[RFC,1/1] scripts: Build per module Rust crates

  Enables compiling Rust crates as dependencies of kernel modules.

When a composite object depends on an `.rlib` file, which by the way is
a current ar archive, Kbuild will compile it from its base Rust source
and link it.

This makes possible to have Rust bindings for a subsystem that is
compiled as a module.

Signed-off-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
---
 Makefile               |  4 ++--
 scripts/Makefile.build | 10 +++++++++-
 scripts/Makefile.lib   | 19 +++++++++++++------
 3 files changed, 24 insertions(+), 9 deletions(-)
  

On Sat, Oct 7, 2023 at 12:57 AM Martin Rodriguez Reboredo
<yakoyoku@gmail.com> wrote:
>
> Enables compiling Rust crates as dependencies of kernel modules.
>
> When a composite object depends on an `.rlib` file, which by the way is
> a current ar archive, Kbuild will compile it from its base Rust source
> and link it.
>
> This makes possible to have Rust bindings for a subsystem that is
> compiled as a module.
>
> Signed-off-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>


I could not understand how this will work because
there is no explanation how to use *.rlib in the later
build steps.

If I understand correctly, does this intend to link *.rlib
as a part of modules?

In C, there was a discussion about
"that would be nice to be able to link static libraries",
but we do not do it any more. [1]

Following that discussion, linking libraries does not
seem what we want to do.


[1]: https://lore.kernel.org/lkml/20200106032324.3147-1-masahiroy@kernel.org/




> ---
>  Makefile               |  4 ++--
>  scripts/Makefile.build | 10 +++++++++-
>  scripts/Makefile.lib   | 19 +++++++++++++------
>  3 files changed, 24 insertions(+), 9 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 7d6be12e4c3e..7774c97e8aa0 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -283,7 +283,7 @@ no-compiler-targets := $(no-dot-config-targets) install dtbs_install \
>                         headers_install modules_install modules_sign kernelrelease image_name
>  no-sync-config-targets := $(no-dot-config-targets) %install modules_sign kernelrelease \
>                           image_name
> -single-targets := %.a %.i %.ko %.lds %.ll %.lst %.mod %.o %.rsi %.s %.symtypes %/
> +single-targets := %.a %.i %.ko %.lds %.ll %.lst %.mod %.o %.rlib %.rsi %.s %.symtypes %/
>
>  config-build   :=
>  mixed-build    :=
> @@ -1919,7 +1919,7 @@ $(clean-dirs):
>  clean: $(clean-dirs)
>         $(call cmd,rmfiles)
>         @find $(or $(KBUILD_EXTMOD), .) $(RCS_FIND_IGNORE) \
> -               \( -name '*.[aios]' -o -name '*.rsi' -o -name '*.ko' -o -name '.*.cmd' \
> +               \( -name '*.[aios]' -o -name '*.rlib' -o -name '*.rsi' -o -name '*.ko' -o -name '.*.cmd' \
>                 -o -name '*.ko.*' \
>                 -o -name '*.dtb' -o -name '*.dtbo' \
>                 -o -name '*.dtb.S' -o -name '*.dtbo.S' \
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index da37bfa97211..627010518b27 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -246,7 +246,9 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
>  # To make this rule robust against "Argument list too long" error,
>  # ensure to add $(obj)/ prefix by a shell command.
>  cmd_mod = printf '%s\n' $(call real-search, $*.o, .o, -objs -y -m) | \
> -       $(AWK) '!x[$$0]++ { print("$(obj)/"$$0) }' > $@
> +       $(AWK) '!x[$$0]++ { print("$(obj)/"$$0) }' > $@ && \
> +       printf '%s\n' $(call real-search, $*.rlib, .rlib, -objs -y -m) | \
> +       $(AWK) '!x[$$0]++ { print("--library=$(obj)/"$$0) }' >> $@
>
>  $(obj)/%.mod: FORCE
>         $(call if_changed,mod)
> @@ -291,6 +293,12 @@ quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
>  $(obj)/%.o: $(src)/%.rs FORCE
>         $(call if_changed_dep,rustc_o_rs)
>
> +quiet_cmd_rustc_rlib_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
> +      cmd_rustc_rlib_rs = $(rust_common_cmd) -Ccodegen-units=1 --emit=link=$@ $<
> +
> +$(obj)/%.rlib: $(src)/%.rs FORCE
> +       $(call if_changed_dep,rustc_rlib_rs)
> +
>  quiet_cmd_rustc_rsi_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
>        cmd_rustc_rsi_rs = \
>         $(rust_common_cmd) -Zunpretty=expanded $< >$@; \
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 68d0134bdbf9..6e8cfbad015d 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -53,14 +53,18 @@ multi-search = $(sort $(foreach m, $1, $(if $(call suffix-search, $m, $2, $3 -),
>  real-search = $(foreach m, $1, $(if $(call suffix-search, $m, $2, $3 -), $(call suffix-search, $m, $2, $3), $m))
>
>  # If $(foo-objs), $(foo-y), $(foo-m), or $(foo-) exists, foo.o is a composite object
> -multi-obj-y := $(call multi-search, $(obj-y), .o, -objs -y)
> -multi-obj-m := $(call multi-search, $(obj-m), .o, -objs -y -m)
> +multi-obj-y := $(call multi-search, $(obj-y), .o, -objs -y) \
> +       $(call multi-search, $(obj-y), .rlib, -objs -y)
> +multi-obj-m := $(call multi-search, $(obj-m), .o, -objs -y -m) \
> +       $(call multi-search, $(obj-m), .rlib, -objs -y -m)
>  multi-obj-ym := $(multi-obj-y) $(multi-obj-m)
>
>  # Replace multi-part objects by their individual parts,
>  # including built-in.a from subdirectories
> -real-obj-y := $(call real-search, $(obj-y), .o, -objs -y)
> -real-obj-m := $(call real-search, $(obj-m), .o, -objs -y -m)
> +real-obj-y := $(call real-search, $(obj-y), .o, -objs -y) \
> +       $(call real-search, $(obj-y), .rlib, -objs -y)
> +real-obj-m := $(call real-search, $(obj-m), .o, -objs -y -m) \
> +       $(call real-search, $(obj-m), .rlib, -objs -y -m)
>
>  always-y += $(always-m)
>
> @@ -107,7 +111,8 @@ endif
>  # Finds the multi-part object the current object will be linked into.
>  # If the object belongs to two or more multi-part objects, list them all.
>  modname-multi = $(sort $(foreach m,$(multi-obj-ym),\
> -               $(if $(filter $*.o, $(call suffix-search, $m, .o, -objs -y -m)),$(m:.o=))))
> +               $(if $(filter $*.o, $(call suffix-search, $m, .o, -objs -y -m)),$(m:.o=)) \
> +               $(if $(filter $*.rlib, $(call suffix-search, $m, .rlib, -objs -y -m)),$(m:.rlib=))))
>
>  __modname = $(or $(modname-multi),$(basetarget))
>
> @@ -210,7 +215,9 @@ _cpp_flags += -I $(srctree)/$(src) -I $(objtree)/$(obj)
>  endif
>  endif
>
> -part-of-module = $(if $(filter $(basename $@).o, $(real-obj-m)),y)
> +part-of-module =                                             \
> +       $(if $(or $(filter $(basename $@).o, $(real-obj-m)), \
> +               $(filter $(basename $@).rlib, $(real-obj-m))),y)
>  quiet_modtag = $(if $(part-of-module),[M],   )
>
>  modkern_cflags =                                          \
> --
> 2.42.0
>


--
Best Regards
Masahiro Yamada
  

On Sun, Oct 22, 2023 at 6:22 PM Masahiro Yamada <masahiroy@kernel.org> wrote:
>
> I could not understand how this will work because
> there is no explanation how to use *.rlib in the later
> build steps.

It seems linux-kbuild was not Cc'd in the cover letter -- I replied to
Martin there: https://lore.kernel.org/rust-for-linux/20231006155739.246381-1-yakoyoku@gmail.com/

Cheers,
Miguel
  

On 10/22/23 13:21, Masahiro Yamada wrote:
> On Sat, Oct 7, 2023 at 12:57 AM Martin Rodriguez Reboredo
> <yakoyoku@gmail.com> wrote:
>>
>> Enables compiling Rust crates as dependencies of kernel modules.
>>
>> When a composite object depends on an `.rlib` file, which by the way is
>> a current ar archive, Kbuild will compile it from its base Rust source
>> and link it.
>>
>> This makes possible to have Rust bindings for a subsystem that is
>> compiled as a module.
>>
>> Signed-off-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> 
> 
> I could not understand how this will work because
> there is no explanation how to use *.rlib in the later
> build steps.
> 
> If I understand correctly, does this intend to link *.rlib
> as a part of modules?

Yes, the purpose of this RFC is to link the *.rlib, its members
specifically, as part of modules to avoid the scenario of having
Rust bindings linked statically and their C counterparts dynamically.

I'll write a v2 that'll clarify its usage. I've made more progress
locally and had run tests with success.