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Subject: Re: [syzbot] kernel BUG in assertfail
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     syzbot <syzbot+a6bf271c02e4fe66b4e4@syzkaller.appspotmail.com>,
        Chris Mason <clm@fb.com>, dsterba@suse.com,
        Josef Bacik <josef@toxicpanda.com>,
        linux-btrfs@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>
References: <000000000000f9136f05c39b84e4@google.com>
 <21666193-5ad7-2656-c50f-33637fabb082@suse.com>
 <CACT4Y+bqevMT3cD5sXjSv9QYM_7CwjYmN_Ne5LSj=3-REZ+oTw@mail.gmail.com>
From:   Nikolay Borisov <nborisov@suse.com>
Message-ID: <224f1e6a-76fa-6356-fe11-af480cee5cf2@suse.com>
Date:   Mon, 31 May 2021 11:57:32 +0300
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On 31.05.21 г. 11:55, Dmitry Vyukov wrote:
> On Mon, May 31, 2021 at 10:44 AM 'Nikolay Borisov' via syzkaller-bugs
> <syzkaller-bugs@googlegroups.com> wrote:
>> On 31.05.21 г. 10:53, syzbot wrote:
>>> Hello,
>>>
>>> syzbot found the following issue on:
>>>
>>> HEAD commit:    1434a312 Merge branch 'for-5.13-fixes' of git://git.kernel..
>>> git tree:       upstream
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=162843f3d00000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=9f3da44a01882e99
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=a6bf271c02e4fe66b4e4
>>>
>>> Unfortunately, I don't have any reproducer for this issue yet.
>>>
>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>> Reported-by: syzbot+a6bf271c02e4fe66b4e4@syzkaller.appspotmail.com
>>>
>>> assertion failed: !memcmp(fs_info->fs_devices->fsid, fs_info->super_copy->fsid, BTRFS_FSID_SIZE), in fs/btrfs/disk-io.c:3282
>>
>> This means a device contains a btrfs filesystem which has a different
>> FSID in its superblock than the fsid which all devices part of the same
>> fs_devices should have. This can happen in 2 ways - memory corruption
>> where either of the ->fsid member are corrupted or if there was a crash
>> while a filesystem's fsid was being changed. We need more context about
>> what the test did?
> 
> Hi Nikolay,
> 
> From a semantic point of view we can consider that it just mounts /dev/random.
> If syzbot comes up with a reproducer it will post it, but you seem to
> already figure out what happened, so I assume you can write a unit
> test for this.
> 

Well no, under normal circumstances this shouldn't trigger. So if syzbot
is doing something stupid as mounting /dev/random then I don't see a
problem here. The assert is there to catch inconsistencies during normal
operation which doesn't seem to be the case here.