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

[Christian Hesse]

Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx> on Mon, 2025/04/28 22:36:
> Some file systems do not support read_iter/write_iter, such as selinuxfs
> in this issue.
> So before calling them, first confirm that the interface is supported and
> then call it.
> 
> It is releavant in that vfs_iter_read/write have the check, and removal
> of their used caused szybot to be able to hit this issue.
> 
> Fixes: f2fed441c69b ("loop: stop using vfs_iter__{read,write} for buffered
> I/O") Reported-by: syzbot+6af973a3b8dfd2faefdc@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=6af973a3b8dfd2faefdc
> Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
> Reviewed-by: Christoph Hellwig <hch@xxxxxx>
> ---
> V1 -> V2: move check to loop_configure and loop_change_fd
> V2 -> V3: using helper for this check
> V3 -> V4: remove input parameters change and mode
> V4 -> V5: remove braces around !file->f_op->write_iter

This introduced a regression for Arch Linux, breaking boot media generated
with archiso [0]. More specifically it's this call of losetup [1].

There's a squashfs inside iso9660. Mounting the iso9660 filesystem works
fine, but losetup complains when setting up:

$ 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

This has been bisected to commit d278164832618bf2775c6a89e6434e2633de1eed in
mainline (and 9bd3feb324fce2e93e09d0a5b00887e81d337a8c for linux-6.14.y,
184b147b9f7f07577567a80fcc9314f2bd0b0b00 for linux-6.12.y). Thanks to
Christian Heusel for his work on this.

As the call tries to setup in read-only mode the check for
(file->f_op->read_iter) fails here, returning the -EINVAL we see.

Reported-by: Christian Hesse <mail@xxxxxxxx>
Bisected-by: Christian Heusel <christian@xxxxxxxxx>

[0] https://gitlab.archlinux.org/archlinux/mkinitcpio/mkinitcpio-archiso
[1] https://gitlab.archlinux.org/archlinux/mkinitcpio/mkinitcpio-archiso/-/blob/master/hooks/archiso?ref_type=heads#L88
-- 
Mit freundlichen Gruessen
Christian Hesse

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