Re: [OE-core] [RFC PATCH v2 1/2] bitbake.conf: Pad rpath and remove build ID in native binaries

[Jacob Kroon <jacob.kroon@gmail.com>]

On 12/2/21 00:11, Richard Purdie wrote:
> On Tue, 2021-11-30 at 23:37 +0100, Jacob Kroon wrote:
>> Try to make sure that the RUNTIME dynamic entry size is the same for all
>> binaries produced with the native compiler. This is necessary in order to
>> produce identical binaries when using differently sized buildpaths. I've
>> tried using only patchelf, and keeping the linker flags as they are, but
>> I am unable to produce identical binaries. Has anyone else managed to do
>> this with patchelf ? If not, maybe we can write a new tool that can handle it ?
>>
>> The build-id also needs to be removed since it is calculated based on
>> the data present at link time. This includes STAGING_LIBDIR_NATIVE
>> and STAGING_BASE_LIBDIR_NATIVE. Both will differ and they need to be temporarily
>> preserved since some recipes will execute the binaries during do_install()
>> (for example python3-native). Later on these are removed in chrpath.bbclass.
>>
>> This hack is the first step for producing identical native binaries when using
>> different build paths. 'zstd-native' is a working example.
>>
>> Signed-off-by: Jacob Kroon <jacob.kroon@gmail.com>
>> ---
>>  meta/classes/chrpath.bbclass | 3 +++
>>  meta/conf/bitbake.conf       | 5 ++++-
>>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> I'm a little torn on this. Our other option would be to hardcoded a specific
> dummy path and then edit it later to the correct value. That may be neater than
> adding the padding. It will change the end binaries but hopefully only after
> they're installed so should give the same net end result more neatly?
> 

Hmm not sure I follow. This patch adds a new dummy rpath entry,
"/rpath-padding-xxx...", then we remove it in chrpath. I don't know what
other value we would like to put there. If I understand you correctly,
we could perhaps pad one of the ones we already pass

-Wl,-rpath,${STAGING_LIBDIR_NATIVE}
-Wl,-rpath,${STAGING_BASE_LIBDIR_NATIVE}

with spaces, like:

-Wl,-rpath,${STAGING_LIBDIR_NATIVE}
-Wl,-rpath,"${STAGING_BASE_LIBDIR_NATIVE}${RPATH_PADDING}"

If that works that would be less intrusive I think.

> If we separate out the build-id patch we could hopefully get that piece merged
> as that shouldn't be controversial? 
> 

Yes, I can split it out into a separate patch.

But now that I've looked at this for a while, I've asked myself what
good does all this do ? The only optimization I can think of is that if
we rebuild a native recipes, and the sysroot component turns out the
same, then we don't need to create a new sstate cache entry. So we save
disk space, but disk space is cheap. We still need to build it. What I
would like is to have a common sstate dir for multiple build
directories. So if I build libtool-native in one build path, then at my
other build path it would just pick it up from sstate cache when I build
there. In the end, is that something that would be possible ?

Jacob