Re: [PATCH V5] loop: Add sanity check for read/write_iter

[Christian Hesse]

Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx> on Tue, 2025/05/20 11:00:
> On Mon, 19 May 2025 17:56:40 +0200, Christian Hesse wrote:
> > $ losetup --find --show --read-only --
> > /run/archiso/bootmnt/arch/x86_64/airootfs.sfs losetup:
> > /run/archiso/bootmnt/arch/x86_64/airootfs.sfs: failed to set up loop
> > device: Invalid argument
>
> I tried to reproduce the problem you mentioned using the kernel containing
> "commit:f5c84eff", but failed to reproduce it.
> The complete reproduction steps are as follows:
> 
> sudo apt install squashfs-tools debootstrap
> sudo debootstrap --arch=amd64 focal rootfs http://archive.ubuntu.com/ubuntu/
> sudo mksquashfs rootfs rootfs.sfs -comp xz -e boot
> [...]

That's the wrong end of the stack. After all squashfs is not directly
involved here (that was just an etxra info on why we have a loopback file
inside iso9660).

The issue is setting up the loopback file inside a mounted iso9660 filesystem.
Take these steps for easy reproduction:

root@leda ~ # mkdir iso.d 
root@leda ~ # truncate -s 10m iso.d/loopback.img
root@leda ~ # mkisofs -o iso.iso iso.d/
Setting input-charset to 'UTF-8' from locale.
 94,75% done, estimate finish Tue May 20 07:34:52 2025
Total translation table size: 0
Total rockridge attributes bytes: 0
Total directory bytes: 0
Path table size(bytes): 10
Max brk space used 0
5294 extents written (10 MB)
root@leda ~ # mount -o loop iso.iso /mnt/tmp 
mount: /mnt/tmp: WARNING: source write-protected, mounted read-only.
root@leda ~ # losetup --find --show --read-only -- /mnt/tmp/loopback.img 
losetup: /mnt/tmp/loopback.img: failed to set up loop device: Invalid argument

Hope that helps, let me know if you need more assistance.
-- 
Best regards,
Chris

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