Re: [PATCH v2 1/5] 9p: Fix refcounting during full path walks for fid lookups

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Freitag, 27. Mai 2022 01:59:59 CEST Tyler Hicks wrote:
> Decrement the refcount of the parent dentry's fid after walking
> each path component during a full path walk for a lookup. Failure to do
> so can lead to fids that are not clunked until the filesystem is
> unmounted, as indicated by this warning:
> 
>  9pnet: found fid 3 not clunked

That explains why I saw so many fids not being clunked with recent Linux 
kernel versions while doing some 9p protocol debugging with QEMU recently.

> The improper refcounting after walking resulted in open(2) returning
> -EIO on any directories underneath the mount point when using the virtio
> transport. When using the fd transport, there's no apparent issue until
> the filesytem is unmounted and the warning above is emitted to the logs.

Actually I never saw that open() = -EIO error. Do you have a reproducer?

> In some cases, the user may not yet be attached to the filesystem and a
> new root fid, associated with the user, is created and attached to the
> root dentry before the full path walk is performed. Increment the new
> root fid's refcount to two in that situation so that it can be safely
> decremented to one after it is used for the walk operation. The new fid
> will still be attached to the root dentry when
> v9fs_fid_lookup_with_uid() returns so a final refcount of one is
> correct/expected.
> 
> Fixes: 6636b6dcc3db ("9p: add refcount to p9_fid struct")
> Cc: stable@vger.kernel.org
> Signed-off-by: Tyler Hicks <tyhicks@linux.microsoft.com>
> ---
>  fs/9p/fid.c | 17 +++++------------
>  1 file changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/9p/fid.c b/fs/9p/fid.c
> index 79df61fe0e59..5a469b79c1ee 100644
> --- a/fs/9p/fid.c
> +++ b/fs/9p/fid.c
> @@ -152,7 +152,7 @@ static struct p9_fid *v9fs_fid_lookup_with_uid(struct
> dentry *dentry, const unsigned char **wnames, *uname;
>  	int i, n, l, clone, access;
>  	struct v9fs_session_info *v9ses;
> -	struct p9_fid *fid, *old_fid = NULL;
> +	struct p9_fid *fid, *old_fid;
> 
>  	v9ses = v9fs_dentry2v9ses(dentry);
>  	access = v9ses->flags & V9FS_ACCESS_MASK;
> @@ -194,13 +194,12 @@ static struct p9_fid *v9fs_fid_lookup_with_uid(struct
> dentry *dentry, if (IS_ERR(fid))
>  			return fid;
> 
> +		refcount_inc(&fid->count);
>  		v9fs_fid_add(dentry->d_sb->s_root, fid);
>  	}
>  	/* If we are root ourself just return that */
> -	if (dentry->d_sb->s_root == dentry) {
> -		refcount_inc(&fid->count);
> +	if (dentry->d_sb->s_root == dentry)
>  		return fid;
> -	}

Hmm, wouldn't it then be possible that the root fid is returned with refcount 
being 2 here?

>  	/*
>  	 * Do a multipath walk with attached root.
>  	 * When walking parent we need to make sure we
> @@ -212,6 +211,7 @@ static struct p9_fid *v9fs_fid_lookup_with_uid(struct
> dentry *dentry, fid = ERR_PTR(n);
>  		goto err_out;
>  	}
> +	old_fid = fid;
>  	clone = 1;
>  	i = 0;
>  	while (i < n) {
> @@ -221,15 +221,8 @@ static struct p9_fid *v9fs_fid_lookup_with_uid(struct
> dentry *dentry, * walk to ensure none of the patch component change
>  		 */
>  		fid = p9_client_walk(fid, l, &wnames[i], clone);
> +		p9_client_clunk(old_fid);
>  		if (IS_ERR(fid)) {
> -			if (old_fid) {
> -				/*
> -				 * If we fail, clunk fid which are 
mapping
> -				 * to path component and not the last 
component
> -				 * of the path.
> -				 */
> -				p9_client_clunk(old_fid);
> -			}
>  			kfree(wnames);
>  			goto err_out;
>  		}

So this is the actual fix mentioned in the commit log. Makes sense.

Nitpicking: Wouldn't it be a bit cleaner to set old_fid solely within the 
while loop and just before overwriting fid? And as we now have bumped to
-std=C11, probably making old_fid a local variable within loop scope only?

Best regards,
Christian Schoenebeck