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

[Xu, Lizhi]

I figured out your steps to reproduce, and yes, this problem will occur if you do losetup with a file in a filesystem that does not support read_iter, which is what this patch does.

________________________________________
发件人: Christian Hesse
已发送: 2025 年 5 月 20 日 星期二 13:39
收件人: Xu, Lizhi
抄送: axboe@xxxxxxxxx; christian@xxxxxxxxx; hch@xxxxxxxxxxxxx; linux-block@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; ming.lei@xxxxxxxxxx; syzbot+6af973a3b8dfd2faefdc@xxxxxxxxxxxxxxxxxxxxxxxxx; syzkaller-bugs@xxxxxxxxxxxxxxxx
主题: Re: [PATCH V5] loop: Add sanity check for read/write_iter


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