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>