Arch Linux

Brief description:
A loopback filesystem with "user" option which has been mounted by a non-root
user cannot subsequently be unmounted by that user, only by root. Fails with
"operation not permitted".

Possibly related to:
 FS#27948 
 FS#28936 

Pertinent version info:
util-linux:
util-linux 2.21.2-5

Kernel:
Linux 3.5.3-1-ARCH #1 SMP PREEMPT Sun Aug 26 08:15:06 UTC 2012 \
i686 GNU/Linu

Other:
/etc/mtab is symlinked to /proc/self/mounts. Not sure if this is relevant
or not, just mentioning it. Some other reporters of similar-sounding bugs
have opined that it might be relevant.


How to reproduce:
Minimal example attached below. 100% reproducible.


When appeared:
Unfortunately I do not have a very good idea when this bug first appeared, but
suspect it is recent (last 6 months?). With certainty though, umount did
used to behave correctly and allow user unmounting of loopback filesystem
which had been mounted by the same user.

Commentary:
IMO, this is a serious issue because it prevents users from being able to
freely mount/umount filesystems without either root privs or assistance from
an admin (as root). On personal systems where the user is also the admin and has
root privs, it is merely annoying; but on systems where users have no root privs
and frequently do mount/umount cycles, it's painful, and eventually "uses up"
all the loop devices.

I realize that this is most likely an upstream issue, but reporting it here
first just for completeness. If you determine it is upstream, please let me
know, I will (re)file it upstream.



#========================= BEGIN MINIMAL EXAMPLE ===========================
#
# As root, create a loopback filesystem for testing:
#

$ cd /tmp
$ dd bs=4096 count=1000 if=/dev/zero of=test.lfs
$ losetup /dev/loop1 test.lfs
$ mke2fs /dev/loop1
$ losetup -d /dev/loop1


#
# View /etc/fstab entry for above:
#
$ grep test.lfs /etc/fstab
/tmp/test.lfs /mnt/test ext2 defaults,user 0 0


#
# As user 'foo', mount the filesystem and verify that the reported mount
# options include "user=foo"
#
$ id
uid=501(foo) gid=501(foo) groups=501(foo),19(log),91(video),92(audio)
$ mount /mnt/test
$ mount | grep test
/tmp/test.lfs on /mnt/test type ext2 \
(rw,nosuid,nodev,noexec,relatime,user=foo)
$ losetup -a
/dev/loop0: []: (/tmp/test.lfs)


#
# As user 'foo', attempt to umount the filesystem. Fails. Loop device is still
# reported as in-use, and filesystem still reported as mounted. (Attempting
# the umount as root succeeds normally.)
#
$ id
uid=501(foo) gid=501(foo) groups=501(foo),19(log),91(video),92(audio)
$ umount /mnt/test
$ umount: /mnt/test: umount failed: Operation not permitted
$ mount | grep test.lfs
/tmp/test.lfs on /mnt/test type ext2 \
(rw,nosuid,nodev,noexec,relatime,user=foo)
$ losetup -a
/dev/loop0: []: (/tmp/test.lfs)
#=========================== END MINIMAL EXAMPLE ===========================