[RFC PATCH v2 1/2] kbuild: Build Rust crates as libraries

From: Martin Rodriguez Reboredo
Date: Thu Oct 26 2023 - 20:35:31 EST

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Enables compiling Rust crates as dependencies of kernel modules. These
are also meant to be used as libraries for other parts of the kernel.

For these crates `bindings` is also exposed to them and they get their
symbols exported for them to be used by other code.

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 archive it.

This makes possible to have Rust bindings for a subsystem that is
compiled either built-in or as a module. They can also be made into
modules by themselves too.

Signed-off-by: Martin Rodriguez Reboredo <yakoyoku@xxxxxxxxx>
---
v1 -> v2:
- Fixed kernel compilation.
- Added support for building crates as modules.

.gitignore | 2 ++
Makefile | 4 ++--
scripts/Makefile.build | 49 +++++++++++++++++++++++++++++++++++----
scripts/Makefile.lib | 18 +++++++++++---
scripts/Makefile.modfinal | 7 +++++-
5 files changed, 70 insertions(+), 10 deletions(-)

diff --git a/.gitignore b/.gitignore
index 0bbae167bf93..8353b01e2915 100644
--- a/.gitignore
+++ b/.gitignore
@@ -38,6 +38,7 @@
*.o
*.o.*
*.patch
+*.rlib
*.rmeta
*.rpm
*.rsi
@@ -53,6 +54,7 @@
*.zst
Module.symvers
modules.order
+exports_*_generated.c

#
# Top-level generic files
diff --git a/Makefile b/Makefile
index 8a40530868ff..4113a5b93ddc 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..11045ddba1a9 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -20,6 +20,7 @@ always-m :=
targets :=
subdir-y :=
subdir-m :=
+rust-libs :=
EXTRA_AFLAGS :=
EXTRA_CFLAGS :=
EXTRA_CPPFLAGS :=
@@ -243,9 +244,13 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) FORCE
$(call if_changed_rule,cc_o_c)
$(call cmd,force_checksrc)

+mod_objs = $(call real-search, $*.o, .o, -objs -y -m)
+exports_objs = $(foreach m, $(sort $(basename \
+ $(filter $(notdir $(rlib-m)), $(mod_objs)))), exports_$m_generated.o)
+
# 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) | \
+cmd_mod = printf '%s\n' $(mod_objs) $(exports_objs) | \
$(AWK) '!x[$$0]++ { print("$(obj)/"$$0) }' > $@

$(obj)/%.mod: FORCE
@@ -274,7 +279,7 @@ rust_common_cmd = \
-Zcrate-attr='feature($(rust_allowed_features))' \
--extern alloc --extern kernel \
--crate-type rlib -L $(objtree)/rust/ \
- --crate-name $(basename $(notdir $@)) \
+ --crate-name $(basename $(notdir $<)) \
--out-dir $(dir $@) --emit=dep-info=$(depfile)

# `--emit=obj`, `--emit=asm` and `--emit=llvm-ir` imply a single codegen unit
@@ -285,11 +290,42 @@ rust_common_cmd = \
# i.e. the outputs we would get for the different single targets (e.g. `.ll`)
# would not match each other.

+rlib_obj = $(dir $@)$*.$*.o
+rlib_meta = $(dir $@)$(patsubst %.rlib,%.rmeta,$(notdir $@))
+
quiet_cmd_rustc_o_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
cmd_rustc_o_rs = $(rust_common_cmd) --emit=obj=$@ $<

+quiet_cmd_ln_rlib_o = SYMLINK $@
+ cmd_ln_rlib_o = ln -s -f $*.$*.o $@
+
$(obj)/%.o: $(src)/%.rs FORCE
- $(call if_changed_dep,rustc_o_rs)
+ $(if $(findstring $@, $(rlib-y) $(rlib-m)), \
+ $(call if_changed,ln_rlib_o), $(call if_changed_dep,rustc_o_rs))
+
+quiet_cmd_rustc_rlib_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
+ cmd_rustc_rlib_rs = $(rust_common_cmd) --extern bindings \
+ --emit=obj=$(rlib_obj) --emit=metadata=$(rlib_meta) $<; \
+ rm -f $@; $(AR) cDPrST $@ $(rlib_obj) $(rlib_meta)
+
+quiet_cmd_rlib_exports = EXPORTS $@
+ cmd_rlib_exports = \
+ echo "\#include <linux/module.h>" > $@; \
+ $(NM) -p --defined-only $(rlib_obj) \
+ | grep -P ' (T|R|D) (?!(init|cleanup)_module)' | cut -d ' ' -f 3 \
+ | xargs -Isymbol \
+ echo 'extern int symbol; EXPORT_SYMBOL_GPL(symbol);' >> $@
+
+$(rlib-m): $(obj)/%.o: $(obj)/exports_%_generated.o $(obj)/lib%.rlib $(src)/%.rs FORCE
+
+$(obj)/lib%.rlib: $(src)/%.rs FORCE
+ $(call if_changed_dep,rustc_rlib_rs)
+
+$(obj)/exports_%_generated.c: $(obj)/lib%.rlib FORCE
+ $(call if_changed,rlib_exports)
+
+$(obj)/exports_%_generated.o: $(obj)/exports_%_generated.c FORCE
+ $(call if_changed,cc_o_c)

quiet_cmd_rustc_rsi_rs = $(RUSTC_OR_CLIPPY_QUIET) $(quiet_modtag) $@
cmd_rustc_rsi_rs = \
@@ -394,9 +430,14 @@ $(subdir-modorder): $(obj)/%/modules.order: $(obj)/% ;
# To make this rule robust against "Argument list too long" error,
# remove $(obj)/ prefix, and restore it by a shell command.

+# If we have a .rlib for the archive change it for its object member
+crate-obj = $(foreach m, $1, $(if $(findstring .rlib, $m), \
+ $(basename $(m:lib%=%)).$(m:lib%.rlib=%.o),$m))
+
quiet_cmd_ar_builtin = AR $@
cmd_ar_builtin = rm -f $@; \
- $(if $(real-prereqs), printf "$(obj)/%s " $(patsubst $(obj)/%,%,$(real-prereqs)) | xargs) \
+ $(if $(real-prereqs), printf "$(obj)/%s " \
+ $(call crate-obj, $(patsubst $(obj)/%,%,$(real-prereqs))) | xargs) \
$(AR) cDPrST $@

$(obj)/built-in.a: $(real-obj-y) FORCE
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 68d0134bdbf9..3454b05dbbdc 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -45,12 +45,17 @@ else
obj-y := $(filter-out %/, $(obj-y))
endif

+rlib-y := $(foreach m, $(filter lib%.rlib, $(obj-y)), $(m:lib%.rlib=%.o))
+rlib-m := $(foreach m, $(filter lib%.rlib, $(obj-m)), $(m:lib%.rlib=%.o))
+
# Expand $(foo-objs) $(foo-y) etc. by replacing their individuals
suffix-search = $(strip $(foreach s, $3, $($(1:%$(strip $2)=%$s))))
# List composite targets that are constructed by combining other targets
multi-search = $(sort $(foreach m, $1, $(if $(call suffix-search, $m, $2, $3 -), $m)))
# List primitive targets that are compiled from source files
real-search = $(foreach m, $1, $(if $(call suffix-search, $m, $2, $3 -), $(call suffix-search, $m, $2, $3), $m))
+# List exported symbols from .rlib targets
+rlib-exports = $(foreach m, $1, $(if $(findstring .rlib, $m),$(m:lib%.rlib=exports_%_generated.o)) $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)
@@ -59,7 +64,7 @@ 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-y := $(call rlib-exports, $(call real-search, $(obj-y), .o, -objs -y))
real-obj-m := $(call real-search, $(obj-m), .o, -objs -y -m)

always-y += $(always-m)
@@ -95,6 +100,8 @@ extra-y := $(addprefix $(obj)/,$(extra-y))
always-y := $(addprefix $(obj)/,$(always-y))
targets := $(addprefix $(obj)/,$(targets))
obj-m := $(addprefix $(obj)/,$(obj-m))
+rlib-y := $(addprefix $(obj)/,$(rlib-y))
+rlib-m := $(addprefix $(obj)/,$(rlib-m))
lib-y := $(addprefix $(obj)/,$(lib-y))
real-obj-y := $(addprefix $(obj)/,$(real-obj-y))
real-obj-m := $(addprefix $(obj)/,$(real-obj-m))
@@ -210,7 +217,11 @@ _cpp_flags += -I $(srctree)/$(src) -I $(objtree)/$(obj)
endif
endif

-part-of-module = $(if $(filter $(basename $@).o, $(real-obj-m)),y)
+_rust_libs = $(foreach l, $(rust-libs), -L $(obj)/$(dir $l) --extern $(notdir $(basename $l)))
+
+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 = \
@@ -232,7 +243,8 @@ c_flags = -Wp,-MMD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) \
$(_c_flags) $(modkern_cflags) \
$(basename_flags) $(modname_flags)

-rust_flags = $(_rust_flags) $(modkern_rustflags) @$(objtree)/include/generated/rustc_cfg
+rust_flags = $(_rust_flags) $(modkern_rustflags) $(_rust_libs) \
+ @$(objtree)/include/generated/rustc_cfg

a_flags = -Wp,-MMD,$(depfile) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) \
$(_a_flags) $(modkern_aflags)
diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
index b3a6aa8fbe8c..e05749a5ca0a 100644
--- a/scripts/Makefile.modfinal
+++ b/scripts/Makefile.modfinal
@@ -30,11 +30,16 @@ quiet_cmd_cc_o_c = CC [M] $@

ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)

+# Symlinks of foo.o that point to foo.foo.o are members of a .rlib archives,
+# so their exports should be added to the module
+ko_objs = $(foreach o, $(filter %.o, $^),$(if \
+ $(filter-out %$o,$(realpath $o)),$(dir $o)exports_$(notdir $*)_generated.o) $o)
+
quiet_cmd_ld_ko_o = LD [M] $@
cmd_ld_ko_o += \
$(LD) -r $(KBUILD_LDFLAGS) \
$(KBUILD_LDFLAGS_MODULE) $(LDFLAGS_MODULE) \
- -T scripts/module.lds -o $@ $(filter %.o, $^); \
+ -T scripts/module.lds -o $@ $(ko_objs); \
$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)

quiet_cmd_btf_ko = BTF [M] $@
--
2.42.0

Next message: Jeff Johnson: "Re: [PATCH v2] airo: replace deprecated strncpy with strscpy_pad"
Previous message: Martin Rodriguez Reboredo: "[RFC PATCH v2 0/2] rust: crates in other kernel directories"
In reply to: Martin Rodriguez Reboredo: "[RFC PATCH v2 0/2] rust: crates in other kernel directories"
Next in thread: Martin Rodriguez Reboredo: "[RFC PATCH v2 2/2] samples: rust: Add USB sample bindings"
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