Re: WARNING in ovl_instantiate

[Amir Goldstein]

On Tue, Mar 26, 2019 at 2:10 PM syzbot
<syzbot+9c69c282adc4edd2b540@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> syzbot has bisected this bug to:
>
> commit 01b39dcc95680b04c7af5de7f39f577e9c4865e3
> Author: Amir Goldstein <amir73il@xxxxxxxxx>
> Date:   Fri May 11 08:15:15 2018 +0000
>
>      ovl: use inode_insert5() to hash a newly created inode
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=176da0cd200000
> start commit:   de6629eb Merge tag 'pci-v5.0-fixes-1' of git://git.kernel...
> git tree:       upstream
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=14eda0cd200000
> console output: https://syzkaller.appspot.com/x/log.txt?x=10eda0cd200000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=edf1c3031097c304
> dashboard link: https://syzkaller.appspot.com/bug?extid=9c69c282adc4edd2b540
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12c7a94f400000
>
> Reported-by: syzbot+9c69c282adc4edd2b540@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: 01b39dcc9568 ("ovl: use inode_insert5() to hash a newly created
> inode")
>

Dmitry,

The root cause of this bug is that repro is mounting overlapping overlay
layers (i.e. upperdir=./file0,lowerdir=.:file0).
Miklos claimed that the fix should be to fail such mounts.
Below is a patch to test:

#syz test: https://github.com/amir73il/linux.git ovl-check-overlap

However, I see that this specific overlapping layers mount has already
mutated to several other repros out there, like the ones in this bug:

https://syzkaller.appspot.com/bug?extid=a55ccfc8a853d3cff213

I believe that disallowing overlapping layers will silence some
bugs, whose root cause may be different.

Besides doing the overlapping layers mount, this repro family also
does extensive access to overlay underlying layers concurrently
with overlay access and *that* is the root cause for most of these
"possible deadlock" bugs (some false positives and some real).

Assuming that ovl-check-overlap will get merged, you may need to
hint syzkaller that overlapping layers is no longer a valid input or
maybe it will figure it out on its own?...

Thanks,
Amir.