Re: overlayfs: overlapping upperdir path

From: Amir Goldstein <amir73il@gmail.com>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	overlayfs <linux-unionfs@vger.kernel.org>,
	Vivek Goyal <vgoyal@redhat.com>
Subject: Re: overlayfs: overlapping upperdir path
Date: Thu, 1 Apr 2021 17:30:41 +0300	[thread overview]
Message-ID: <CAOQ4uxgcO-Wvjwbmjme+OwVz6bZnVz4C87dgJDJQY1u55BWGjw@mail.gmail.com> (raw)
In-Reply-To: <CAJfpegtUOVF-_GWk8Z-zUHUss0=GAd7HOY_qPSNroUx9og_deA@mail.gmail.com>

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?

>
> 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.

Thanks,
Amir.