[RESEND RFC PATCH 04/10] FWU: Add metadata access functions for GPT partitioned block devices

[Ilias Apalodimas]

Hi Etienne,


[...]
> > > > +
> > > > +     /* And now the replica */
> > > > +     ret = gpt_write_metadata_partition(desc, metadata,
> > > secondary_mpart);
> > > > +     if (ret < 0) {
> > > > +             log_err("Updating secondary metadata partition failed\n");
> > > > +             return ret;
> > > > +     }
> > >
> > > So shouldn't we do something about this case?  The first partition was
> > > correctly written and the second failed.  Now if the primary GPT somehow
> > > gets corrupted the device is now unusable.
>
> The replica is not there to overcome bitflips of the storage media.
> It's here to allow updates while reliable info a still available in
> the counter part.

Sure but with this piece of code this assumption is broken.  At the
point the secondary partition fails to write, you loose that
reliability. When the next update happens you are left with one valid
and one invalid partition of metadata.

> The scheme could be to rely on only 1 instance of the fwu-metadata
> (sorry Simon) image is valid.
> A first load: load 1st instance, crap the second.
> At update: find the crapped one: write it with new data. Upon success
> crapped the alternate one.
> This is a suggestion. There are many ways to handle that.

We could change to something like that, however this is not what's
currently happening.  gpt_check_metadata_validity() is trying to check
and make sure both of the partitions are sane.  If they aren't it
tries to recover those looking at a sane partition. So the question
for really is, should we do something *here* or rely on the fact that
the next update will try to fix the broken metadata.

Cheers
/Ilias