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 <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

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