fsnotify: implement a default maximum queue depth

Currently fanotify has no maximum queue depth.  Since fanotify is
CAP_SYS_ADMIN only this does not pose a normal user DoS issue, but it
certianly is possible that an fanotify listener which can't keep up could
OOM the box.  This patch implements a default 16k depth.  This is the same
default depth used by inotify, but given fanotify's better queue merging in
many situations this queue will contain many additional useful events by
comparison.
Signed-off-by: Eric Paris <eparis@redhat.com>

fs/notify/fanotify/fanotify_user.c

@@ -16,6 +16,8 @@
 
 #include <asm/ioctls.h>
 
+#define FANOTIFY_DEFAULT_MAX_EVENTS	16384
+
 extern const struct fsnotify_ops fanotify_fsnotify_ops;
 
 static struct kmem_cache *fanotify_mark_cache __read_mostly;
@@ -689,6 +691,8 @@ SYSCALL_DEFINE2(fanotify_init, unsigned int, flags, unsigned int, event_f_flags)
 		goto out_put_group;
 	}
 
+	group->max_events = FANOTIFY_DEFAULT_MAX_EVENTS;
+
 	fd = anon_inode_getfd("[fanotify]", &fanotify_fops, group, f_flags);
 	if (fd < 0)
 		goto out_put_group;

include/linux/fanotify.h

@@ -12,7 +12,6 @@
 
 #define FAN_EVENT_ON_CHILD	0x08000000	/* interested in child events */
 
-/* FIXME currently Q's have no limit.... */
 #define FAN_Q_OVERFLOW		0x00004000	/* Event queued overflowed */
 
 #define FAN_OPEN_PERM		0x00010000	/* File open in perm check */