linux-kernel - Re: [PATCH] ovl: check dentry positiveness in ovl_cleanup

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Date:	Mon, 16 Nov 2015 19:26:11 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	David Howells <dhowells@...hat.com>,
	Miklos Szeredi <miklos@...redi.hu>
Cc:	Al Viro <viro@...iv.linux.org.uk>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, linux-unionfs@...r.kernel.org
Subject: Re: [PATCH] ovl: check dentry positiveness in ovl_cleanup_whiteouts()

Note: kernels starting from 4.0 prints this
[ 72.925147] overlayfs: cleanup of '#ffff88022da16280/a' failed (-2)
instead of crashing, because of this part

--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -19,7 +19,7 @@ void ovl_cleanup(struct inode *wdir, struct dentry 
*wdentry)
         int err;

         dget(wdentry);
-       if (S_ISDIR(wdentry->d_inode->i_mode))
+       if (d_is_dir(wdentry))
                 err = ovl_do_rmdir(wdir, wdentry);
         else
                 err = ovl_do_unlink(wdir, wdentry);


of e36cb0b89ce20b4f8786a57e8a6bc8476f577650
("VFS: (Scripted) Convert S_ISLNK/DIR/REG(dentry->d_inode) to 
d_is_*(dentry)")

in older kernels crash happens at dereferencing wdentry->d_inode
ovl_do_rmdir/unlink calls vfs_unlink/vfs_rmdir which checks positiveness
in may_delete(). both returns -ENOENT (-2) in that case.

So, patch is still required: at least for avoiding flood in kernel log.

On 16.11.2015 18:44, Konstantin Khlebnikov wrote:
> This patch fixes kernel crash at removing directory which contains
> whiteouts from lower layers.
>
> Cache of directory content passed as "list" contains entries from all
> layers, including whiteouts from lower layers. So, lookup in upper dir
> (moved into work at this stage) will return negative entry. Plus this
> cache is filled long before and we can race with external removal.
>
> Example:
>   mkdir -p lower0/dir lower1/dir upper work overlay
>   touch lower0/dir/a lower0/dir/b
>   mknod lower1/dir/a c 0 0
>   mount -t overlay none overlay -o lowerdir=lower1:lower0,upperdir=upper,workdir=work
>   rm -fr overlay/dir
>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
> Cc: Stable <stable@...r.kernel.org> # 3.18+
> ---
>   fs/overlayfs/readdir.c |    3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index 70e9af551600..adcb1398c481 100644
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -571,7 +571,8 @@ void ovl_cleanup_whiteouts(struct dentry *upper, struct list_head *list)
>   			       (int) PTR_ERR(dentry));
>   			continue;
>   		}
> -		ovl_cleanup(upper->d_inode, dentry);
> +		if (dentry->d_inode)
> +			ovl_cleanup(upper->d_inode, dentry);
>   		dput(dentry);
>   	}
>   	mutex_unlock(&upper->d_inode->i_mutex);
>


-- 
Konstantin
--
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