kbuild: refactor cross-compiling linux-headers package

Since commit 13b2548 ("kbuild: change working directory to external
module directory with M="), when cross-building host programs for the
linux-headers package, the "Entering directory" and "Leaving directory"
messages appear multiple times, and each object path shown is relative
to the working directory. This makes it difficult to track which objects
are being rebuilt.

In hindsight, using the external module build (M=) was not a good idea.

This commit simplifies the script by leveraging the run-command target,
resulting in a cleaner build log again.

[Before]

  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bindeb-pkg
    [ snip ]
  Rebuilding host programs with aarch64-linux-gnu-gcc...
  make[5]: Entering directory '/home/masahiro/linux'
  make[6]: Entering directory '/home/masahiro/linux/debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+'
    HOSTCC  scripts/kallsyms
    HOSTCC  scripts/sorttable
    HOSTCC  scripts/asn1_compiler
  make[6]: Leaving directory '/home/masahiro/linux/debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+'
  make[5]: Leaving directory '/home/masahiro/linux'
  make[5]: Entering directory '/home/masahiro/linux'
  make[6]: Entering directory '/home/masahiro/linux/debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+'
    HOSTCC  scripts/basic/fixdep
    HOSTCC  scripts/mod/modpost.o
    HOSTCC  scripts/mod/file2alias.o
    HOSTCC  scripts/mod/sumversion.o
    HOSTCC  scripts/mod/symsearch.o
    HOSTLD  scripts/mod/modpost
  make[6]: Leaving directory '/home/masahiro/linux/debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+'
  make[5]: Leaving directory '/home/masahiro/linux'

[After]

  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bindeb-pkg
    [ snip ]
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/basic/fixdep
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/kallsyms
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/sorttable
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/asn1_compiler
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/mod/modpost.o
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/mod/file2alias.o
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/mod/sumversion.o
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/mod/symsearch.o
    HOSTLD  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/mod/modpost

Signed-off-by: Masahiro Yamada <[email protected]>

Author: masahir0y

scripts/package/install-extmod-build

@@ -49,38 +49,23 @@ mkdir -p "${destdir}"
 # This caters to host programs that participate in Kbuild. objtool and
 # resolve_btfids are out of scope.
 if [ "${CC}" != "${HOSTCC}" ]; then
-	echo "Rebuilding host programs with ${CC}..."
-
-	# This leverages external module building.
-	# - Clear sub_make_done to allow the top-level Makefile to redo sub-make.
-	# - Filter out --no-print-directory to print "Entering directory" logs
-	#   when Make changes the working directory.
-	unset sub_make_done
-	MAKEFLAGS=$(echo "${MAKEFLAGS}" | sed s/--no-print-directory//)
-
-	cat <<-'EOF' >  "${destdir}/Kbuild"
-	subdir-y := scripts
+	cat "${destdir}/scripts/Makefile" - <<-'EOF' > "${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
 
 	# 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_OUTPUT=. 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="${CC}" KBUILD_OUTPUT=. KBUILD_EXTMOD="${destdir}" scripts/
+	# Clear VPATH and srcroot because the source files reside in the output
+	# directory.
+	# shellcheck disable=SC2016 # $(MAKE), $(CC), and $(build) will be expanded by Make
+	"${MAKE}" run-command KBUILD_RUN_COMMAND='+$(MAKE) HOSTCC=$(CC) VPATH= srcroot=. $(build)='"${destdir}"/scripts
 
-	rm -f "${destdir}/Kbuild" "${destdir}/scripts/Kbuild"
+	rm -f "${destdir}/scripts/Kbuild"
 fi
 
 find "${destdir}" \( -name '.*.cmd' -o -name '*.o' \) -delete