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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 20:24:08 +0200


i riddled over UEFI appendix A:
> > Do you mean that one has to interpret the statement of UEFI 2.4
> >   "All EFI GUIDs (Globally Unique Identifiers) have the format described
> >    in RFC 4122 [...]
> >    It should also be noted that TimeLow, TimeMid, TimeHighAndVersion fields
> >    in the EFI are encoded as little endian."
> > as
> >   The text format of EFI GUIDs is the same as in RFC 4122, but the binary
> >   representation is byte-swapped at the first 4, 2, and 2 bytes.

Andrei Borzenkov wrote:
> Yes. See other mail I just sent :)

I have to give in, since i am not in the position to raise protest against
Intel because of obfuscating english language. :))

> > Value "modification-date" produces a low
> > quality GUID from the value set by option --modification-date=.

> How? Time base UUID (Version 1) requires "node data" 

I am aware that the result is a poor fake of an UUID. Its time granularity
is below any standard. No site id. So i let it pose as random.
The reason for offering it is to reduce the number of parameters which
one has to remember for reproducibly producing an ISO.
(--modification-date= must be used in any case to override the automatic
 "now" timestamping of the ISO filesystem.)

> >  The partition GUIDs will be generated in a reproducible way by exoring a
> >  little-endian 32 bit counter with the disk GUID beginning at byte offset 9.

> Not sure I follow - what counter?

Partition number. I shall clarify this in the API.

In result the GUID byte strings of a grub-mkrescue GPT will look like

  GPT disk GUID      :      2303cd2a73c7424aa29825632da7f446
  GPT partition GUID :   1  2303cd2a73c7424aa29925632da7f446
  GPT partition GUID :   2  2303cd2a73c7424aa29a25632da7f446
  GPT partition GUID :   3  2303cd2a73c7424aa29b25632da7f446
  GPT partition GUID :   4  2303cd2a73c7424aa29c25632da7f446

In current xorriso, the first partition GUID is the original random
and disk GUID is derived from it as last one:

  GPT disk GUID      :      6d359a15f8763e47b9ee78207025c200
  GPT partition GUID :   1  6d359a15f8763e47b9ea78207025c200
  GPT partition GUID :   4  6d359a15f8763e47b9e978207025c200

> My copy of 2.4 RevC still does not mention IEEE 802

I have a PDF with title
  Unified Extensible Firmware Interface Specification
  Version 2.4 Errata B
  April, 2014
In appendix A, table 218, row "Node" i read:
  "Byte Offset: 10"
  "Byte Length: 6"
   The spatially unique node identifier. This can be based
   on any IEEE 802 address obtained from a network
   card. If no network card exists in the system, a
   cryptographic-quality random number can be used."

> Yes it could be more clear.

They should pay you a reward for compensating this flaw.

Have a nice day :)


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