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

[Richard Purdie <richard.purdie@linuxfoundation.org>]

On Thu, 2021-12-02 at 11:19 +0100, Jacob Kroon wrote:
> 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}"


I'm wondering if:

-Wl,-rpath,/not/exist/our-native-libdir-marker
-Wl,-rpath,/not/exist/our-native-base-libdir-marker

would work.

> 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 ?

We originally started here with gcc-cross so lets consider that and multiple
build directories where a patch changes gcc-cross in a way that is irrelavent to
the output.

The "win" is that regardless of whether I build in location A or B, I get the
same gcc-cross binary. Hash-equiv will then not rebuild the target binaries.
Yes, I pay the price of a gcc-cross rebuild but hashequiv saves the targets
rebuilding.

Currently it would only happen if you always build gcc-cross in a specific build
path.

Like everything, it is a question of looking at the changes and deciding whether
they are worth any maintenance burden/code complication or additional overhead
they generate. I don't know the answer here yet but I do appreciate the research
in helping get us data to make decisions on!

Cheers,

Richard