Re: [PATCH v2 05/11] kbuild: change working directory to external module directory with M=

[Charlie Jenkins <charlie@rivosinc.com>]

On Thu, Dec 05, 2024 at 11:48:08AM +0900, Masahiro Yamada wrote:
> On Thu, Dec 5, 2024 at 8:35 AM Charlie Jenkins <charlie@rivosinc.com> wrote:
> >
> > On Sun, Nov 10, 2024 at 10:34:33AM +0900, Masahiro Yamada wrote:
> > > Currently, Kbuild always operates in the output directory of the kernel,
> > > even when building external modules. This increases the risk of external
> > > module Makefiles attempting to write to the kernel directory.
> > >
> > > This commit switches the working directory to the external module
> > > directory, allowing the removal of the $(KBUILD_EXTMOD)/ prefix from
> > > some build artifacts.
> > >
> > > The command for building external modules maintains backward
> > > compatibility, but Makefiles that rely on working in the kernel
> > > directory may break. In such cases, $(objtree) and $(srctree) should
> > > be used to refer to the output and source directories of the kernel.
> > >
> > > The appearance of the build log will change as follows:
> > >
> > > [Before]
> > >
> > >   $ make -C /path/to/my/linux M=/path/to/my/externel/module
> > >   make: Entering directory '/path/to/my/linux'
> > >     CC [M]  /path/to/my/externel/module/helloworld.o
> > >     MODPOST /path/to/my/externel/module/Module.symvers
> > >     CC [M]  /path/to/my/externel/module/helloworld.mod.o
> > >     CC [M]  /path/to/my/externel/module/.module-common.o
> > >     LD [M]  /path/to/my/externel/module/helloworld.ko
> > >   make: Leaving directory '/path/to/my/linux'
> > >
> > > [After]
> > >
> > >   $ make -C /path/to/my/linux M=/path/to/my/externel/module
> > >   make: Entering directory '/path/to/my/linux'
> > >   make[1]: Entering directory '/path/to/my/externel/module'
> > >     CC [M]  helloworld.o
> > >     MODPOST Module.symvers
> > >     CC [M]  helloworld.mod.o
> > >     CC [M]  .module-common.o
> > >     LD [M]  helloworld.ko
> > >   make[1]: Leaving directory '/path/to/my/externel/module'
> > >   make: Leaving directory '/path/to/my/linux'
> > >
> > > Printing "Entering directory" twice is cumbersome. This will be
> > > addressed later.
> >
> > This change has caused O=<relative directory> to fail.
> >
> > For example:
> >
> > make O=build defconfig
> > make -j$(nproc) V=1 O=build bindeb-pkg
> >
> > outputs:
> >
> > make ARCH=x86 KERNELRELEASE=6.13.0-rc1 KBUILD_BUILD_VERSION=3  run-command KBUILD_RUN_COMMAND='+$(srctree)/scripts/package/builddeb linux-libc-dev'
> > dh_installchangelogs -plinux-image-6.13.0-rc1
> > ../scripts/package/builddeb linux-headers-6.13.0-rc1
> > dh_compress -plinux-image-6.13.0-rc1
> > dh_fixperms -plinux-image-6.13.0-rc1
> > dh_gencontrol -plinux-image-6.13.0-rc1 -- -fdebian/image.files
> > Rebuilding host programs with x86_64-linux-gnu-gcc...
> > make[6]: Entering directory '/scratch/kernels/linux/build'
> > /scratch/kernels/linux/Makefile:190: *** specified kernel directory "build" does not exist.  Stop.
> >
> > It is stepping into this directory and then trying to find the directory
> > it just stepped into so $(realpath $(KBUILD_OUTPUT)) returns an empty
> > string.
> >
> > Using an absolute directory resolves this problem, but I believe it
> > shouldn't be necessary.
> 
> 
> Agree.
> 
> I will apply the following fixup unless I have a better idea.
> 
> 
> 
> 
> diff --git a/scripts/package/install-extmod-build
> b/scripts/package/install-extmod-build
> index 64d958ee45f3..85af1573db31 100755
> --- a/scripts/package/install-extmod-build
> +++ b/scripts/package/install-extmod-build
> @@ -69,7 +69,7 @@ if [ "${CC}" != "${HOSTCC}" ]; then
>         #
>         # Use the single-target build to avoid the modpost invocation, which
>         # would overwrite Module.symvers.
> -       "${MAKE}" HOSTCC="${CC}" KBUILD_EXTMOD="${destdir}" scripts/
> +       "${MAKE}" O=. HOSTCC="${CC}" KBUILD_EXTMOD="${destdir}" scripts/
> 
>         cat <<-'EOF' >  "${destdir}/scripts/Kbuild"
>         subdir-y := basic
> @@ -78,7 +78,7 @@ if [ "${CC}" != "${HOSTCC}" ]; then
>         EOF
> 
>         # Run once again to rebuild scripts/basic/ and scripts/mod/modpost.
> -       "${MAKE}" HOSTCC="${CC}" KBUILD_EXTMOD="${destdir}" scripts/
> +       "${MAKE}" O=. HOSTCC="${CC}" KBUILD_EXTMOD="${destdir}" scripts/
> 
>         rm -f "${destdir}/Kbuild" "${destdir}/scripts/Kbuild"
>  fi
> 

Looks fine to me :)

If this is what you decide on, feel free to add my tags.

Tested-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>

> 
> 
> --
> Best Regards
> Masahiro Yamada