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Date:   Sat, 11 Apr 2020 09:14:39 -0700
From:   Matthew Wilcox <willy@infradead.org>
To:     syzbot <syzbot+77fa5bdb65cc39711820@syzkaller.appspotmail.com>
Cc:     darrick.wong@oracle.com, hch@infradead.org, jack@suse.cz,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-xfs@vger.kernel.org, riteshh@linux.ibm.com,
        syzkaller-bugs@googlegroups.com, tytso@mit.edu
Subject: Re: WARNING in iomap_apply
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On Sat, Apr 11, 2020 at 12:39:13AM -0700, syzbot wrote:
> The bug was bisected to:
> 
> commit d3b6f23f71670007817a5d59f3fbafab2b794e8c
> Author: Ritesh Harjani <riteshh@linux.ibm.com>
> Date:   Fri Feb 28 09:26:58 2020 +0000
> 
>     ext4: move ext4_fiemap to use iomap framework
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=16c62a57e00000
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=15c62a57e00000
> console output: https://syzkaller.appspot.com/x/log.txt?x=11c62a57e00000
> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+77fa5bdb65cc39711820@syzkaller.appspotmail.com
> Fixes: d3b6f23f7167 ("ext4: move ext4_fiemap to use iomap framework")
> 
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 7023 at fs/iomap/apply.c:51 iomap_apply+0xa0c/0xcb0 fs/iomap/apply.c:51

This is:

        if (WARN_ON(iomap.length == 0))
                return -EIO;

and the call trace contains ext4_fiemap() so the syzbot bisection looks
correct.

>  iomap_fiemap+0x184/0x2c0 fs/iomap/fiemap.c:88
>  _ext4_fiemap+0x178/0x4f0 fs/ext4/extents.c:4860
>  ovl_fiemap+0x13f/0x200 fs/overlayfs/inode.c:467
>  ioctl_fiemap fs/ioctl.c:226 [inline]
>  do_vfs_ioctl+0x8d7/0x12d0 fs/ioctl.c:715