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

[Nicolas Schier <nicolas@fjasle.eu>]

On Thu, Aug 01, 2024 at 11:37:30AM +0900, Masahiro Yamada wrote:
> On Thu, Aug 1, 2024 at 6:10 AM Nicolas Schier <nicolas@fjasle.eu> wrote:
> >
> > 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@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.
> >
> > 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?
> 
> 
> I meant the former.
> 
> 
> Debian applies a tricky patch to the tools build system
> in order to cross-compile objtool:
> 
> https://salsa.debian.org/kernel-team/linux/-/blob/debian/6.9.9-1/debian/patches/debian/fixdep-allow-overriding-hostcc-and-hostld.patch
> 
> It is not an elegant solution, though.
> 
> 
> I still believe the right thing to do is
> converting Makefiles for objtool and resolve_btfids into Kbuild style.
> 
> 
> objtool and resolve_btfids are necessary for building external modules,
> when CONFIG_OBJTOOL=y and CONFIG_DEBUG_INFO_BTF=y, respectively.
> If these comments are confusing, I can delete them.

I think it's good to mention that cross-built linux-headers package is
still broken for CONFIG_OBJTOOL=y and CONFIG_DEBUG_INFO_BTF=y.  I think
I'd add a sentence to the commit message and keep the comment here as it
is.

Kind regards,
Nicolas