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Re: Grub module to return partuuid of a device such as (hd0, gpt1) at bo

From: Thomas Schmitt
Subject: Re: Grub module to return partuuid of a device such as (hd0, gpt1) at boot time
Date: Sun, 14 Aug 2016 11:09:43 +0200


after having freshly re-read the specs of UEFI and RFC4122,
here is some nitpicking:

Steve Kenton wrote:
> >     3F2504E0-4F89-41D3-9A0C-0305E82C3301

Andrei Borzenkov wrote:
> Usually GUIDs are displayed in lower case

RFC4122 prescribes to read them independently of case, but to produce
them with lowercase letters.

> GPT GUID are pretty well defined by UEFI spec which in turn is
> based on RFC 4122.

UEFI 2.4 Appendix A claims to be in sync with RFC 4122. But that's
obviously not true.

They seem to have evolved in parallel. UEFI only documents the GUID variant
with MAC address and finely granulated timestamp, which imposes a privacy
problem. Other sources blame Microsoft for it.
RFC 4122 describes that variant with different endianness, a pseudo-random
variant (which about everybody is using), and a variant with crypto-grade
hash of user input. The abstract of RFC 4122 names Distributed Computing
Environment by Open Software Foundation as ancestor of its definition.

While trying to grok the conversion from text format to binary format
i could not reliably determine where to put the version nibble. RFC 4122
prescribes big-endian representation of 16 bit "time_hi_and_version",
whereas UEFI prescribes little-endian "TimeHighAndVersion".

uuidgen(1) obviously follows RFC 4122.
xorriso currently generates them UEFI style but in next release will
cowardly waste 4 bits of entropy by writing the nibble into both bytes.

It seems that nobody interprets the entrails of GUIDs but rather everybody
uses them as opaque byte strings.
Nevertheless one has to expect byte swapping to happen when converting
forth and back between text representation and binary representation
by using different conversion software.

Have a nice day :)


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