Re: [RFC] FUSE permission modell (Was: fuse review bits)

["Bodo Eggert <harvested.in.lkml@posting.7eggert.dyndns.org>" <7eggert@gmx.de>]

Jamie Lokier <jamie@shareable.org> wrote:
> Miklos Szeredi wrote:

>>   4) Access should not be further restricted for the owner of the
>>      mount, even if permission bits, uid or gid would suggest
>>      otherwise
> 
> Why?  Surely you want to prevent writing to files which don't have the
> writable bit set?  A filesystem may also create append-only files -
> and all users including the mount owner should be bound by that.

That will depend on the situation. If the user is mounting a tgz owned
by himself, FUSE should default to being a convenient hex-editor.

>>   5) As much of the available information should be exported via the
>>      filesystem as possible
> 
> This is the root of the conflict.  You are trying to overload the
> permission bits and uid/gid to mean something different than they
> normally do.
> 
> While it's convenient to see some "remote" information such as the
> uid/gid in a tar file, are you sure it's a good idea to break the unix
> permissions model - which will break some programs?  (For example, try
> editing a file with the broken semantics in an editor which checks the
> uid/gid of the file against the current user).

The editor will try to keep the original permissions, and saving will be
less effective.

>>   1) Only allow mount over a directory for which the user has write
>>      access (and is not sticky)
> 
> Seems good - but why not sticky?  Mounting a user filesystem in
> /tmp/user-xxx/my-mount-point seems not unreasonable - provided the
> administrator can delete the directory (which is possible with
> detachable mount points).

I once mounted a filesystem in ~/tmp after forgetting about it being a
symlink to /tmp/$me/tmp, and I had to promise never to do that again.
Ng zvqavtug, gur pyrnahc-grzc-fpevcg xvpxrq va.

>>   5) The filesystem daemon is free to fill in all file attributes to
>>      any (sane) value, and the kernel won't modify these.
> 
> Dangerous, because an administrative program might actually trust the
> attributes to mean what they normally mean in the unix permissions model.

The same risk applies to smbmounted file systems.

Sane daemons will do no check besides matching the owner of a file in the
user's home against the expected UID and checking the permission mask,
since you can't trust users not to mess with files in directories they own.
The "best" they can do should be shoothing their own feet.

(If the user doesn't own the directory, FUSE shouldn't mount.)
-- 
Top 100 things you don't want the sysadmin to say:
80. I cleaned up the root partition and now there's LOTS of free space.

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