From f1d87664b82aeeaa1be9ee22dc85a59fd5a60d63 Mon Sep 17 00:00:00 2001 From: Masahiro Yamada Date: Sat, 27 Jul 2024 16:42:04 +0900 Subject: [PATCH] kbuild: cross-compile linux-headers package when possible 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". - objtool and resolve_btfids These are built by the tools build system. They are not covered by the current solution. The resulting linux-headers package is broken if CONFIG_OBJTOOL or CONFIG_DEBUG_INFO_BTF is enabled. 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 Reviewed-by: Nicolas Schier --- 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 9fee4a3236cc..d2c9cacecc0c 100755 --- a/scripts/package/install-extmod-build +++ b/scripts/package/install-extmod-build @@ -44,4 +44,38 @@ 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. +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 after + # 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