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Date: Sat, 13 Apr 2024 12:47:21 +0200
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Subject: Re: [PATCH] efivarfs: Iterate variables with increasing name buffer
 sizes
To: Peter Jones <pjones@redhat.com>
Cc: Ard Biesheuvel <ardb@kernel.org>, Matthew Garrett <mjg59@srcf.ucam.org>,
 linux-efi@vger.kernel.org, Jeremy Kerr <jk@ozlabs.org>,
 linux-kernel@vger.kernel.org
References: <20240122231507.1307793-1-timschumi@gmx.de>
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From: Tim Schumacher <timschumi@gmx.de>
In-Reply-To: <CA+g+hrh_REk-bcTt-D+eSngofxdejeRXuXKhf1O15wzn+qMy6Q@mail.gmail.com>
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On 24.01.24 22:25, Peter Jones wrote:
> On Tue, Jan 23, 2024 at 12:33=E2=80=AFPM Tim Schumacher <timschumi@gmx.d=
e> wrote:
>>
>> On 23.01.24 15:09, Ard Biesheuvel wrote:
>>> On Tue, 23 Jan 2024 at 14:55, Tim Schumacher <timschumi@gmx.de> wrote:
>>>>
>>>> I'd rather avoid introducing deviations from the specifications on th=
e
>>>> kernel side as well.
>>>
>>> Which specification would this deviate from?
>>
>> The preexisting comment claims "Per EFI spec", and it appears that I go=
t
>> mislead by that. Neither the UEFI specification, nor the newest revisio=
n
>> of the EFI specification (which I guess is what would have been current
>> back in 2004, when this comment was introduced) seem to make any mentio=
n
>> of a maximum length for the variable name.
>
> Curiously, I can't find it in the 1.02 spec (the oldest I can find)
> either.  When I inherited efibootmgr around 2013, this was a
> limitation there, but I don't see anything in that tree that claims
> it's a spec limitation either.  My suspicion is this is a former
> Itanium firmware limit that got promoted to "the spec says" by word of
> mouth, or was in some very early ia64 implementation spec.

In case anyone is still curious about this, I managed to track down where
the supposed limit actually came from.

The efivarfs documentation claims that "The old sysfs EFI variables code o=
nly
supported variables of up to 1024 bytes. This limitation existed in versio=
n
0.99 of the EFI specification, but was removed before any full releases."

With some effort I managed to track down a copy of EFI v0.99 [1], which
indeed says the following:

"The size of the VariableName, including the Unicode Null in bytes plus th=
e
  DataSize is limited to a maximum size of 1024 bytes."

This note was there at least in version 0.92 and 0.99, and gone in at leas=
t
version 1.02. I haven't been able to find a copy of version 1.01, but it m=
ost
likely never even existed online, given that 1.02 happened only 12 days la=
ter
(and for the sole reason of "legal and trademarking requirements").
The EFI 0.99 errata (which might have included more details) sadly doesn't=
 seem
to have been backed up anywhere by third-parties.

Tim

[1] Searching for "EFISpec_V099" on your preferred search engine should
     find it. I doubt that Intel will care about copyright assignments for
     feedback on 0.99 now, but the agreement prompt sadly prevented the We=
b
     Archive from reaching it.