_mostly;
>  
>  /*
>   * This mutex is used to serialize ep_free() and eventpoll_release_file().
> @@ -243,16 +243,18 @@ static struct kmem_cache *pwq_cache __re
>  
>  #include <linux/sysctl.h>
>  
> -static int zero;
> +static long zero;
> +static long long_max = LONG_MAX;
>  
>  ctl_table epoll_table[] = {
>  	{
>  		.procname	= "max_user_watches",
>  		.data		= &max_user_watches,
> -		.maxlen		= sizeof(int),
> +		.maxlen		= sizeof(max_user_watches),
>  		.mode		= 0644,
> -		.proc_handler	= proc_dointvec_minmax,
> +		.proc_handler	= proc_doulongvec_minmax,
>  		.extra1		= &zero,
> +		.extra2		= &long_max,
>  	},
>  	{ }
>  };
> @@ -564,7 +566,7 @@ static int ep_remove(struct eventpoll *e
>  	/* At this point it is safe to free the eventpoll item */
>  	kmem_cache_free(epi_cache, epi);
>  
> -	atomic_dec(&ep->user->epoll_watches);
> +	atomic_long_dec(&ep->user->epoll_watches);
>  
>  	return 0;
>  }
> @@ -900,12 +902,15 @@ static int ep_insert(struct eventpoll *e
>  {
>  	int error, revents, pwake = 0;
>  	unsigned long flags;
> +	long user_watches;
>  	struct epitem *epi;
>  	struct ep_pqueue epq;
>  
> -	if (unlikely(atomic_read(&ep->user->epoll_watches) >=
> -		     max_user_watches))
> +	user_watches = atomic_long_inc_return(&ep->user->epoll_watches);
> +	if (unlikely(user_watches > max_user_watches)) {
> +		atomic_long_dec(&ep->user->epoll_watches);
>  		return -ENOSPC;
> +	}
>  	if (!(epi = kmem_cache_alloc(epi_cache, GFP_KERNEL)))
>  		return -ENOMEM;
>  
> @@ -968,8 +973,6 @@ static int ep_insert(struct eventpoll *e
>  
>  	spin_unlock_irqrestore(&ep->lock, flags);
>  
> -	atomic_inc(&ep->user->epoll_watches);
> -
>  	/* We have to call this outside the lock */
>  	if (pwake)
>  		ep_poll_safewake(&ep->poll_wait);
> @@ -1422,6 +1425,7 @@ static int __init eventpoll_init(void)
>  	 */
>  	max_user_watches = (((si.totalram - si.totalhigh) / 25) << PAGE_SHIFT) /
>  		EP_ITEM_COST;
> +	BUG_ON(max_user_watches < 0);
>  
>  	/* Initialize the structure used to perform safe poll wait head wake ups */
>  	ep_nested_calls_init(&poll_safewake_ncalls);
> Index: pv1010933/include/linux/sched.h
> ===================================================================
> --- pv1010933.orig/include/linux/sched.h	2010-10-01 10:27:07.000000000 -0500
> +++ pv1010933/include/linux/sched.h	2010-10-04 10:44:11.287823312 -0500
> @@ -666,7 +666,7 @@ struct user_struct {
>  	atomic_t inotify_devs;	/* How many inotify devs does this user have opened? */
>  #endif
>  #ifdef CONFIG_EPOLL
> -	atomic_t epoll_watches;	/* The number of file descriptors currently watched */
> +	atomic_long_t epoll_watches; /* The number of file descriptors currently watched */
>  #endif
>  #ifdef CONFIG_POSIX_MQUEUE
>  	/* protected by mq_lock	*/
> 
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