linux-kernel - Re: request for patches: showing mount options

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Message-Id: <E1IG84S-00064z-00@dorka.pomaz.szeredi.hu>
Date:	Wed, 01 Aug 2007 08:52:36 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	kzak@...hat.com
CC:	chuck.lever@...cle.com, miklos@...redi.hu,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	sfrench@...ba.org, trond.myklebust@....uio.no,
	mark.fasheh@...cle.com, kurt.hackel@...cle.com
Subject: Re: request for patches: showing mount options

> > The removal of /etc/mtab in favor of /proc/mounts is a new requirement, 
> > and is not as trivial as you might hope.  Internally the NFS client 
> > represents the mount options as a binary data structure, and it contains 
> > only the information that has traditionally been passed into the kernel 
> > by the current mount command.  The user-space-only options are not 
> > passed to the kernel nor stored in the data structure.
> > 
> > Adding facilities to store information about every possible mount 
> > option, including the user-space-only ones, will take a bit of time, but 
> > is possible, if not straightforward.  We just have to understand all the 
> > dependencies.
> 
>  I still have doubts. The removal of /etc/mtab is nice, but a little
>  unreal wish. Do we really want to store non-kernel data (options) in
>  kernel? What about options that are not closely related to any
>  filesystem -- for example "loop="?

"loop=DEV" is special, we've already talked about that.

For example if we have an mtab entry like this:

  /dir/foo.iso /mnt/image ext3 rw,loop=/dev/loop/0 0 0

It says: /dev/loop/0 has been automatically set up by mount(8) from
/dir/foo.iso and so needs to be torn down by umount(8).  It is not a
property of the mount, rather the property of the device.

So what we really want is a file to store this, and only this info:

/var/lib/mount/loop:

/dev/loop/0 /dir/foo.iso
/dev/loop/2 /dir/bar.img
...

Notice, there's nothing about the _mountpoint_ in this file.

>  Maybe we can replace /etc/mtab with something more useful
>  (e.g. /var/run/mount/<mntid>.tab) for really user-space-only information.

The problem with storing mount options in userspace is that
unprivileged mounts/umounts won't work in the presense of those, we
cannot _force_ an unprivileged user to update a file.

So we want to avoid storing options in userspace as long as possible.

Miklos
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