linux-kernel - Re: [PATCH V5] loop: Add sanity check for read/write

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Message-ID: <f17dfb4c-8c18-4a49-b8f9-532aacd4df8f@heusel.eu>
Date: Tue, 20 May 2025 18:53:31 +0200
From: "christian@...sel.eu" <christian@...sel.eu>
To: "hch@...radead.org" <hch@...radead.org>
Cc: Christian Hesse <mail@...rm.de>, "Xu, Lizhi" <Lizhi.Xu@...driver.com>, 
	"axboe@...nel.dk" <axboe@...nel.dk>, "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "ming.lei@...hat.com" <ming.lei@...hat.com>, 
	"syzbot+6af973a3b8dfd2faefdc@...kaller.appspotmail.com" <syzbot+6af973a3b8dfd2faefdc@...kaller.appspotmail.com>, 
	"syzkaller-bugs@...glegroups.com" <syzkaller-bugs@...glegroups.com>
Subject: Re: [PATCH V5] loop: Add sanity check for read/write_iter

On 25/05/20 05:49AM, hch@...radead.org wrote:
> On Tue, May 20, 2025 at 02:46:22PM +0200, Christian Hesse wrote:
> > "Xu, Lizhi" <Lizhi.Xu@...driver.com> on Tue, 2025/05/20 12:27:
> > > If it is a regular file, generic_ro_fops is used. In this case,
> > > isofs supports read_iter. When a regular file has a writable attribute,
> > 
> > Just tested with an iso file where writable flag from loopback file inside
> > was explicitly removed. No change.
> > 
> > > the problem will recur because isofs does not support write_iter.
> > 
> > We have two indications here that setup should happen in read-only mode:
> > 
> > * The underlaying filesystem is read-only
> > * `losetup` is called with switch `--read-only`
> > 
> > I would expect both to make this happy.
> 
> Can you test this patch?
> 
> We historically allow a writable fd on block devices even when they
> are read-only.  I suspect your use case is doing that and the new
> check for write_iter is interfering with that:

I have tested the patch and can confirm that it fixes the usecase as
represented by the reproducer that I have used to bisect the bug.

If you turn this into an actual patch you can add my

Tested-by: Christian Heusel <christian@...sel.eu>

if you want :)

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