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Re: Do grub-mkrescue GPT GUIDs need more entropy than --fs-uuid gets ?

From: Andrei Borzenkov
Subject: Re: Do grub-mkrescue GPT GUIDs need more entropy than --fs-uuid gets ?
Date: Sun, 14 Aug 2016 09:29:31 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.2.0

14.08.2016 08:03, Michael Zimmermann пишет:
> couldn't we generate GUID's based on the current git revision?
> this way you reproduce the ISO without even looking at the timestamp.
> I don't know anything about the entropy requirements though. Lets wait for
> a reply of the maintainers about that.

EFI GUIDs are compliant with RFC 4122 which means that technically
deriving GUID from e.g. or even simply
timestamp string is absolutely legal as well as basing GUID on "GIT hash

For the purpose of reproducible builds I am not sure what having GUIDs
based on GIT revision buys us. It can't distinguish between different
instances of generated ISO, it cannot be traced back to GIT hash anyway.
So as long as generated GUID has reasonable chance to be different from
any other GUID on the system where ISO was booted, it should be good.

For GRUB itself it does not matter anyway - it does not use GUID, so FS
UUID collision is worse problem.

> Thanks
> Michael
> On Thu, Aug 11, 2016 at 9:55 PM, Thomas Schmitt <address@hidden> wrote:
>> Hi,
>> i am discussing with Chris Lamb on address@hidden
>> how to make production of bootable ISOs reproducible. The last (yet known)
>> obstacle are the pseudo-random GUIDs of the GPT which is produced for EFI
>> bootability.
>> Up to this obstacle it turned out that it will suffice to use the same
>> input file tree and the same overall timestamp with xorriso -as mkisofs
>> option
>>   --modification-date=YYYYMMDDhhmmsscc
>> which was originally introduced for grub-mkrescue to match in grub.cfg
>>   search --fs-uuid --set YYYY-MM-DD-hh-mm-ss-cc
>> I am now wondering whether it would be ok for grub-mkrescue if the GUIDs
>> of the GPT would be derived reproducibly from this timestamp by default.
>> (Currently they stem from /dev/urandom.)
>> These GUIDs will of course be unique inside the GPT. But their entropy
>> will be low and collisions with other ISOs could happen systematically
>> because of nearly identical production times.
>> Well, this can happen to the ISO 9660 --fs-uuid string under the same
>> circumstances.
>> So my question:
>> Is there any reason known why the GPT GUID needs to have better randomness
>> than the "search --fs-uuid" string ?
>> Have a nice day :)
>> Thomas
>> _______________________________________________
>> Grub-devel mailing list
>> address@hidden
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