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

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

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.

Jan

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