NFS: Automounted filesystems should inherit ro,noexec,nodev,sync flags

When a filesystem is being automounted, it needs to preserve the
user-set superblock mount options, such as the "ro" flag.

Reported-by: Li Lingfeng <lilingfeng3@huawei.com>
Link: https://lore.kernel.org/all/20240604112636.236517-3-lilingfeng@huaweicloud.com/
Fixes: f2aedb7 ("NFS: Add fs_context support.")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

Author: Trond Myklebust

fs/nfs/namespace.c

@@ -149,6 +149,7 @@ struct vfsmount *nfs_d_automount(struct path *path)
 	struct vfsmount *mnt = ERR_PTR(-ENOMEM);
 	struct nfs_server *server = NFS_SB(path->dentry->d_sb);
 	struct nfs_client *client = server->nfs_client;
+	unsigned long s_flags = path->dentry->d_sb->s_flags;
 	int timeout = READ_ONCE(nfs_mountpoint_expiry_timeout);
 	int ret;
 
@@ -174,6 +175,11 @@ struct vfsmount *nfs_d_automount(struct path *path)
 		fc->net_ns = get_net(client->cl_net);
 	}
 
+	/* Inherit the flags covered by NFS_SB_MASK */
+	fc->sb_flags_mask |= NFS_SB_MASK;
+	fc->sb_flags &= ~NFS_SB_MASK;
+	fc->sb_flags |= s_flags & NFS_SB_MASK;
+
 	/* for submounts we want the same server; referrals will reassign */
 	memcpy(&ctx->nfs_server._address, &client->cl_addr, client->cl_addrlen);
 	ctx->nfs_server.addrlen	= client->cl_addrlen;

fs/nfs/super.c

@@ -1315,10 +1315,6 @@ int nfs_get_tree_common(struct fs_context *fc)
 	if (server->flags & NFS_MOUNT_NOAC)
 		fc->sb_flags |= SB_SYNCHRONOUS;
 
-	if (ctx->clone_data.sb)
-		if (ctx->clone_data.sb->s_flags & SB_SYNCHRONOUS)
-			fc->sb_flags |= SB_SYNCHRONOUS;
-
 	/* Get a superblock - note that we may end up sharing one that already exists */
 	fc->s_fs_info = server;
 	s = sget_fc(fc, compare_super, nfs_set_super);