linux-kernel - Re: [PATCH 5/8] fuse: don't use ->d

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Message-ID: <20160622164612.GP14480@ZenIV.linux.org.uk>
Date:	Wed, 22 Jun 2016 17:46:12 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Miklos Szeredi <mszeredi@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 5/8] fuse: don't use ->d_time

On Wed, Jun 22, 2016 at 04:35:07PM +0200, Miklos Szeredi wrote:
> Store in memory pointed to by ->d_fsdata.  Use ->d_allocate() to allocate
> the storage.
> 
> We could cast ->d_fsdata directly on 64bit archs, but I don't think this is
> worth the extra complexity.

Now, _that_ is interesting:

> +static void fuse_dentry_release(struct dentry *dentry)
> +{
> +	kfree(dentry->d_fsdata);
> +}

What happens to fuse_dentry_revalidate() called on dentry in process of
getting dropped?  Unlike freeing struct dentry itself, ->d_release() is
not RCU-delayed.  So you are risking dereference of ->d_fsdata after
kfree(); at the very least, it needs RCU-delayed freeing...