add filesystem subtype support

There's a slight problem with filesystem type representation in fuse
based filesystems.

From the kernel's view, there are just two filesystem types: fuse and
fuseblk.  From the user's view there are lots of different filesystem
types.  The user is not even much concerned if the filesystem is fuse based
or not.  So there's a conflict of interest in how this should be
represented in fstab, mtab and /proc/mounts.

The current scheme is to encode the real filesystem type in the mount
source.  So an sshfs mount looks like this:

  sshfs#user@server:/   /mnt/server    fuse   rw,nosuid,nodev,...

This url-ish syntax works OK for sshfs and similar filesystems.  However
for block device based filesystems (ntfs-3g, zfs) it doesn't work, since
the kernel expects the mount source to be a real device name.

A possibly better scheme would be to encode the real type in the type
field as "type.subtype".  So fuse mounts would look like this:

  /dev/hda1       /mnt/windows   fuseblk.ntfs-3g   rw,...
  user@server:/   /mnt/server    fuse.sshfs        rw,nosuid,nodev,...

This patch adds the necessary code to the kernel so that this can be
correctly displayed in /proc/mounts.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 27 insertions, 0 deletions

diff --git a/fs/super.c b/fs/super.c
index 8341e4e1d738..5260d620c555 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -107,6 +107,7 @@ out:
 static inline void destroy_super(struct super_block *s)
 {
         security_sb_free(s);
+        kfree(s->s_subtype);
         kfree(s);
 }
 
@@ -907,6 +908,29 @@ out:
 
 EXPORT_SYMBOL_GPL(vfs_kern_mount);
 
+static struct vfsmount *fs_set_subtype(struct vfsmount *mnt, const char *fstype)
+{
+        int err;
+        const char *subtype = strchr(fstype, '.');
+        if (subtype) {
+                subtype++;
+                err = -EINVAL;
+                if (!subtype[0])
+                        goto err;
+        } else
+                subtype = "";
+
+        mnt->mnt_sb->s_subtype = kstrdup(subtype, GFP_KERNEL);
+        err = -ENOMEM;
+        if (!mnt->mnt_sb->s_subtype)
+                goto err;
+        return mnt;
+
+ err:
+        mntput(mnt);
+        return ERR_PTR(err);
+}
+
 struct vfsmount *
 do_kern_mount(const char *fstype, int flags, const char *name, void *data)
 {
@@ -915,6 +939,9 @@ do_kern_mount(const char *fstype, int flags, const char *name, void *data)
         if (!type)
                 return ERR_PTR(-ENODEV);
         mnt = vfs_kern_mount(type, flags, name, data);
+        if (!IS_ERR(mnt) && (type->fs_flags & FS_HAS_SUBTYPE) &&
+            !mnt->mnt_sb->s_subtype)
+                mnt = fs_set_subtype(mnt, fstype);
         put_filesystem(type);
         return mnt;
 }