nfs: clear SB_RDONLY before getting superblock

As described in the link, commit 52cb7f8 ("nfs: ignore SB_RDONLY when
mounting nfs") removed the check for the ro flag when determining whether
to share the superblock, which caused issues when mounting different
subdirectories under the same export directory via NFSv3. However, this
change did not affect NFSv4.

For NFSv3:
1) A single superblock is created for the initial mount.
2) When mounted read-only, this superblock carries the SB_RDONLY flag.
3) Before commit 52cb7f8 ("nfs: ignore SB_RDONLY when mounting nfs"):
Subsequent rw mounts would not share the existing ro superblock due to
flag mismatch, creating a new superblock without SB_RDONLY.
After the commit:
  The SB_RDONLY flag is ignored during superblock comparison, and this leads
  to sharing the existing superblock even for rw mounts.
  Ultimately results in write operations being rejected at the VFS layer.

For NFSv4:
1) Multiple superblocks are created and the last one will be kept.
2) The actually used superblock for ro mounts doesn't carry SB_RDONLY flag.
Therefore, commit 52cb7f8 doesn't affect NFSv4 mounts.

Clear SB_RDONLY before getting superblock when NFS_MOUNT_UNSHARED is not
set to fix it.

Fixes: 52cb7f8 ("nfs: ignore SB_RDONLY when mounting nfs")
Closes: https://lore.kernel.org/all/[email protected]/T/
Signed-off-by: Li Lingfeng <[email protected]>
Signed-off-by: Anna Schumaker <[email protected]>

Author: Li Lingfeng

fs/nfs/super.c

@@ -1308,8 +1308,17 @@ int nfs_get_tree_common(struct fs_context *fc)
 	if (IS_ERR(server))
 		return PTR_ERR(server);
 
+	/*
+	 * When NFS_MOUNT_UNSHARED is not set, NFS forces the sharing of a
+	 * superblock among each filesystem that mounts sub-directories
+	 * belonging to a single exported root path.
+	 * To prevent interference between different filesystems, the
+	 * SB_RDONLY flag should be removed from the superblock.
+	 */
 	if (server->flags & NFS_MOUNT_UNSHARED)
 		compare_super = NULL;
+	else
+		fc->sb_flags &= ~SB_RDONLY;
 
 	/* -o noac implies -o sync */
 	if (server->flags & NFS_MOUNT_NOAC)