On Thu, Apr 1, 2021 at 4:30 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Thu, Apr 1, 2021 at 4:37 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > Commit 146d62e5a586 ("ovl: detect overlapping layers") made sure we
> > don't have overapping layers, but it also broke the arguably valid use
> > case of
> >
> >  mount -olowerdir=/,upperdir=/subdir,..
> >
> > where subdir also resides on the root fs.
>
> How is 'ls /merged/subdir' expected to behave in that use case?
> Error?

-ELOOP is the error returned.

>
> >
> > I also see that we check for a trap at lookup time, so the question is
> > what does the up-front layer check buy us?
> >
>
> I'm not sure. I know it bought us silence from syzbot that started
> mutating many overlapping layers repos....
> Will the lookup trap have stopped it too? maybe. We did not try.
>
> In general I think that if we can error out to user on mount time
> it is preferred, but if we need to make that use case work, I'd try
> to relax as minimum as possible from the check.

Certainly.  Like lower inside upper makes zero sense, OTOH upper
inside lower does.   So I think we just need to relax the
upperdir/workdir layer check in this case.

Like attached patch.

Thanks,
Miklos