Re: INFO: task hung in xlog_grant_head_check

[Eric Biggers]

On Wed, May 23, 2018 at 08:26:20AM +1000, Dave Chinner wrote:
> On Tue, May 22, 2018 at 08:31:08AM -0400, Brian Foster wrote:
> > On Mon, May 21, 2018 at 10:55:02AM -0700, syzbot wrote:
> > > Hello,
> > > 
> > > syzbot found the following crash on:
> > > 
> > > HEAD commit:    203ec2fed17a Merge tag 'armsoc-fixes' of git://git.kernel...
> > > git tree:       upstream
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=11c1ad77800000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=f3b4e30da84ec1ed
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=568245b88fbaedcb1959
> > > compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> > > syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=122c7427800000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10387057800000
> > > 
> > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > Reported-by: syzbot+568245b88fbaedcb1959@xxxxxxxxxxxxxxxxxxxxxxxxx
> > > 
> > >         (ptrval): 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > ................
> > >         (ptrval): 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> > > ................
> > > XFS (loop0): metadata I/O error in "xfs_trans_read_buf_map" at daddr 0x2 len
> > > 1 error 117
> > > XFS (loop0): xfs_imap_lookup: xfs_ialloc_read_agi() returned error -117,
> > > agno 0
> > > XFS (loop0): failed to read root inode
> > 
> > FWIW, the initial console output is actually:
> > 
> > [  448.028253] XFS (loop0): Mounting V4 Filesystem
> > [  448.033540] XFS (loop0): Log size 9371840 blocks too large, maximum size is 1048576 blocks
> > [  448.042287] XFS (loop0): Log size out of supported range.
> > [  448.047841] XFS (loop0): Continuing onwards, but if log hangs are experienced then please report this message in the bug report.
> > [  448.060712] XFS (loop0): totally zeroed log
> > 
> > ... which warns about an oversized log and resulting log hangs. Not
> > having dug into the details of why this occurs so quickly in this mount
> > failure path,
> 
> I suspect that it is a head and/or log tail pointer overflow, so when it
> tries to do the first trans reserve of the mount - to write the
> unmount record - it says "no log space available, please wait".
> 
> > it does look like we'd never have got past this point on a
> > v5 fs (i.e., the above warning would become an error and we'd not enter
> > the xfs_log_mount_cancel() path).
> 
> And this comes back to my repeated comments about fuzzers needing
> to fuzz properly made V5 filesystems as we catch and error out on
> things like this. Fuzzing random collections of v4 filesystem
> fragments will continue to trip over problems we've avoided with v5
> filesystems, and this is further evidence to point to that.
>
> 
> I'd suggest that at this point, syzbot XFS reports should be
> redirected to /dev/null. It's not worth our time to triage
> unreviewed bot generated bug reports until the syzbot developers
> start listening and acting on what we have been telling them
> about fuzzing filesystems and reproducing bugs that are meaningful
> and useful to us.

The whole point of fuzzing is to provide improper inputs.  A kernel bug is a
kernel bug, even if it's in deprecated/unmaintained code, or involves userspace
doing something unexpected.  If you have known buggy code in XFS that you refuse
to fix, then please provide a kernel config option so that users can disable the
unmaintained XFS formats/features, leaving the maintained ones.  As-is, you seem
to be forcing everyone who enables CONFIG_XFS_FS to build known
buggy/unmaintained code into their kernel.

- Eric