A share can be mounted multiple times on the same mountpoint when using the option 'noac'

[Sachin Prabhu <sprabhu@redhat.com>]

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.

Could the existing behavior be considered to be a bug? If yes, one possible fix for this issue is to move the 'sync' flag assignment before the sget() is called to obtain an already existing superblock structure.

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>

diff -up linux-2.6/fs/nfs/super.c.noac_multiple_mount linux-2.6/fs/nfs/super.c
--- linux-2.6/fs/nfs/super.c.noac_multiple_mount	2010-04-22 17:08:25.000000000 +0100
+++ linux-2.6/fs/nfs/super.c	2010-04-22 17:27:48.000000000 +0100
@@ -1992,9 +1992,6 @@ static inline void nfs_initialise_sb(str
 		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);
@@ -2202,6 +2199,9 @@ static int nfs_get_sb(struct file_system
 	if (server->flags & NFS_MOUNT_UNSHARED)
 		compare_super = NULL;
 
+	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)) {
@@ -2317,6 +2317,9 @@ static int nfs_xdev_get_sb(struct file_s
 	if (server->flags & NFS_MOUNT_UNSHARED)
 		compare_super = NULL;
 
+	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)) {
@@ -2572,6 +2575,9 @@ static int nfs4_remote_get_sb(struct fil
 	if (server->flags & NFS4_MOUNT_UNSHARED)
 		compare_super = NULL;
 
+	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)) {
@@ -2808,6 +2814,9 @@ static int nfs4_xdev_get_sb(struct file_
 	if (server->flags & NFS4_MOUNT_UNSHARED)
 		compare_super = NULL;
 
+	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)) {
@@ -2893,6 +2902,9 @@ static int nfs4_remote_referral_get_sb(s
 	if (server->flags & NFS4_MOUNT_UNSHARED)
 		compare_super = NULL;
 
+	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)) {