linux-kernel - Re: [PATCH 1/3] ovl: check fs features

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Date:   Sat, 5 Nov 2016 22:40:40 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Miklos Szeredi <mszeredi@...hat.com>
Cc:     linux-unionfs@...r.kernel.org, Guillem Jover <guillem@...ian.org>,
        Raphael Hertzog <hertzog@...ian.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH 1/3] ovl: check fs features

On Tue, Oct 25, 2016 at 2:24 PM, Amir Goldstein <amir73il@...il.com> wrote:
> On Tue, Oct 25, 2016 at 10:34 AM, Miklos Szeredi <mszeredi@...hat.com> wrote:
>> To allow adding new, backward incompatible features to overlayfs, we need a
>> way to store the list of features in the overlay.  This is done via
>> "trusted.overlay.features" xattr on the root of the upper layer (or one of
>> the lower layers, that previously acted as an upper layer).  It's a comma
>> separated list of case sensitive strings.
>>
>> If an overlay has an unknown feature, mount shall return an error.  So
>> mechanism should only be used for backward incompatible features.
>
> So maybe be explicit and call the attribute trusted.overlay.incompat_features,
> to allow future addition of compat and rocompat feature sets?
>

On top of the proposed features xattr,
for the sake of being backward compatible with old kernels,
how about creating a file 3 levels down from work dir (e.g. /work/work/a/b/c)
This would cause old kernels to mount overlay read-only,
which is sufficient to keep them from corrupting the redirect structure.

And once again, I suggest learning from the elders fs, who have
successfully gone through many on-disk format upgrades,
and copy the design of the feature trio (compat/incompat/rocompat)

Amir.