lockd: don't allow locking on reexported NFSv2/3

Since commit 9254c8ae9b81 ("nfsd: disallow file locking and delegations
for NFSv4 reexport"), file locking when reexporting an NFS mount via
NFSv4 is expressly prohibited by nfsd. Do the same in lockd:

Add a new nlmsvc_file_cannot_lock() helper that will test whether file
locking is allowed for a given file, and return nlm_lck_denied_nolocks
if it isn't.

Signed-off-by: Jeff Layton <[e-mail]>
Tested-by: Olga Kornievskaia <[e-mail]>
Signed-off-by: Chuck Lever <[e-mail]>

fs/lockd/svclock.c

···
 				(long long)lock->fl.fl_end,
 				wait);
 
+	if (nlmsvc_file_cannot_lock(file))
+		return nlm_lck_denied_nolocks;
+
 	if (!locks_can_async_lock(nlmsvc_file_file(file)->f_op)) {
 		async_block = wait;
 		wait = 0;
···
 				(long long)lock->fl.fl_start,
 				(long long)lock->fl.fl_end);
 
+	if (nlmsvc_file_cannot_lock(file))
+		return nlm_lck_denied_nolocks;
+
 	if (locks_in_grace(SVC_NET(rqstp))) {
 		ret = nlm_lck_denied_grace_period;
 		goto out;
···
 				(long long)lock->fl.fl_start,
 				(long long)lock->fl.fl_end);
 
+	if (nlmsvc_file_cannot_lock(file))
+		return nlm_lck_denied_nolocks;
+
 	/* First, cancel any lock that might be there */
 	nlmsvc_cancel_blocked(net, file, lock);
 
···
 				lock->fl.c.flc_pid,
 				(long long)lock->fl.fl_start,
 				(long long)lock->fl.fl_end);
+
+	if (nlmsvc_file_cannot_lock(file))
+		return nlm_lck_denied_nolocks;
 
 	if (locks_in_grace(net))
 		return nlm_lck_denied_grace_period;

fs/lockd/svcshare.c

···
 	struct xdr_netobj	*oh = &argp->lock.oh;
 	u8			*ohdata;
 
+	if (nlmsvc_file_cannot_lock(file))
+		return nlm_lck_denied_nolocks;
+
 	for (share = file->f_shares; share; share = share->s_next) {
 		if (share->s_host == host && nlm_cmp_owner(share, oh))
 			goto update;
···
 {
 	struct nlm_share	*share, **shpp;
 	struct xdr_netobj	*oh = &argp->lock.oh;
+
+	if (nlmsvc_file_cannot_lock(file))
+		return nlm_lck_denied_nolocks;
 
 	for (shpp = &file->f_shares; (share = *shpp) != NULL;
 					shpp = &share->s_next) {

include/linux/lockd/lockd.h

···
 
 /* XXX: a lot of this should really be under fs/lockd. */
 
+#include <linux/exportfs.h>
 #include <linux/in.h>
 #include <linux/in6.h>
 #include <net/ipv6.h>
···
 int           nlmsvc_unlock_all_by_sb(struct super_block *sb);
 int           nlmsvc_unlock_all_by_ip(struct sockaddr *server_addr);
 
-static inline struct file *nlmsvc_file_file(struct nlm_file *file)
+static inline struct file *nlmsvc_file_file(const struct nlm_file *file)
 {
 	return file->f_file[O_RDONLY] ?
 	       file->f_file[O_RDONLY] : file->f_file[O_WRONLY];
···
 static inline struct inode *nlmsvc_file_inode(struct nlm_file *file)
 {
 	return file_inode(nlmsvc_file_file(file));
+}
+
+static inline bool
+nlmsvc_file_cannot_lock(const struct nlm_file *file)
+{
+	return exportfs_cannot_lock(nlmsvc_file_file(file)->f_path.dentry->d_sb->s_export_op);
 }
 
 static inline int __nlm_privileged_request4(const struct sockaddr *sap)