linux-kernel - [PATCH] Fix incorrect refcount while dispatching unmount event

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Date:	Wed, 26 Nov 2008 17:08:29 +0300
From:	Dmitri Monakhov <dmonakhov@...nvz.org>
To:	linux-kernel@...r.kernel.org
Cc:	rml@...ell.com, Dmitri Monakhov <dmonakhov@...nvz.org>
Subject: [PATCH] Fix incorrect refcount while dispatching unmount event

On umount two event will be dispatched to watcher:
1: inotify_dev_queue_event(.., IN_UNMOUNT,..)
2: remove_watch(watch, dev)
    ->inotify_dev_queue_event(.., IN_IGNORED, ..)
But if watcher has IN_ONESHOT bit set then the watcher will be released
inside first event. Which result in accessing invalid object later.
IMHO it is not pure regression. This bug wasn't triggered while initial
inotify interface testing phase because of another bug in IN_ONESHOT
handling logic :)
  commit ac74c00e499ed276a965e5b5600667d5dc04a84a
  Author: Ulisses Furquim <ulissesf@...il.com>
  Date:   Fri Feb 8 04:18:16 2008 -0800
    inotify: fix check for one-shot watches before destroying them
    As the IN_ONESHOT bit is never set when an event is sent we must check it
    in the watch's mask and not in the event's mask.

TESTCASE:
mkdir mnt
mount -ttmpfs none mnt
mkdir mnt/d
./inotify mnt/d&
umount mnt ## << lockup or crash here

TESTSOURCE:
/* gcc -oinotify inotify.c */
#include <stdio.h>
#include <stdlib.h>
#include <sys/inotify.h>

int main(int argc, char **argv)
{
        char buf[1024];
        struct inotify_event *ie;
        char *p;
        int i;
        ssize_t l;

        p = argv[1];
        i = inotify_init();
        inotify_add_watch(i, p, ~0);

        l = read(i, buf, sizeof(buf));
        printf("read %d bytes\n", l);
        ie = (struct inotify_event *) buf;
        printf("event mask: %d\n", ie->mask);
	return 0;
}

Signed-off-by: Dmitri Monakhov <dmonakhov@...nvz.org>
---
 fs/inotify.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/fs/inotify.c b/fs/inotify.c
index 7bbed1b..dae3f28 100644
--- a/fs/inotify.c
+++ b/fs/inotify.c
@@ -428,11 +428,13 @@ void inotify_unmount_inodes(struct list_head *list)
 		watches = &inode->inotify_watches;
 		list_for_each_entry_safe(watch, next_w, watches, i_list) {
 			struct inotify_handle *ih= watch->ih;
+			get_inotify_watch(watch);
 			mutex_lock(&ih->mutex);
 			ih->in_ops->handle_event(watch, watch->wd, IN_UNMOUNT, 0,
 						 NULL, NULL);
 			inotify_remove_watch_locked(ih, watch);
 			mutex_unlock(&ih->mutex);
+			put_inotify_watch(watch);
 		}
 		mutex_unlock(&inode->inotify_mutex);
 		iput(inode);		
-- 
1.5.4.3

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