linux-kernel - Re: [PATCH -v3 7/8] fsnotify: add in inode fsnotify markings

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Date:	Fri, 28 Nov 2008 05:42:41 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Eric Paris <eparis@...hat.com>
Cc:	linux-kernel@...r.kernel.org, malware-list@...ts.printk.net,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	arjan@...radead.org, hch@...radead.org, a.p.zijlstra@...llo.nl
Subject: Re: [PATCH -v3 7/8] fsnotify: add in inode fsnotify markings

On Tue, Nov 25, 2008 at 12:21:28PM -0500, Eric Paris wrote:

> +void fsnotify_mark_get(struct fsnotify_mark_entry *entry)
> +{
> +	spin_lock(&entry->lock);
> +	entry->refcnt++;
> +	spin_unlock(&entry->lock);
> +}

> +void fsnotify_mark_put(struct fsnotify_mark_entry *entry)
> +{
> +	spin_lock(&entry->lock);
> +	entry->refcnt--;
> +	/* if (!refcnt && killme) we are off both lists and nothing else can find us. */
> +	if ((!entry->refcnt) && (entry->killme)) {
> +		spin_unlock(&entry->lock);
> +		fsnotify_mark_kill(entry);
> +		return;
> +	}
> +	spin_unlock(&entry->lock);
> +}

Uh-huh...  And what happens if fsnotify_mark_get() comes in the middle
of final fsnotify_mark_put()?  You spin on entry->lock, gain it just before
fsnotify_mark_kill() which proceeds to kfree entry under you just as you
increment its refcnt...

> +void fsnotify_clear_mark_group(struct fsnotify_group *group)
> +{
> +	struct fsnotify_mark_entry *entry;
> +	struct inode *inode;
> +
> +	mutex_lock(&group->mark_mutex);
> +	while (!list_empty(&group->mark_entries)) {
> +		entry = list_first_entry(&group->mark_entries, struct fsnotify_mark_entry, g_list);
> +
> +		/* make sure the entry survives until it is off both lists */
> +		fsnotify_mark_get(entry);
> +
> +		/* remove from g_list */
> +		list_del_init(&entry->g_list);
> +		mutex_unlock(&group->mark_mutex);
> +
> +		inode = entry->inode;
> +
> +		spin_lock(&entry->lock);
> +		entry->killme = 1;
> +		spin_unlock(&entry->lock);
> 
> +		/* remove from i_list */
> +		spin_lock(&inode->i_fsnotify_lock);

... and just what would keep the inode from being freed under you here?
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