[J/L][PATCH v2 0/2] loop: fix regression from max_loop default value change

[Mauricio Faria de Oliveira]

BugLink: https://bugs.launchpad.net/bugs/2015400

[ Changes ]

 * v2: Jammy: use fix as in Lunar, instead of a revert.

[ Impact ]

 * Regression in the loop block driver in Jammy kernel
   between 5.15.0-67 to 5.15.0-68 (in v5.15.86 stable),
   due to a change in the default behavior (value) of
   kernel parameter `max_loop` (from 0 to 8) in commit:
   `loop: Fix the max_loop commandline argument treatment
   when it is set to 0` (comment #6).

 * Users of loop devices (major 7) with minor >= 8 now
   fail to `open()` a loop device created with `mknod()`.

 * This is a corner case, as most people use `losetup`
   with usual /dev/loopNUMBER (or `--find`) which are
   not affected as it uses a different code path.
   (`losetup` for `/dev/loopNOT-A-NUMBER` is affected.)

 * Workaround: kernel parameter `max_loop=0`.

[ Test Steps ]

 * Run the test cases (losetup and test-loop.c in comment #6):
   - max_loop not set (default)
   - max_loop=0
   - max_loop=8

 * Verify the default behavior (max_loop not set) is restored.

 * Verify the modified behavior (max_loop is set) is unchanged.

 * Patches verified with jammy/lunar-proposed.
 
[ Regression Potential ]

 * Regressions would be limited to the loop block driver,
   more specifically its default behavior (no parameters)
   or specific usage of max_loop parameter (tested; looks OK).

[ Other Info ]

 * Patch 1 [1] is not quite a fix, but adds CONFIG guards that
   Patch 2 [2] depends on. (Alternatively, a Patch 2 backport
   with that could be done, but Patch 1 seems trivial enough.)

 * The fix on Jammy is only Patch 2, with a trivial backport
   (that CONFIG option is not in Jammy/v5.15, only in v5.18).

 * The fix on Lunar is both patches; clean cherry-picks.

 * The fix on Mantic is both patches too,
   now in v6.5-rc3, which should be automatically incorporated
   as Mantic apparently will release with the 6.5 kernel [3]

   [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=23881aec85f3219e8462e87c708815ee2cd82358
   [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=bb5faa99f0ce40756ab7bbbce4f16c01ca5ebd5a
   [3] https://discourse.ubuntu.com/t/introducing-kernel-6-5-for-the-23-10-mantic-minotaur-release

Mauricio Faria de Oliveira (2):
  loop: deprecate autoloading callback loop_probe()
  loop: do not enforce max_loop hard limit by (new) default

 drivers/block/loop.c | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)

-- 
2.39.2