ACK: [PATCH][SRU][Disco] UBUNTU: SAUCE: shiftfs: fix buggy unlink logic

[Connor Kuehl]

On 8/29/19 11:45 AM, Christian Brauner wrote:
> BugLink: https://bugs.launchpad.net/bugs/1841977
> 
> The way we messed with setting i_nlink was brittle and wrong. We used to
> set the i_nlink of the shiftfs dentry to be deleted to the i_nlink count
> of the underlay dentry of the directory it resided in which makes no
> sense whatsoever. We also missed drop_nlink() which is crucial since
> i_nlink affects whether a dentry is cleaned up on dput().
> With this I cannot reproduce the bug anymore where shiftfs misleads zfs
> into believing that a deleted file can not be removed from disk because
> it is still referenced.
> 
> Fixes: commit 87011da41961 ("shiftfs: rework and extend")
> Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>

Acked-by: Connor Kuehl <connor.kuehl at canonical.com>

> ---
>   fs/shiftfs.c | 15 ++++++++++-----
>   1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/shiftfs.c b/fs/shiftfs.c
> index 57e04a02d74c..084aa7a25566 100644
> --- a/fs/shiftfs.c
> +++ b/fs/shiftfs.c
> @@ -585,6 +585,7 @@ static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
>   {
>   	struct dentry *lowerd = dentry->d_fsdata;
>   	struct inode *loweri = dir->i_private;
> +	struct inode *inode = d_inode(dentry);
>   	int err;
>   	const struct cred *oldcred;
>   
> @@ -594,15 +595,19 @@ static int shiftfs_rm(struct inode *dir, struct dentry *dentry, bool rmdir)
>   		err = vfs_rmdir(loweri, lowerd);
>   	else
>   		err = vfs_unlink(loweri, lowerd, NULL);
> -	inode_unlock(loweri);
>   	revert_creds(oldcred);
>   
> -	shiftfs_copyattr(loweri, dir);
> -	set_nlink(d_inode(dentry), loweri->i_nlink);
> -	if (!err)
> +	if (!err) {
>   		d_drop(dentry);
>   
> -	set_nlink(dir, loweri->i_nlink);
> +		if (rmdir)
> +			clear_nlink(inode);
> +		else
> +			drop_nlink(inode);
> +	}
> +	inode_unlock(loweri);
> +
> +	shiftfs_copyattr(loweri, dir);
>   
>   	return err;
>   }
>