linux-kernel - Re: root=PARTUUID for MBR/NT disk signatures?

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Message-ID: <50328258.9030800@wwwdotorg.org>
Date:	Mon, 20 Aug 2012 12:30:48 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	Tejun Heo <tj@...nel.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Will Drewry <wad@...omium.org>,
	"cross-distro@...ts.linaro.org" <cross-distro@...ts.linaro.org>,
	Jens Axboe <axboe@...nel.dk>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: root=PARTUUID for MBR/NT disk signatures?

On 08/20/2012 12:22 PM, Tejun Heo wrote:
> Hello,
> 
> On Fri, Aug 17, 2012 at 04:10:52PM -0600, Stephen Warren wrote:
>> I was considering extending the kernel command-line option
>> root=PARTUUID= to also support MBR (NT disk signatures). I was thinking
>> of a syntax along the lines of:
>>
>> root=PARTUUID=UUUUUUUU-PP[/PARTNROFF=%d]
>>
>> ... where UUUUUUUU is the hex representation of the NT disk signature,
>> and PP is the hex representation of the partition number. Like GPT,
>> /PARTNROFF could be used too if desired.
>>
>> Related, I was thinking of changing struct partition_meta_info's uuid
>> field to be a string, so that it could simply be strcmp'd against the
>> UUID value on the kernel command-line. That way, the type of the UUID is
>> irrelevant.
>>
>> Does anyone have any objection to that?
> 
> Wouldn't that be able to break setups which work currently?

I don't believe so:

Since the newly supported UUID syntax wouldn't ever match any EFI UUID
(the lengths differ in all cases), I don't believe the new syntax would
affect behavior for any existing usage.

Obviously, part_efi.c would be modified to initialize struct
partition_meta_info's uuid field to the appropriate string
representation of the UUID so that the str(case)cmp would still succeed
for existing command-lines. I ended up coding up that part of the change
late Friday, and the feature was certainly still working OK.
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