[RFC][patch] Add to GEOM_PART ability to detect GPT with corrupted PMBR

[Andrey V. Elsukov]

Hi All,

Currently GEOM_PART doesn't try to detect GPT in case when it doesn't
find PMBR's signature. Thus the backup GPT header and table are useless
until PMBR will not be restored first.

The proposed patch changes this behavior. When we probing GPT and didn't
find the PMBR's signature, we will read the backup GPT header and table.
And if it is valid, GPT will be detected.

The same changes also added to loader(8). If you will be able to load
the loader from some media, it can detect damaged GPT and it is possible
to choose partition from damaged disk for loading.

	https://reviews.freebsd.org/D4282

-- 
WBR, Andrey V. Elsukov

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