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Subject: Re: [RFCv2 PATCH 0/3] Salted build ids via linker sections
To:     Masahiro Yamada <yamada.masahiro@socionext.com>
Cc:     Andy Lutomirski <luto@kernel.org>, mjw@fedoraproject.org,
        "H . J . Lu" <hjl.tools@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        X86 ML <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nick Clifton <nickc@redhat.com>,
        Cary Coutant <ccoutant@gmail.com>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>
References: <20180329180112.11055-1-labbott@redhat.com>
 <CAK7LNATTfjpkG0PLR5_anZOW5zfSZxKgFO6sHXuMf50OEtx0PQ@mail.gmail.com>
From:   Laura Abbott <labbott@redhat.com>
Message-ID: <fb123c1f-7680-9e75-125a-e4e66f0f9c93@redhat.com>
Date:   Mon, 14 May 2018 14:02:07 -0700
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On 05/06/2018 11:28 PM, Masahiro Yamada wrote:
> 2018-03-30 3:01 GMT+09:00 Laura Abbott <labbott@redhat.com>:
>> Hi,
>>
>> This is v2 of my proposal to allow unique build-ids in the kernel. from
>> last time:
>>
>> ""
>> In Fedora, the debug information is packaged separately (foo-debuginfo) and
>> can be installed separately. There's been a long standing issue where only one
>> version of a debuginfo info package can be installed at a time. Mark Wielaard
>> made an effort for Fedora 27 to allow parallel installation of debuginfo (see
>> https://fedoraproject.org/wiki/Changes/ParallelInstallableDebuginfo for
>> more details)
>>
>> Part of the requirement to allow this to work is that build ids are
>> unique between builds. The existing upstream rpm implementation ensures
>> this by re-calculating the build-id using the version and release as a
>> seed. This doesn't work 100% for the kernel because of the vDSO which is
>> its own binary and doesn't get updated. After poking holes in a few of my
>> ideas, there was a discussion with some people from the binutils team about
>> adding --build-id-salt to let ld do the calculation debugedit is doing. There
>> was a counter proposal made about adding some extra information via a .comment
>> which will affect the build id calculation but just get stripped out.
>> ""
> 
> 
> I think you already know '--build-id=uuid' linker option.
> 
> Doesn't this solve your problem?
> 
> The disadvantage of this option is,
> we will lose reproducible building because  --build-id=uuid
> adds every time random salt.
> 
> The advantage is, the implementation is even simpler,
> and easier to migrate to --build-id-salt once it is supported
> in the future.
> 
> 

It could, theoretically. The reproducibility is nice though and
I'd like to keep the kernel close to matching what other packages
do though.

Thanks,
Laura

>> This v2 cleans up the naming to be consistent and also switches to a
>> config option vs. an environment variable. I've seen some sporadic
>> failures about missing the generated header so I think I'm still missing
>> a dependency somewhere.
> 
> Right.
> 
> There is no dependency between 'prepare' and 'scripts'
> in the top Makefile.
> Therefore, Kbuild can run them simultaneously,
> which would cause a race in parallel building.
> 
> 
> 
> 
> 
> 
> 
> 
>   I'm still mostly looking for feedback whether
>> this would be acceptable for merging or if we should just persue a
>> --build-id-salt in binutils.
>>
>> Thanks,
>> Laura
>>
>>
>> Laura Abbott (3):
>>    kbuild: Introduce build-salt generated header
>>    kbuild: Link with generated build-salt header
>>    x86/vdso: Add build salt to the vDSO
>>
>>   Makefile                              | 13 +++++++++++--
>>   arch/x86/entry/vdso/vdso-layout.lds.S |  3 +++
>>   init/Kconfig                          |  8 ++++++++
>>   scripts/.gitignore                    |  1 +
>>   scripts/Makefile                      |  2 +-
>>   scripts/build-salt.lds.S              |  5 +++++
>>   scripts/gensalt                       | 21 +++++++++++++++++++++
>>   scripts/link-vmlinux.sh               |  3 ++-
>>   8 files changed, 52 insertions(+), 4 deletions(-)
>>   create mode 100644 scripts/build-salt.lds.S
>>   create mode 100755 scripts/gensalt
>>
>> --
>> 2.16.2
>>
>> --
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> 
> 
>