nfsd: disallow file locking and delegations for NFSv4 reexport

We do not and cannot support file locking with NFS reexport over
NFSv4.x for the same reason we don't do it for NFSv3: NFS reexport
server reboot cannot allow clients to recover locks because the source
NFS server has not rebooted, and so it is not in grace. Since the
source NFS server is not in grace, it cannot offer any guarantees that
the file won't have been changed between the locks getting lost and
any attempt to recover/reclaim them. The same applies to delegations
and any associated locks, so disallow them too.

Clients are no longer allowed to get file locks or delegations from a
reexport server, any attempts will fail with operation not supported.

Update the "Reboot recovery" section accordingly in
Documentation/filesystems/nfs/reexport.rst

Signed-off-by: Mike Snitzer <[e-mail]>
Reviewed-by: Jeff Layton <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

Documentation/filesystems/nfs/reexport.rst

···
 ---------------
 
 The NFS protocol's normal reboot recovery mechanisms don't work for the
-case when the reexport server reboots.  Clients will lose any locks
-they held before the reboot, and further IO will result in errors.
-Closing and reopening files should clear the errors.
+case when the reexport server reboots because the source server has not
+rebooted, and so it is not in grace.  Since the source server is not in
+grace, it cannot offer any guarantees that the file won't have been
+changed between the locks getting lost and any attempt to recover them.
+The same applies to delegations and any associated locks.  Clients are
+not allowed to get file locks or delegations from a reexport server, any
+attempts will fail with operation not supported.
 
 Filehandle limits
 -----------------

fs/nfs/export.c

···
 		 EXPORT_OP_CLOSE_BEFORE_UNLINK	|
 		 EXPORT_OP_REMOTE_FS		|
 		 EXPORT_OP_NOATOMIC_ATTR	|
-		 EXPORT_OP_FLUSH_ON_CLOSE,
+		 EXPORT_OP_FLUSH_ON_CLOSE	|
+		 EXPORT_OP_NOLOCKS,
 };

fs/nfsd/nfs4state.c

···
 	if (!nf)
 		return ERR_PTR(-EAGAIN);
 
+	/*
+	 * File delegations and associated locks cannot be recovered if the
+	 * export is from an NFS proxy server.
+	 */
+	if (exportfs_cannot_lock(nf->nf_file->f_path.mnt->mnt_sb->s_export_op)) {
+		nfsd_file_put(nf);
+		return ERR_PTR(-EOPNOTSUPP);
+	}
+
 	spin_lock(&state_lock);
 	spin_lock(&fp->fi_lock);
 	if (nfs4_delegation_exists(clp, fp))
···
 	status = fh_verify(rqstp, &cstate->current_fh, S_IFREG, 0);
 	if (status != nfs_ok)
 		return status;
+	if (exportfs_cannot_lock(cstate->current_fh.fh_dentry->d_sb->s_export_op)) {
+		status = nfserr_notsupp;
+		goto out;
+	}
 
 	if (lock->lk_is_new) {
 		if (nfsd4_has_session(cstate))
···
 		status = nfserr_lock_range;
 		goto put_stateid;
 	}
+	if (exportfs_cannot_lock(nf->nf_file->f_path.mnt->mnt_sb->s_export_op)) {
+		status = nfserr_notsupp;
+		goto put_file;
+	}
+
 	file_lock = locks_alloc_lock();
 	if (!file_lock) {
 		dprintk("NFSD: %s: unable to allocate lock!\n", __func__);

include/linux/exportfs.h

···
 						  atomic attribute updates
 						*/
 #define EXPORT_OP_FLUSH_ON_CLOSE	(0x20) /* fs flushes file data on close */
-#define EXPORT_OP_ASYNC_LOCK		(0x40) /* fs can do async lock request */
+#define EXPORT_OP_NOLOCKS		(0x40) /* no file locking support */
 	unsigned long	flags;
 };
+
+/**
+ * exportfs_cannot_lock() - check if export implements file locking
+ * @export_ops:	the nfs export operations to check
+ *
+ * Returns true if the export does not support file locking.
+ */
+static inline bool
+exportfs_cannot_lock(const struct export_operations *export_ops)
+{
+	return export_ops->flags & EXPORT_OP_NOLOCKS;
+}
 
 extern int exportfs_encode_inode_fh(struct inode *inode, struct fid *fid,
 				    int *max_len, struct inode *parent,