Re [PATCH v5 2/5] x86/pkeys: Add helper functions to update PKRU on sigframe

[jeffxu]

The orig_pkru & init_pkru_value is quite difficult to understand.

case 1> init_pkru: 00 (allow all)
orig_pkru all cases  => allow all

case 2> init_pkru: 01 (disable all)
Orig_pkru:
allow all 00 => 00 allow all.
disable all 01 => 01 disable all.
disable write 10 => 00 allow all <--- *** odd ***
disable all 11 => 01 disable all

case 3> init pkru: 10 (disable write)
allow all 00 => 00 allow all.
disable all 01 => 00 (allow all) <----*** odd ***
disable write 10 => 10 allow all
disable all 11 => 10 disable write <--- *** odd ***

case 4> init pkru: 11 (disable all)
orig_pkru all cases => unchanged. 

set PKRU(0) seems to be better, easy to understand.

In addition, kernel overwrites the PKRU during the
signal handleing is a new ABI, it might be the best
to add a flag during sigaltstack(), similar to
how SS_AUTODISARM is set.

> +	return orig_pkru;
> +}
> +
 
-Jeff