[6.12.y regression] loosetup: failed to set up loop device: Invalid argument after 184b147b9f7f ("loop: Add sanity check for read/write_iter")

[Salvatore Bonaccorso <carnil@debian.org>]

Hi

In Debian Roland Clobus reported a regression with setting up loop
devices from a backing squashfs file lying on read-only mounted target
directory from a iso.

The original report is at:
https://bugs.debian.org/1106070

Quoting the report:

On Mon, May 19, 2025 at 12:15:10PM +0200, Roland Clobus wrote:
> Package: linux-image-6.12.29-amd64
> Version: 6.12.29-1
> Severity: important
> X-Debbugs-Cc: debian-amd64@lists.debian.org
> User: debian-amd64@lists.debian.org
> Usertags: amd64
> X-Debbugs-Cc: phil@hands.com
> User: debian-qa@lists.debian.org
> Usertags: openqa
> X-Debbugs-Cc: debian-boot
> 
> Hello maintainers of the kernel,
> 
> The new kernel (6.12.29) has a modified behaviour (compared to 6.12.27) for
> the loop device.
> 
> This causes the Debian live images (for sid) to fail to boot.
> 
> The change happened between 20250518T201633Z and 20250519T021902Z, which
> matches the upload of 6.12.29 (https://tracker.debian.org/news/1646619/accepted-linux-signed-amd64-612291-source-into-unstable/)
> at 20250518T230426Z.
> 
> To reproduce:
> * Download the daily live image from https://openqa.debian.net/tests/396941/asset/iso/smallest-build_sid_20250519T021902Z.iso
> * Boot into the live image (the first boot option)
> * Result: an initramfs shell (instead of a live system) -> FAIL
> * Try: `losetup -r /dev/loop1 /run/live/medium/live/filesystem.squashfs`
> * Result: `failed to set up loop device: invalid argument` -> FAIL
> * Try: `cp /run/live/medium/live/filesystem.squashfs /`
> * Try: `losetup -r /dev/loop2 /filesystem.squashfs`
> * Result: `loop2: detected capacity change from 0 to 1460312` -> PASS
> 
> It appears that the loopback device cannot be used any more with the mount
> /run/live/medium (which is on /dev/sr0).
> 
> I've verified: the md5sum of the squashfs file is OK.
> 
> The newer kernel is not in trixie yet.
> 
> With kind regards,
> Roland Clobus

A short reproducer is as follows:

iso="netinst.iso"
url="https://openqa.debian.net/tests/396941/asset/iso/smallest-build_sid_20250519T021902Z.iso"
if [ ! -e "${iso}" ]; then
        wget "${url}" -O "${iso}"
fi
mountdir="$(mktemp -d)"
mount -v "./${iso}" "${mountdir}"
losetup -v -r -f "${mountdir}/live/filesystem.squashfs"
loosetup -l

resulting in:

mount: /tmp/tmp.HgbNe7ek3h: WARNING: source write-protected, mounted read-only.
mount: /dev/loop0 mounted on /tmp/tmp.HgbNe7ek3h.
losetup: /tmp/tmp.HgbNe7ek3h/live/filesystem.squashfs: failed to set up loop device: Invalid argument
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE         DIO LOG-SEC
/dev/loop0         0      0         1  0 /root/netinst.iso   0     512

Reverting 184b147b9f7f ("loop: Add sanity check for read/write_iter")
on top of 6.12.29 fixes the issue:

mount: /tmp/tmp.ACkkdCdYvB: WARNING: source write-protected, mounted read-only.
mount: /dev/loop0 mounted on /tmp/tmp.ACkkdCdYvB.
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE                                    DIO LOG-SEC
/dev/loop1         0      0         0  1 /tmp/tmp.ACkkdCdYvB/live/filesystem.squashfs   0     512
/dev/loop0         0      0         1  0 /root/netinst.iso                              0     512

For completeness, netinst.iso is a iso9660 fstype with mount options
"ro,relatime,nojoliet,check=s,map=n,blocksize=2048,iocharset=utf8".

#regzbot introduced: 184b147b9f7f
#regzbot link: https://bugs.debian.org/1106070

Regards,
Salvatore