Testing on Lunar:
---
The default behavior before the regression is restored/fixed.
The modified behaviors are unchanged.
$ lsb_release -cs
No LSB modules are available.
lunar
Original:
$ uname -rv
6.2.0-27-generic #28-Ubuntu SMP PREEMPT_DYNAMIC Wed Jul 12 22:39:51 UTC
2023
$ cat /proc/cmdline
... root=... ro
$ sudo ./test-loop
open: /dev/loop8: No such device or address
...
$ cat /proc/cmdline
... root=... ro max_loop=0
$ sudo ./test-loop
$
...
$ cat /proc/cmdline
... root=... ro max_loop=8
$ sudo ./test-loop
open: /dev/loop8: No such device or address
Patched:
$ uname -rv
6.2.0-27-generic #28+lp2015400 SMP PREEMPT_DYNAMIC Mon Jul 24 22:16:20
UTC 2023
$ cat /proc/cmdline
... root=... ro
$ sudo ./test-loop
$
...
$ cat /proc/cmdline
... root=... ro max_loop=0
$ sudo ./test-loop
$
...
$ cat /proc/cmdline
... root=... ro max_loop=8
$ sudo ./test-loop
open: /dev/loop8: No such device or address
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You received this bug notification because you are a member of Kernel
Packages, which is subscribed to linux-hwe-5.15 in Ubuntu.
https://bugs.launchpad.net/bugs/2015400
Title:
losetup with mknod fails on jammy with kernel 5.15.0-69-generic
Status in linux package in Ubuntu:
Invalid
Status in linux-hwe-5.15 package in Ubuntu:
Invalid
Status in linux source package in Focal:
Invalid
Status in linux-hwe-5.15 source package in Focal:
In Progress
Status in linux source package in Jammy:
In Progress
Status in linux-hwe-5.15 source package in Jammy:
Invalid
Status in linux source package in Lunar:
In Progress
Status in linux-hwe-5.15 source package in Lunar:
Invalid
Status in linux source package in Mantic:
Invalid
Status in linux-hwe-5.15 source package in Mantic:
Invalid
Bug description:
[ 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.
[ Regression Potential ]
* Regressions would be limited to the loop block driver,
more specifically its default behavior (but it's that now)
or specific usage of max_loop parameter (tested; looks OK).
[ Other Info ]
* The fix on Jammy is just a Revert, since it had not been
released with the offending patch.
* The fix on Lunar is the recently accepted 2 patches [1, 2]
as it was released with the offending patch, so let's keep
that patch's improvement/behavior for the max_loop=0 case,
and "fix"/restore the historical no-limit default behavior.
* The fix on Mantic is the recently accepted 2 patches too,
now in v6.5-rc3, which should be automatically incorporated
as Mantic apparently will release with the 6.5 kernel [3]
* 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.)
[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
Original Description:
---
losetup fails with devices created manually by mknod on kernel
5.15.0-69-generic
# fallocate -l 1G test
# mknod -m 660 /dev/loop8 b 7 8
# chown root:disk /dev/loop8
# losetup /dev/loop8 ./test
losetup: ./test: failed to set up loop device: Device or resource busy
Possibly as a result of this patch:
https://lore.kernel.org/lkml/20221208200605.756287-1-isaacmanjar...@google.com/T/
which was introduced in 5.15.0-68:
http://launchpadlibrarian.net/653145495/linux_5.15.0-67.74_5.15.0-68.75.diff.gz
On a machine prior to this change (no issue with losetup):
# cat /sys/module/loop/parameters/max_loop
0
# uname -r
5.15.0-58-generic
On a machine after the change (has losetup issue as described above):
# cat /sys/module/loop/parameters/max_loop
8
# uname -r
5.15.0-69-generic
So it looks like the default changed and the max amount of loop
devices that can be created with mknod (not loop-control) is 8. If we
set max_loop=0 on the kernel command line, it works as before. Cannot
unload and reload module on a running system to change the parameter
because it is built into the kernel.
Another workaround is to use `losetup --find` but that means you
cannot create with named devices created with mknod.
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/2015400/+subscriptions
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