fstab: allow regular user to unmount a loopback file?

rjlee · 01-17-2009, 10:01 AM

I can mount a loopback squashfs file image as a user, but cannot then unmount it again.

I have the following line in my /etc/fstab file:

Code:

/home/ubuntu/dos/cd.squashfs /home/ubuntu/dos/cd squashfs user,loop,ro 0 0

When I try and mount the directory (as a regular user), it mounts okay.

mount then reports:

Code:

/dev/loop0 on /home/ubuntu/dos/cd type squashfs (ro,noexec,nosuid,nodev,user=ubuntu)

The exact same line appears in /etc/mtab.

But when I try to unmount it as the same regular user, I get the following error:

Code:

umount: /home/ubuntu/dos/cd mount disagrees with the fstab

I then have to unmount the file using root permissions.

Having read the mount manpage, it suggests that the user option will fail if /etc/mtab is a symlink to /proc/mtab (which it isn't).

I have also tried with various fstab options and seen the same error. The options I've tried include:

Code:

user,owner,loop,ro,noexec,nosuid,nodev
users,owner,loop,ro,noexec,nosuid,nodev
users,loop,ro,noexec,nosuid,nodev

Is there any way to debug why I'm getting this error when unmounting?

Thanks in advance for any help,

— Robert J Lee

eggixyz · 01-17-2009, 08:15 PM

Hey There,

The problem may be because

Quote:

/home/ubuntu/dos/cd.squashfs

from the /etc/fstab gets translated into

Quote:

/dev/loop0

once it's mounted.

Have you tried using this line in /etc/fstab instead?:

Quote:

/dev/loop0 /home/ubuntu/dos/cd squashfs user,loop,ro 0 0

It may or may not work, but it's worth a shot

Best wishes,

Mike

rjlee · 01-18-2009, 05:52 PM

Hi Mike,

Thanks for that.

Your suggestion did actually work, in that you could then unmount the partition. But that meant that mounting the filesystem would just sit there, since it didn't know what file /dev/loop0 should point to.

Which made me realise how to fix it:

I put two lines in the fstab for it. My original line came first, and so it would be found by mount. Then I added a second copy of the line, with the file replaced with /dev/loop0, right after it, and unmount would find that and let me unmount it.

It doesn't seem very tidy, and I don't think I've ever seen that approach documented anywhere, but it does work.

Thanks again,

—Robert J Lee

eggixyz · 01-18-2009, 08:16 PM

Nice,

That is unusual. I've never seen it done that way, either.

If it's not a bug, it must be a feature

Nice work - Glad I could help you out

, Mike

rjlee · 01-21-2009, 02:48 PM

One further point, in case anyone else has the same problem: I needed to add the noauto option to one line to stop it from mounting twice.

Pixmix · 06-16-2015, 04:56 AM

Just a brief update if someone stomps here having the same problem with tagsistant:

I got the same problem and I solved it adding a dummy line in the /etc/fstab file conform to the naming reported by mount as suggested.

Quick and dirty, but works.