linux-ext4 - Re: [syzbot] [ext4?] WARNING in bdev

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Message-ID: <f634msrtbc75cspm3pysavmxc5gfzlut56bee7qtc72ypmd4ap@p7tmmjisdc72>
Date: Tue, 15 Jul 2025 12:36:59 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: Jan Kara <jack@...e.cz>
Cc: syzbot <syzbot+01ef7a8da81a975e1ccd@...kaller.appspotmail.com>, 
	adilger.kernel@...ger.ca, anna.luese@...ien.de, brauner@...nel.org, 
	jfs-discussion@...ts.sourceforge.net, libaokun1@...wei.com, linkinjeon@...nel.org, 
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	p.raghav@...sung.com, shaggy@...nel.org, sj1557.seo@...sung.com, 
	syzkaller-bugs@...glegroups.com, tytso@....edu
Subject: Re: [syzbot] [ext4?] WARNING in bdev_getblk

> > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=127d8d82580000
> > start commit:   835244aba90d Add linux-next specific files for 20250709
> > git tree:       linux-next
> > final oops:     https://syzkaller.appspot.com/x/report.txt?x=117d8d82580000
> > console output: https://syzkaller.appspot.com/x/log.txt?x=167d8d82580000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=8396fd456733c122
> > dashboard link: https://syzkaller.appspot.com/bug?extid=01ef7a8da81a975e1ccd
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=115c40f0580000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11856a8c580000
> > 
> > Reported-by: syzbot+01ef7a8da81a975e1ccd@...kaller.appspotmail.com
> > Fixes: 77eb64439ad5 ("fs/buffer: remove the min and max limit checks in __getblk_slow()")
> > 
> > For information about bisection process see: https://goo.gl/tpsmEJ#bisection
> 
> Ah, I see what's going on here. The reproducer mounts ext4 filesystem and
> sets block size on loop0 loop device to 32k using LOOP_SET_BLOCK_SIZE. Now
> because there are multiple reproducer running using various loop devices it
> can happen that we're setting blocksize during mount which obviously
> confuses the filesystem (and makes sb mismatch the bdev block size). It is
> really not a good idea to allow setting block size (or capacity for that
> matter) underneath an exclusive opener. The ioctl should have required
> exclusive open from the start but now it's too late to change that so we
> need to perform a similar dance with bd_prepare_to_claim() as in
> loop_configure() to grab temporary exclusive access... Sigh.
> 
> Anyway, the commit 77eb64439ad5 is just a victim that switched KERN_ERR
> messages in the log to WARN_ON so syzbot started to notice this breakage.

I was also thinking the change we did from KERN_ERR to WARN_ON was catching
a different bug.

Thanks for taking a look and fixing the issue Jan.

-- 
Pankaj Raghav