linux-kernel - Re: [PATCH] ext4: do not BUG when INLINE_DATA

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Message-ID: <20250721135142.GA1415603@mit.edu>
Date: Mon, 21 Jul 2025 09:51:42 -0400
From: "Theodore Ts'o" <tytso@....edu>
To: Jan Kara <jack@...e.cz>
Cc: Moon Hee Lee <moonhee.lee.ca@...il.com>,
        syzbot+544248a761451c0df72f@...kaller.appspotmail.com,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org
Subject: Re: [PATCH] ext4: do not BUG when INLINE_DATA_FL lacks system.data
 xattr

On Mon, Jul 21, 2025 at 02:49:57PM +0200, Jan Kara wrote:
> > We need to do something similar for LOOP_CLR_FD, LOOP_SET_STATUS,
> > LOOP_SET_STATUS64, LOOP_CHANGE_FD, and LOOP_SET_CAPACITY ioctls.
> 
> Well, careful here. Changing loop device underneath mounted filesystem is a
> valid usecase in active use (similarly as changing DM device underneath a
> filesystem). So don't think we can play similar tricks as with
> LOOP_SET_BLOCK_SIZE where changing block device block size just doesn't
> make sense while the device is in use. Similarly LOOP_CLR_FD is an
> equivalent of device going away. LOOP_CHANGE_FD is a legacy of the past but
> it was *designed* to be used to swap backing file under a life filesystem
> (old days of Wild West :)) during boot. We may get away with dropping that
> these days but so far I'm not convinced it's worth the risk. So in this case
> I don't see anything here that couldn't happen with say DM device and thus
> I wouldn't really restrict the loop device functionality...

Sure, and LOOP_SET_CAPACITY might be used to grow a file system image
which the file system could then grow into.  Fair.

This is related to BLK_DEV_WRITE_MOUNTED=n which the syzkaller folks
have agreed to use to prevent noisy syzkaller reports.  We're seeing a
bunch of syzkaller reports now that syzkaller has been taught how to
use these loop ioctls and so we're seeing loop device hijinks.  Which
is fine; I can just start filtering any syzkaller report that uses
loop device ioctls as false positive noise and call it a day.
Unfortunately, that won't help deal with researchers that are taking
the syzkaller code and then sending reports without any dashboards or
reproducers.  :-(

However, I do think that if the file system has advertised that they
don't support random underlying block device hijinks, such as XFS for
example, we should honor this and disallow those "wild west" loop
device operations.  And perhaps we should similarly disallow them if
BLK_DEV_WRITE_MOUNTED=n.

						- Ted