Date:	Mon, 13 Apr 2015 17:06:56 +0200
From:	Michal Marek <mmarek@...e.cz>
To:	Nicholas Mc Guire <der.herr@...r.at>
CC:	Jonathan Corbet <corbet@....net>,
	Nicholas Mc Guire <hofrat@...dl.org>,
	linux-kbuild@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kbuild: add documentation of KBUILD_BUILD_VERSION

On 2015-04-12 06:08, Nicholas Mc Guire wrote:
> On Sat, 11 Apr 2015, Michal Marek wrote:
> 
>> Dne 11.4.2015 v 15:20 Nicholas Mc Guire napsal(a):
>>> On Sat, 11 Apr 2015, Jonathan Corbet wrote:
>>>
>>>> On Sun,  5 Apr 2015 08:44:28 +0200
>>>> Nicholas Mc Guire <hofrat@...dl.org> wrote:
>>>>
>>>>> KBUILD_BUILD_VERSION is currently not documented but it is
>>>>> needed when rebuilding a kernel that should result in the identical
>>>>> binary. This is a brief documentation of KBUILD_BUILD_VERSION.
>>>>
>>>> Can we add something like the above to the document itself so that
>>>> readers have an idea of why they might want to tweak this?
>>>>
>>>> Either way, I can take it in the docs tree if that's best..Michal?
>>>>
>>> I thought of that but it would be inconsistent as all other descriptions
>>> here are only the function not the use.
>>
>> Most of the entries in this file predate efforts at deterministic
>> builds, so I'd prefer usefulness over consistency here :-).
>>
>>
>>> I did not find a file where 
>>> the problem of identical rebuild would really fit.
>>
>> ... unless, of course, you want to start a new file covering this topic.
>> Because it's not just the few override variables, but also some options
>> have to be turned off (I remember CONFIG_GCOV_KERNEL) and the paths must
>> be the same, or relative paths must be used.
>>
> The identical path really only should be needed for GCOV as it compiles
> path info into the binary - are you aware of any other cnfig option that
> would need the same path ?

The paths appear in BUG() messages if you are using O=<path>. This can
be solver by either not using O= or using O=<subdir>, in which case
kbuild uses '..' as path to the srctree.

IIRC the problem with GCOV was that gcc was adding a randomly named
symbol to each object file.


> It could be in a short document but it could also just be placed right
> under the descriptions of the relevant variables - something like:
> 
> 
> Use of KBUILD_BUILD_{TIMESTAMP,VERSION,USER,HOST}
> --------------------------------------------------
> 
> When the kernel is built, Kbuild stores the build timestamp, version, user
> and hostname in include/generated/compile.h. By default this information
> is extracted from the build-environment on every build. To override this
> behavior and rebuild an identical binary kernel, given a running kernel,
> the original compile time information needs to be passed to Kbuild.
> 
> Prerequisites:
>  * Original kernel config available (e.g extract it from /proc/config.gz)
>  * Identical source tree
>  * Identical toolchain
>  * Same source path if building with CONFIG_GCOV_KERNEL
>  * Module signature (CONFIG_MODULE_SIG) must be disabled

I think that only MODULE_SIG_ALL is problematic, the signature checking
code itself is OK.


> If that makes sense to cleanup I'll try to figure out what
> other config options could cause problems and submit it
> as a patch to kbuild.txt

Michal
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