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

[Nicolas Schier]

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?

> +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>