Re: WARNING in ovl_instantiate

[Dmitry Vyukov <dvyukov@google.com>]

On Mon, Dec 17, 2018 at 2:30 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > On Sun, Dec 16, 2018 at 6:00 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > On Sat, Dec 15, 2018 at 9:34 PM syzbot
> > > <syzbot+9c69c282adc4edd2b540@syzkaller.appspotmail.com> wrote:
> > > >
> > > > syzbot has found a reproducer for the following crash on:
> > > >
> > > > HEAD commit:    d14b746c6c1c Add linux-next specific files for 20181214
> > > > git tree:       linux-next
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=143f9a15400000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1da6d2d18f803140
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=9c69c282adc4edd2b540
> > > > compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12a6e543400000
> > > >
> > > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > > Reported-by: syzbot+9c69c282adc4edd2b540@syzkaller.appspotmail.com
> > > >
> > > > overlayfs: filesystem on './file0' not supported as upperdir
> > > > overlayfs: filesystem on './file0' not supported as upperdir
> > > > overlayfs: filesystem on './file0' not supported as upperdir
> > > > overlayfs: filesystem on './file0' not supported as upperdir
> > > > overlayfs: filesystem on './file0' not supported as upperdir
> > > > WARNING: CPU: 1 PID: 28918 at fs/overlayfs/dir.c:263
> > > > ovl_instantiate+0x369/0x400 fs/overlayfs/dir.c:263
> > >
> > > Looks like some corner case race when using same dir as upper and lower.
> > > Doesn't look like a critical issue, I just don't know how to explain
> > > getting to this
> > > state. Couldn't reproduce on my target machine.
> > >
> > > It would have been interesting for me to see the strace of the repro threads
> > > when that WARN happens. I wonder if anyone else has already asked for it and
> > > how hard would it be to make that information available with the bug report.
> >
> > Hi Amir,
> >
> > By strace you mean return values of syscalls, or something else?
> >
>
> I do mean return values.
> Some of the commands in the repro are obviously going to fail and
> some will fail conditionally depending on who wins the race.
> It could have been good for analysis of the bug to know when the
> race happened which syscall sequence took place.
>
> > We had only 1 strace-related request, and it was related to better
> > static decoding of inputs rather then dynamic behavior:
> > https://github.com/google/syzkaller/issues?utf8=%E2%9C%93&q=is%3Aissue+is%3Aopen+strace
> >
> > I don't immediately see how to capture runtime behavior. It would work
> > if we dump everything onto console right away. But this will produce
> > tons of output (really lots). And that output will be intermixed
> > across parallel processes. And it will be hard to understand which
> > exactly syscalls participated in the process that provoked the crash.
> > Or maybe it's exactly syscalls from several processes interacted. Lots
> > of output can also slow down and perturb execution.
>
> Yeh, I figured. Maybe the return values of syscalls is something that syzkaller
> should cache and in case of failure, report recent run sequences in format
> similar to the repro program. Just a though. Much easier said than done.

Yes, not so easy to do. This info needs to be piped through several
processes and then sent off the machine. But then again we won't know
which exactly execution provoked the crash. Reporting just any
execution can work simpler cases, but in these cases this info is not
so useful because all executions are the same and a developer can
probably predict outcome of syscalls, or re-reproduce locally and
strace. And it won't exactly for harder cases like this where 1/10000
executions trigger the crash. And in such cases reporting a wrong info
may be worse then not reporting it at all.
Part of the idea is to provide enough info for a developer to
reproduce the crash locally and then they can dump any kind of info,
add additional checks, debugging output, etc. We can't cover debugging
problem in all its generality, there is no common recipe. And for
simpler cases a developer frequently does not need anything other than
kernel crash message.


> > But meanwhile I was able to reproduce this on the first run within 4
> > minutes. Maybe you need to wait longer, it does not happen
> > immediately.
>
> Oh! I wonder if this type of information, how long or how many repeats before
> crash happens is available in the bug report and I missed it - if not, could be
> useful to add it.

There are some implicit signals like the log saying that it took 7 minutes:

2018/12/15 18:45:07 executed programs: 0
[ 1266.911390] IPVS: ftp: loaded support on port[0] = 21
...
[ 1665.610555] overlayfs: filesystem on './file0' not supported as upperdir
[ 1665.617831] WARNING: CPU: 1 PID: 28918 at fs/overlayfs/dir.c:263
ovl_instantiate+0x369/0x400

or mentions of any of these in the repro (which are all present in this repro):

"threaded":true,"collide":true,"repeat":true,"procs":6

But I filed https://github.com/google/syzkaller/issues/885 for a more
verbal signal.



> Anyway, The reason that WARN_ON is there is because I wasn't sure
> if that could happen. Apparently it can with this weird setup. Once I am able to
> understand how it happens most likely the result will be to covert the WARN_ON
> to pr_warn. User anyway gets an error, so there is probably nothing to
> worry about
> (famous last words).
>
> Thanks,
> Amir.