[RFC] refactor mnt_fstype_is_pseudofs()?

[Stanislav Brabec <sbrabec@suse.cz>]

Hi.

I got a but report about a dumpfs FUSE filesystem reporting bad mount output.

The same problem affects other pseudo file systems not listed in mnt_fstype_is_pseudofs():
# mount | grep encfs
encfs on /Encrypted type fuse.encfs (rw,nosuid,nodev,relatime,user_id=10027,group_id=100,default_permissions)
# mkdir encfs
# mount | grep encfs
/root/encfs on /Encrypted type fuse.encfs (rw,nosuid,nodev,relatime,user_id=10027,group_id=100,default_permissions)
^^^^^^^^^^^

Debugging shows two reasons why it happens:
1) encfs is not listed in mnt_fstype_is_pseudofs()
2) mnt_resolve_path() calls canonicalize_path_and_cache() on the kernel output, which makes only a little sense

Currently I am aware of three file systems affected at least (i. e. not listed): gvfs (subtype was renamed from gvfs-fuse-daemon to gvfsd-fuse), encfs, dumpfs.

Easy fix would be adding all of them to the list. But probably much more exists.

More complex would allow to add fuse.*. But it is not correct, because FUSE can be a regular file system based on block device (e. g. fuse ntfs). Yes, such reports "fuseblk" as a default. But still, it can be changed.

That is why I started thinking about the better fix than adding these three to the list:

If the column 10 (mount source) of a particular line of /proc/<pid>/mountinfo does not start by "/", then it is a pseudofs. Actually, it could be generalized:
- If it does not contain "/" (or maybe another character) at all, it is a pseudofs.
- If it contains "/" (or maybe another character), but it does not start by it, it is a network file system.
- If it starts by "/", it is a regular file system.

Before implementing, I want to make a wider discussion, whether it could work.

I talked with a kernel developers, and he told that there is no solution "correct by principle". Filesystem developer can provide virtually anything as a source. But it could work in most cases.

What do you think about such change?

And additionally: Does canonicalization of the kernel output make any sense? If not, it could be skipped for commands like a bare "mount". (But I am not sure how complicated it would be.)

-- 
Best Regards / S pozdravem,

Stanislav Brabec
software developer
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