linux-kernel - [Patch] Convert max_user

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Date:	Mon, 4 Oct 2010 14:44:11 -0500
From:	Robin Holt <holt@....com>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Robin Holt <holt@....com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [Patch] Convert max_user_watches to long.



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.

 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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