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

[Nicolas Schier <nicolas@fjasle.eu>]

On Thu, Oct 17, 2024 at 07:45:57AM -0700 Ron Economos wrote:
> On 7/27/24 12:42 AM, 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@kernel.org>
> > ---
> > 
> >   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.
> > +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
> 
> This patch causes a build error when cross-compiling for RISC-V. I'm using
> the cross-compiler from https://github.com/riscv-collab/riscv-gnu-toolchain.
> When trying to build .debs with:
> 
> make CROSS_COMPILE=riscv64-unknown-linux-gnu- ARCH=riscv INSTALL_MOD_STRIP=1
> "KCFLAGS=-mtune=sifive-7-series" LOCALVERSION= bindeb-pkg
> 
> I get the following error:
> 
> Rebuilding host programs with riscv64-unknown-linux-gnu-gcc...
>   HOSTCC debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/genksyms/genksyms.o
>   YACC debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/genksyms/parse.tab.[ch]
>   HOSTCC debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/genksyms/parse.tab.o
>   LEX debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/genksyms/lex.lex.c
>   HOSTCC debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/genksyms/lex.lex.o
>   HOSTLD debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/genksyms/genksyms
>   HOSTCC debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/selinux/genheaders/genheaders
>   HOSTCC debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/selinux/mdp/mdp
>   HOSTCC debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/kallsyms
>   HOSTCC debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/sorttable
>   HOSTCC debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/asn1_compiler
>   HOSTCC debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/sign-file
> 
> debian/linux-headers-6.12.0-rc3/usr/src/linux-headers-6.12.0-rc3/scripts/sign-file.c:25:10:
> fatal error: openssl/opensslv.h: No such file or directory
>    25 | #include <openssl/opensslv.h>
>       |          ^~~~~~~~~~~~~~~~~~~~
> compilation terminated.

I guess you have openssl/opensslv.h available on your system, do you?  (In
Debian/Ubuntu package libssl-dev or similar)

Can you natively build a kernel with a similar kernel config?

Kind regards,
Nicolas