nfs: Do not allow multiple mounts on same mountpoint when using -o noac

Do not allow multiple mounts on same mountpoint when using -o noac

When you normally attempt to mount a share twice on the same mountpoint,
a check in do_add_mount causes it to return an error

# mount localhost:/nfsv3 /mnt
# mount localhost:/nfsv3 /mnt
mount.nfs: /mnt is already mounted or busy

However when using the option 'noac', the user is able to mount the same
share on the same mountpoint multiple times. This happens because a
share mounted with the noac option is automatically assigned the 'sync'
flag MS_SYNCHRONOUS in nfs_initialise_sb(). This flag is set after the
check for already existing superblocks is done in sget(). The check for
the mount flags in nfs_compare_mount_options() does not take into
account the 'sync' flag applied later on in the code path. This means
that when using 'noac', a new superblock structure is assigned for every
new mount of the same share and multiple shares on the same mountpoint
are allowed.

ie.
# mount -onoac localhost:/nfsv3 /mnt
can be run multiple times.

The patch checks for noac and assigns the sync flag before sget() is
called to obtain an already existing superblock structure.
Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/super.c

@@ -2035,9 +2035,6 @@ static inline void nfs_initialise_sb(struct super_block *sb)
 		sb->s_blocksize = nfs_block_bits(server->wsize,
 						 &sb->s_blocksize_bits);
 
-	if (server->flags & NFS_MOUNT_NOAC)
-		sb->s_flags |= MS_SYNCHRONOUS;
-
 	sb->s_bdi = &server->backing_dev_info;
 
 	nfs_super_set_maxbytes(sb, server->maxfilesize);
@@ -2249,6 +2246,10 @@ static struct dentry *nfs_fs_mount(struct file_system_type *fs_type,
 	if (server->flags & NFS_MOUNT_UNSHARED)
 		compare_super = NULL;
 
+	/* -o noac implies -o sync */
+	if (server->flags & NFS_MOUNT_NOAC)
+		sb_mntdata.mntflags |= MS_SYNCHRONOUS;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
 	s = sget(fs_type, compare_super, nfs_set_super, &sb_mntdata);
 	if (IS_ERR(s)) {
@@ -2361,6 +2362,10 @@ nfs_xdev_mount(struct file_system_type *fs_type, int flags,
 	if (server->flags & NFS_MOUNT_UNSHARED)
 		compare_super = NULL;
 
+	/* -o noac implies -o sync */
+	if (server->flags & NFS_MOUNT_NOAC)
+		sb_mntdata.mntflags |= MS_SYNCHRONOUS;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
 	s = sget(&nfs_fs_type, compare_super, nfs_set_super, &sb_mntdata);
 	if (IS_ERR(s)) {
@@ -2628,6 +2633,10 @@ nfs4_remote_mount(struct file_system_type *fs_type, int flags,
 	if (server->flags & NFS4_MOUNT_UNSHARED)
 		compare_super = NULL;
 
+	/* -o noac implies -o sync */
+	if (server->flags & NFS_MOUNT_NOAC)
+		sb_mntdata.mntflags |= MS_SYNCHRONOUS;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
 	s = sget(&nfs4_fs_type, compare_super, nfs_set_super, &sb_mntdata);
 	if (IS_ERR(s)) {
@@ -2916,6 +2925,10 @@ nfs4_xdev_mount(struct file_system_type *fs_type, int flags,
 	if (server->flags & NFS4_MOUNT_UNSHARED)
 		compare_super = NULL;
 
+	/* -o noac implies -o sync */
+	if (server->flags & NFS_MOUNT_NOAC)
+		sb_mntdata.mntflags |= MS_SYNCHRONOUS;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
 	s = sget(&nfs4_fs_type, compare_super, nfs_set_super, &sb_mntdata);
 	if (IS_ERR(s)) {
@@ -3003,6 +3016,10 @@ nfs4_remote_referral_mount(struct file_system_type *fs_type, int flags,
 	if (server->flags & NFS4_MOUNT_UNSHARED)
 		compare_super = NULL;
 
+	/* -o noac implies -o sync */
+	if (server->flags & NFS_MOUNT_NOAC)
+		sb_mntdata.mntflags |= MS_SYNCHRONOUS;
+
 	/* Get a superblock - note that we may end up sharing one that already exists */
 	s = sget(&nfs4_fs_type, compare_super, nfs_set_super, &sb_mntdata);
 	if (IS_ERR(s)) {