NFS: implement option checking when remounting NFS filesystems (resend)

When remounting an NFS or NFS4 filesystem, the new NFS options are not
respected, yet the remount will still return success. This patch adds
a remount_fs sb op for NFS that checks any new nfs mount options against
the existing ones and fails the mount if any have changed.

This is only implemented for string-based mount options since doing
this with binary options isn't really feasible.

This is essentially the same as the original patch I sent out, but
adds a check to see if the addr= option has changed.
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/super.c

@@ -207,6 +207,7 @@ static int nfs_xdev_get_sb(struct file_system_type *fs_type,
 		int flags, const char *dev_name, void *raw_data, struct vfsmount *mnt);
 static void nfs_kill_super(struct super_block *);
 static void nfs_put_super(struct super_block *);
+static int nfs_remount(struct super_block *sb, int *flags, char *raw_data);
 
 static struct file_system_type nfs_fs_type = {
 	.owner		= THIS_MODULE,
@@ -234,6 +235,7 @@ static const struct super_operations nfs_sops = {
 	.umount_begin	= nfs_umount_begin,
 	.show_options	= nfs_show_options,
 	.show_stats	= nfs_show_stats,
+	.remount_fs	= nfs_remount,
 };
 
 #ifdef CONFIG_NFS_V4
@@ -278,6 +280,7 @@ static const struct super_operations nfs4_sops = {
 	.umount_begin	= nfs_umount_begin,
 	.show_options	= nfs_show_options,
 	.show_stats	= nfs_show_stats,
+	.remount_fs	= nfs_remount,
 };
 #endif
 
@@ -1396,6 +1399,79 @@ out_invalid_fh:
 	return -EINVAL;
 }
 
+static int
+nfs_compare_remount_data(struct nfs_server *nfss,
+			 struct nfs_parsed_mount_data *data)
+{
+	if (data->flags != nfss->flags ||
+	    data->rsize != nfss->rsize ||
+	    data->wsize != nfss->wsize ||
+	    data->retrans != nfss->client->cl_timeout->to_retries ||
+	    data->auth_flavors[0] != nfss->client->cl_auth->au_flavor ||
+	    data->acregmin != nfss->acregmin / HZ ||
+	    data->acregmax != nfss->acregmax / HZ ||
+	    data->acdirmin != nfss->acdirmin / HZ ||
+	    data->acdirmax != nfss->acdirmax / HZ ||
+	    data->timeo != (10U * nfss->client->cl_timeout->to_initval / HZ) ||
+	    data->nfs_server.addrlen != nfss->nfs_client->cl_addrlen ||
+	    memcmp(&data->nfs_server.address, &nfss->nfs_client->cl_addr,
+		   data->nfs_server.addrlen) != 0)
+		return -EINVAL;
+
+	return 0;
+}
+
+static int
+nfs_remount(struct super_block *sb, int *flags, char *raw_data)
+{
+	int error;
+	struct nfs_server *nfss = sb->s_fs_info;
+	struct nfs_parsed_mount_data *data;
+	struct nfs_mount_data *options = (struct nfs_mount_data *)raw_data;
+	struct nfs4_mount_data *options4 = (struct nfs4_mount_data *)raw_data;
+
+	/*
+	 * Userspace mount programs that send binary options generally send
+	 * them populated with default values. We have no way to know which
+	 * ones were explicitly specified. Fall back to legacy behavior and
+	 * just return success.
+	 */
+	if ((sb->s_type == &nfs4_fs_type && options4->version == 1) ||
+	    (sb->s_type == &nfs_fs_type && options->version >= 1 &&
+	     options->version <= 6))
+		return 0;
+
+	data = kzalloc(sizeof(*data), GFP_KERNEL);
+	if (data == NULL)
+		return -ENOMEM;
+
+	/* fill out struct with values from existing mount */
+	data->flags = nfss->flags;
+	data->rsize = nfss->rsize;
+	data->wsize = nfss->wsize;
+	data->retrans = nfss->client->cl_timeout->to_retries;
+	data->auth_flavors[0] = nfss->client->cl_auth->au_flavor;
+	data->acregmin = nfss->acregmin / HZ;
+	data->acregmax = nfss->acregmax / HZ;
+	data->acdirmin = nfss->acdirmin / HZ;
+	data->acdirmax = nfss->acdirmax / HZ;
+	data->timeo = 10U * nfss->client->cl_timeout->to_initval / HZ;
+	data->nfs_server.addrlen = nfss->nfs_client->cl_addrlen;
+	memcpy(&data->nfs_server.address, &nfss->nfs_client->cl_addr,
+		data->nfs_server.addrlen);
+
+	/* overwrite those values with any that were specified */
+	error = nfs_parse_mount_options((char *)options, data);
+	if (error < 0)
+		goto out;
+
+	/* compare new mount options with old ones */
+	error = nfs_compare_remount_data(nfss, data);
+out:
+	kfree(data);
+	return error;
+}
+
 /*
  * Initialise the common bits of the superblock
  */