[Bug 1640823] Re: [trusty] mount -o loop is limited to 8 loop devices

[Martin Pitt]

I successfully ran the test case against ubuntu20.9 in -proposed. I also
verified that using losetup and mount on loop devices still works:

$ sudo losetup -f --show debian-stretch-DI-alpha7-amd64-netinst.iso 
/dev/loop0
$ sudo mount /dev/loop0 /mnt
mount: block device /dev/loop0 is write-protected, mounting read-only
$ ls /mnt
autorun.inf  debian [...]

With the -proposed version, losetup -f also succeeds to use /dev/loop-
control, so you can do the above a few times and it works with
/dev/loop12 too. When moving /dev/loop-control out of the way, the above
also works up to loop7, and then errors out with "losetup: could not
find any free loop device" as expected. "losetup -a" correctly shows the
bound devices/files.


** Tags removed: verification-failed
** Tags added: verification-needed

** Tags removed: verification-needed
** Tags added: verification-done

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https://bugs.launchpad.net/bugs/1640823

Title:
  [trusty] mount -o loop is limited to 8 loop devices

Status in util-linux package in Ubuntu:
  Fix Released
Status in util-linux source package in Trusty:
  Fix Committed

Bug description:
  trusty has a very old util-linux which does not yet know about /dev
  /loop-control to create arbitrarily many loop devices. This feature
  was introduced in Linux 3.1 already (i. e. before precise even). This
  is a showstopper for backporting snappy as that needs a lot of loop
  mounts.

  Support for loop-control got introduced later util-linux versions, but
  backporting full support for it (for losetup) is too intrusive. We
  only need a partial backport for "mount -o loop".

  SRU TEST CASE:
  First, use up all default 8 loop devices:
  $ for i in `seq 8`; do echo $i; sudo losetup --find /etc/issue; done

  Now try to use a 9th:
  $ dd if=/dev/zero of=/tmp/img bs=1M count=50
  $ mkfs.ext2 -F /tmp/img
  $ sudo mount -o loop /tmp/img /mnt

  With current trusty's "mount" package this will fail with "could not
  find any free loop device". With the proposed version, this should
  succeed, and "sudo losetup -a" should show "/dev/loop8: ...
  (/tmp/img)".

  Now, reboot, disable loop-control with

    sudo mv /dev/loop-control{,.disabled}

  and run the test case again. Now "mount -o loop" should fail with
  "could not find any free loop device" (as before). Ensure that there
  are no hangs, infinite loops, etc.

  ADDITIONAL REGRESSION CHECKING TEST CASES

  1. Check that every type of losetup call documented in the losetup
  manpage still works correctly.

  2. Check that mount and umount commands that use loop devices still
  work correctly.

  REGRESSION POTENTIAL: /dev/loop-control and the corresponding util-
  linux support has exited for a long time without known/major issues,
  so this should be fairly safe. Also, the patch falls back to the
  previous "iterate over loop0 to loop7" behaviour if loop-control is
  not available.

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