Re: [PATCH 1/1] add support to set KBUILD_BUILD_TIMESTAMP for custom kernels

[Jan Kiszka <jan.kiszka@siemens.com>]

On 22.03.22 18:27, Moessbauer, Felix wrote:
>> -----Original Message-----
>> From: Kiszka, Jan (T CED) <jan.kiszka@siemens.com>
>> Sent: Tuesday, March 22, 2022 3:49 PM
>> To: Moessbauer, Felix (T CED SES-DE) <felix.moessbauer@siemens.com>
>> Cc: isar-users@googlegroups.com
>> Subject: Re: [PATCH 1/1] add support to set KBUILD_BUILD_TIMESTAMP for
>> custom kernels
>>
>> On 22.03.22 09:54, Moessbauer, Felix (T CED SES-DE) wrote:
>>>> -----Original Message-----
>>>> From: Kiszka, Jan (T CED) <jan.kiszka@siemens.com>
>>>> Sent: Monday, March 21, 2022 6:52 PM
>>>> To: Moessbauer, Felix (T CED SES-DE) <felix.moessbauer@siemens.com>;
>>>> isar- users@googlegroups.com
>>>> Subject: Re: [PATCH 1/1] add support to set KBUILD_BUILD_TIMESTAMP
>>>> for custom kernels
>>>>
>>>> On 21.03.22 18:33, Moessbauer, Felix (T CED SES-DE) wrote:
>>>>> Hi Jan,
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: Kiszka, Jan (T CED) <jan.kiszka@siemens.com>
>>>>>> Sent: Monday, March 21, 2022 6:18 PM
>>>>>> To: Moessbauer, Felix (T CED SES-DE)
>>>>>> <felix.moessbauer@siemens.com>;
>>>>>> isar- users@googlegroups.com
>>>>>> Subject: Re: [PATCH 1/1] add support to set KBUILD_BUILD_TIMESTAMP
>>>>>> for custom kernels
>>>>>>
>>>>>> On 21.03.22 15:40, Felix Moessbauer wrote:
>>>>>>> This patch adds the infrastructure to overwrite the
>>>>>>> KBUILD_BUILD_TIMESTAMP environment variable for custom kernel
>> builds.
>>>>>>> By that, we improve the support for reproducible kernel builds.
>>>>>>>
>>>>>>
>>>>>> Agreed.
>>>>>>
>>>>>>> Setting this to a fixed string further helps to profit from ccache
>>>>>>> caching on repeated builds.
>>>>>>
>>>>>> Hmm, but the practical impact of this variable should have been
>>>>>> limited /wrt ccache. I was seeing significant build-time reduction
>>>>>> before the
>>>> commit already.
>>>>>
>>>>> Yes, but of course depends on configuration.
>>>>>
>>>>>>
>>>>>>>
>>>>>>> Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
>>>>>>> ---
>>>>>>>  meta/recipes-kernel/linux/files/debian/isar/build.tmpl  | 1 +
>>>>>>> meta/recipes-kernel/linux/files/debian/isar/common.tmpl | 2 ++
>>>>>>>  meta/recipes-kernel/linux/linux-custom.inc              | 3 +++
>>>>>>>  3 files changed, 6 insertions(+)
>>>>>>>
>>>>>>> diff --git
>>>>>>> a/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
>>>>>>> b/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
>>>>>>> index b7749cec..32d2eb23 100644
>>>>>>> --- a/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
>>>>>>> +++ b/meta/recipes-kernel/linux/files/debian/isar/build.tmpl
>>>>>>> @@ -34,6 +34,7 @@ print_settings() {  # ---------------  #
>>>>>>> ARCH=${ARCH}  # CROSS_COMPILE=${CROSS_COMPILE}
>>>>>>> +# KBUILD_BUILD_TIMESTAMP=${KBUILD_BUILD_TIMESTAMP}
>>>>>>>  EOF
>>>>>>>  }
>>>>>>>
>>>>>>> diff --git
>>>>>>> a/meta/recipes-kernel/linux/files/debian/isar/common.tmpl
>>>>>>> b/meta/recipes-kernel/linux/files/debian/isar/common.tmpl
>>>>>>> index b7752fb6..ffe84745 100644
>>>>>>> --- a/meta/recipes-kernel/linux/files/debian/isar/common.tmpl
>>>>>>> +++ b/meta/recipes-kernel/linux/files/debian/isar/common.tmpl
>>>>>>> @@ -7,6 +7,7 @@ ARCH=${KERNEL_ARCH}
>>>>>>> KERNEL_PKG_IMAGE=linux-image-${KERNEL_NAME_PROVIDED}
>>>>>>>  KERNEL_PKG_KERN_HEADERS=linux-headers-
>> ${KERNEL_NAME_PROVIDED}
>>>>>>>  KERNEL_PKG_LIBC_HEADERS=linux-libc-dev
>>>>>>> +KBUILD_BUILD_TIMESTAMP="${KBUILD_BUILD_TIMESTAMP}"
>>>>>>>
>>>>>>>  # Constants
>>>>>>>  KCONF=.config
>>>>>>> @@ -36,6 +37,7 @@ main() {
>>>>>>>
>>>>>>>      # variables to be exported
>>>>>>>      export ARCH
>>>>>>> +    export KBUILD_BUILD_TIMESTAMP
>>>>>>>
>>>>>>>      # are we cross-compiling?
>>>>>>>      BUILD_ARCH=$(dpkg-architecture -qDEB_BUILD_ARCH) diff --git
>>>>>>> a/meta/recipes-kernel/linux/linux-custom.inc
>>>>>>> b/meta/recipes-kernel/linux/linux-custom.inc
>>>>>>> index 8a5dbf82..f8e87453 100644
>>>>>>> --- a/meta/recipes-kernel/linux/linux-custom.inc
>>>>>>> +++ b/meta/recipes-kernel/linux/linux-custom.inc
>>>>>>> @@ -33,6 +33,8 @@ KERNEL_HEADERS_DEBIAN_DEPENDS ?= "libc6, \
>>>>>>>
>>>>>>>  KERNEL_LIBC_DEV_DEPLOY ?= "0"
>>>>>>>
>>>>>>> +KBUILD_BUILD_TIMESTAMP ??= "LANG=C $(date)"
>>>>>>
>>>>>> Can't we use SOURCE_DATE_EPOCH like the rest?
>>>>>
>>>>> This variable is still not honored by the kernel (AFAIK).
>>>>> Also the kernel documentation only mentions KBUILD_BUILD_TIMESTAMP.
>>>>> There is a patch from 2015 which has not been applied [1].
>>>>>
>>>>> We could do the mapping in ISAR, but I prefer to have it explicit.
>>>>>
>>>>>
>>>>
>>>> Then how does the regular Debian kernel set a stable date? We should
>>>> align to that and surely not force the user to set anything manually
>>>> (that is my interpretation of the behavior of the current patch).
>>>
>>> They set it in the rules file to "$(SOURCE_DATE)" [1], which itself is computed
>> from the changelog:
>>> SOURCE_DATE := $(shell dpkg-parsechangelog -SDate)
>>>
>>> Unfortunately, this is not possible for us because we generate the changelog
>> on the fly.
>>> Setting "KBUILD_BUILD_TIMESTAMP" to the current build time (as
>>> default) should be reasonable, Simply because we do not have better
>> information. That's also how it was previously.
>>> The only addition here is do give the user support to set it to something
>> different.
>>>
>>
>> There must be more information in the build env because other packages that
>> do evaluate SOURCE_DATE_EPOCH find that properly set, automatically.
> 
> Well... the SOURCE_DATE_EPOCH variable is set by debhelper to the date of the last entry in the changelog [1], [2].
> This is technically the same as stated above, but already happens automatically.
> 
> In our use-case, that does not help much, as the changelog is auto-generated by ISAR.

It does help because it will already deliver a stable (even if
meaningless) timestamp.

And if there is a changelog that is appended, its last entry date is used.

> Actually, it points to the begin of the Unix epoch, which is questionable.
> At least there, we should add the current build time (and add an option to overwrite, otherwise dpkg-raw recipes can no longer be cached).

That would be counterproductive for reproducibility and was the reason
to pin it to 0 not that long ago (53d315fdd3f3).

If you want proper changelog with proper dates, don't use deb_add_changelog.

Jan

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