Re: [RFC][PATCH] nfsd: vfs_mkdir() might succeed leaving dentry negative unhashed

["J. Bruce Fields" <bfields@fieldses.org>]

On Fri, May 11, 2018 at 10:13:39PM +0100, Al Viro wrote:
> [-stable fodder; as it is, one can e.g. add
> /mnt/cgroup     localhost(rw,no_root_squash,fsid=4242)
> to /etc/exports,
> mount -t cgroup none /mnt/cgroup
> mkdir /tmp/a
> mount -t nfs localhost://mnt/cgroup /tmp/a
> mkdir /tmp/a/foo

How is the cgroup filesystem exportable?  That sounds like a bug in
itself.  We don't want people using NFS as some kind of weird remote
configuration protocol.

> and have knfsd oops; the patch below deals with that.
> 
> Questions:
> 	1) is fh_update() use below legitimate, or should we
> do fh_put/fh_compose instead?

fh_update looks OK to me, but do we need it here?  There's already a

	if (!err)
		err = fh_update(reshp);

at the end of nfsd_create_locked.

> 	2) is nfserr_serverfail valid for situation when
> directory created by such vfs_mkdir() manages to disappear
> under us immediately afterwards?  Or should we return nfserr_stale
> instead?

We just got a successful result on the create and the parent's still
locked, so if somebody hits this I think we want them reporting a bug,
not wasting time trying to find a mistake in their own logic.

> It's in vfs.git#for-linus, if you prefer to use git for review...
> ]
> 
> That can (and does, on some filesystems) happen - ->mkdir() (and thus
> vfs_mkdir()) can legitimately leave its argument negative and just
> unhash it, counting upon the lookup to pick the object we'd created
> next time we try to look at that name.
> 
> Some vfs_mkdir() callers forget about that possibility...

I'd rather have this in a helper function with a comment or two, but I
can do that as a followup patch.

--b.

> 
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
> diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
> index 2410b093a2e6..b0555d7d8200 100644
> --- a/fs/nfsd/vfs.c
> +++ b/fs/nfsd/vfs.c
> @@ -1201,6 +1201,28 @@ nfsd_create_locked(struct svc_rqst *rqstp, struct svc_fh *fhp,
>  		break;
>  	case S_IFDIR:
>  		host_err = vfs_mkdir(dirp, dchild, iap->ia_mode);
> +		if (!host_err && unlikely(d_unhashed(dchild))) {
> +			struct dentry *d;
> +			d = lookup_one_len(dchild->d_name.name,
> +					   dchild->d_parent,
> +					   dchild->d_name.len);
> +			if (IS_ERR(d)) {
> +				host_err = PTR_ERR(d);
> +				break;
> +			}
> +			if (unlikely(d_is_negative(d))) {
> +				dput(d);
> +				err = nfserr_serverfault;
> +				goto out;
> +			}
> +			dput(resfhp->fh_dentry);
> +			resfhp->fh_dentry = dget(d);
> +			err = fh_update(resfhp);
> +			dput(dchild);
> +			dchild = d;
> +			if (err)
> +				goto out;
> +		}
>  		break;
>  	case S_IFCHR:
>  	case S_IFBLK: