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

From: Amir Goldstein
Date: Sat Nov 05 2016 - 16:41:12 EST

On Tue, Oct 25, 2016 at 2:24 PM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> On Tue, Oct 25, 2016 at 10:34 AM, Miklos Szeredi <mszeredi@xxxxxxxxxx> 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)