Re: [PATCH 4/4] kbuild: cross-compile linux-headers package when possible

[Masahiro Yamada]

On Thu, Aug 1, 2024 at 6:10 AM Nicolas Schier <nicolas@xxxxxxxxx> wrote:
>
> On Sat, Jul 27, 2024 at 04:42:04PM +0900, Masahiro Yamada wrote:
> > A long standing issue in the upstream kernel packaging is that the
> > linux-headers package is not cross-compiled.
> >
> > For example, you can cross-build Debian packages for arm64 by running
> > the following command:
> >
> >   $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bindeb-pkg
> >
> > However, the generated linux-headers-*_arm64.deb is useless because the
> > host programs in it were built for your build machine architecture
> > (likely x86), not arm64.
> >
> > The Debian kernel maintains its own Makefiles to cross-compile host
> > tools without relying on Kbuild. [1]
> >
> > Instead of adding such full custom Makefiles, this commit adds a small
> > piece of code to cross-compile host programs located under the scripts/
> > directory.
> >
> > A straightforward solution is to pass HOSTCC=${CROSS_COMPILE}gcc, but it
> > would also cross-compile scripts/basic/fixdep, which needs to be native
> > to process the if_changed_dep macro. (This approach may work under some
> > circumstances; you can execute foreign architecture programs with the
> > help of binfmt_misc because Debian systems enable CONFIG_BINFMT_MISC,
> > but it would require installing QEMU and libc for that architecture.)
> >
> > A trick is to use the external module build (KBUILD_EXTMOD=), which
> > does not rebuild scripts/basic/fixdep. ${CC} needs to be able to link
> > userspace programs (CONFIG_CC_CAN_LINK=y).
> >
> > There are known limitations:
> >
> >  - GCC plugins
> >
> >    It would possible to rebuild GCC plugins for the target architecture
> >    by passing HOSTCXX=${CROSS_COMPILE}g++ with necessary packages
> >    installed, but gcc on the installed system emits
> >    "cc1: error: incompatible gcc/plugin versions". I did not find a
> >    solution for this because 'gcc' on a foreign architecture is a
> >    different compiler after all.
> >
> >  - objtool and resolve_btfids
> >
> >    These are built by the tools build system. They are not covered by
> >    the current solution.
> >
> > I only tested this with Debian, but it should work for other package
> > systems as well.
> >
> > [1]: https://salsa.debian.org/kernel-team/linux/-/blob/debian/6.9.9-1/debian/rules.real#L586
> >
> > Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx>
> > ---
> >
> >  scripts/package/install-extmod-build | 34 ++++++++++++++++++++++++++++
> >  1 file changed, 34 insertions(+)
> >
> > diff --git a/scripts/package/install-extmod-build b/scripts/package/install-extmod-build
> > index cc335945dfbc..0b56d3d7b48f 100755
> > --- a/scripts/package/install-extmod-build
> > +++ b/scripts/package/install-extmod-build
> > @@ -43,4 +43,38 @@ mkdir -p "${destdir}"
> >       fi
> >  } | tar -c -f - -T - | tar -xf - -C "${destdir}"
> >
> > +# When ${CC} and ${HOSTCC} differ, we are likely cross-compiling. Rebuild host
> > +# programs using ${CC}. This assumes CC=${CROSS_COMPILE}gcc, which is usually
> > +# the case for package building. It does not cross-compile when CC=clang.
> > +#
> > +# This caters to host programs that participate in Kbuild. objtool and
> > +# resolve_btfids are out of scope.
>
> Just for clarification: Why do you call both "out of scope" here?
> Because they're not being built by kbuild, or because they will never be
> needed for building oot kmods?


I meant the former.


Debian applies a tricky patch to the tools build system
in order to cross-compile objtool:

https://salsa.debian.org/kernel-team/linux/-/blob/debian/6.9.9-1/debian/patches/debian/fixdep-allow-overriding-hostcc-and-hostld.patch

It is not an elegant solution, though.


I still believe the right thing to do is
converting Makefiles for objtool and resolve_btfids into Kbuild style.


objtool and resolve_btfids are necessary for building external modules,
when CONFIG_OBJTOOL=y and CONFIG_DEBUG_INFO_BTF=y, respectively.
If these comments are confusing, I can delete them.







>
> > +if [ "${CC}" != "${HOSTCC}" ] && is_enabled CONFIG_CC_CAN_LINK; then
> > +     echo "Rebuilding host programs with ${CC}..."
> > +
> > +     cat <<-'EOF' >  "${destdir}/Kbuild"
> > +     subdir-y := scripts
> > +     EOF
> > +
> > +     # HOSTCXX is not overridden. The C++ compiler is used to build:
> > +     # - scripts/kconfig/qconf, which is unneeded for external module builds
> > +     # - GCC plugins, which will not work on the installed system even with
> > +     #   being rebuilt.
> > +     #
> > +     # Use the single-target build to avoid the modpost invocation, which
> > +     # would overwrite Module.symvers.
> > +     "${MAKE}" HOSTCC="${CC}" KBUILD_EXTMOD="${destdir}" scripts/
> > +
> > +     cat <<-'EOF' >  "${destdir}/scripts/Kbuild"
> > +     subdir-y := basic
> > +     hostprogs-always-y := mod/modpost
> > +     mod/modpost-objs := $(addprefix mod/, modpost.o file2alias.o sumversion.o symsearch.o)
> > +     EOF
> > +
> > +     # Run once again to rebuild scripts/basic/ and scripts/mod/modpost.
> > +     "${MAKE}" HOSTCC="${CC}" KBUILD_EXTMOD="${destdir}" scripts/
> > +
> > +     rm -f "${destdir}/Kbuild" "${destdir}/scripts/Kbuild"
> > +fi
> > +
> >  find "${destdir}" \( -name '.*.cmd' -o -name '*.o' \) -delete
> > --
> > 2.43.0
> >
> >
>
> Thanks for fixing this; looks good to me.
>
> Reviewed-by: Nicolas Schier <nicolas@xxxxxxxxx>
>


-- 
Best Regards
Masahiro Yamada