From: Sami Tolvanen <samitolvanen@google.com>

Previously, two things stopped Rust from using MODVERSIONS:
1. Rust symbols are occasionally too long to be represented in the
   original versions table
2. Rust types cannot be properly hashed by the existing genksyms
   approach because:
	* Looking up type definitions in Rust is more complex than C
	* Type layout is potentially dependent on the compiler in Rust,
	  not just the source type declaration.

CONFIG_EXTENDED_MODVERSIONS addresses the first point, and
CONFIG_GENDWARFKSYMS the second. If Rust wants to use MODVERSIONS, allow
it to do so by selecting both features.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Co-developed-by: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Matthew Maurer <mmaurer@google.com>
---
 init/Kconfig  |  3 ++-
 rust/Makefile | 32 ++++++++++++++++++++++++++++++--
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/init/Kconfig b/init/Kconfig
index 530a382ee0feb391b4717abdba3672e584a462d0..f5cce579f29b2ed89e97f8075a3bf70e32e71ad0 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1942,7 +1942,8 @@ config RUST
 	bool "Rust support"
 	depends on HAVE_RUST
 	depends on RUST_IS_AVAILABLE
-	depends on !MODVERSIONS
+	select EXTENDED_MODVERSIONS if MODVERSIONS
+	depends on (GENDWARFKSYMS || !MODVERSIONS)
 	depends on !GCC_PLUGIN_RANDSTRUCT
 	depends on !RANDSTRUCT
 	depends on !DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE
diff --git a/rust/Makefile b/rust/Makefile
index b5e0a73b78f3e58fc8fb8c9fab8fb5792406c6d8..b80bc4eb98202f774c493da89c2caee322cffc91 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -303,10 +303,11 @@ $(obj)/bindings/bindings_helpers_generated.rs: private bindgen_target_extra = ;
 $(obj)/bindings/bindings_helpers_generated.rs: $(src)/helpers/helpers.c FORCE
 	$(call if_changed_dep,bindgen)
 
+rust_exports = $(NM) -p --defined-only $(1) | awk '$$2~/(T|R|D|B)/ && $$3!~/__cfi/ { printf $(2),$(3) }'
+
 quiet_cmd_exports = EXPORTS $@
       cmd_exports = \
-	$(NM) -p --defined-only $< \
-		| awk '$$2~/(T|R|D|B)/ && $$3!~/__cfi/ {printf "EXPORT_SYMBOL_RUST_GPL(%s);\n",$$3}' > $@
+	$(call rust_exports,$<,"EXPORT_SYMBOL_RUST_GPL(%s);\n",$$3) > $@
 
 $(obj)/exports_core_generated.h: $(obj)/core.o FORCE
 	$(call if_changed,exports)
@@ -378,11 +379,35 @@ ifneq ($(or $(CONFIG_ARM64),$(and $(CONFIG_RISCV),$(CONFIG_64BIT))),)
 		__ashlti3 __lshrti3
 endif
 
+ifdef CONFIG_MODVERSIONS
+cmd_gendwarfksyms = $(if $(skip_gendwarfksyms),, \
+	$(call rust_exports,$@,"%s\n",$$3) | \
+	scripts/gendwarfksyms/gendwarfksyms \
+		$(if $(KBUILD_SYMTYPES), --symtypes $(@:.o=.symtypes),) \
+		$@ >> $(dot-target).cmd)
+endif
+
 define rule_rustc_library
 	$(call cmd_and_fixdep,rustc_library)
 	$(call cmd,gen_objtooldep)
+	$(call cmd,gendwarfksyms)
 endef
 
+define rule_rust_cc_library
+	$(call if_changed_rule,cc_o_c)
+	$(call cmd,force_checksrc)
+	$(call cmd,gendwarfksyms)
+endef
+
+# helpers.o uses the same export mechanism as Rust libraries, so ensure symbol
+# versions are calculated for the helpers too.
+$(obj)/helpers/helpers.o: $(src)/helpers/helpers.c $(recordmcount_source) FORCE
+	+$(call if_changed_rule,rust_cc_library)
+
+# Disable symbol versioning for exports.o to avoid conflicts with the actual
+# symbol versions generated from Rust objects.
+$(obj)/exports.o: private skip_gendwarfksyms = 1
+
 $(obj)/core.o: private skip_clippy = 1
 $(obj)/core.o: private skip_flags = -Wunreachable_pub
 $(obj)/core.o: private rustc_objcopy = $(foreach sym,$(redirect-intrinsics),--redefine-sym $(sym)=__rust$(sym))
@@ -394,6 +419,7 @@ ifneq ($(or $(CONFIG_X86_64),$(CONFIG_X86_32)),)
 $(obj)/core.o: scripts/target.json
 endif
 
+$(obj)/compiler_builtins.o: private skip_gendwarfksyms = 1
 $(obj)/compiler_builtins.o: private rustc_objcopy = -w -W '__*'
 $(obj)/compiler_builtins.o: $(src)/compiler_builtins.rs $(obj)/core.o FORCE
 	+$(call if_changed_rule,rustc_library)
@@ -404,6 +430,7 @@ $(obj)/alloc.o: private rustc_target_flags = $(alloc-cfgs)
 $(obj)/alloc.o: $(RUST_LIB_SRC)/alloc/src/lib.rs $(obj)/compiler_builtins.o FORCE
 	+$(call if_changed_rule,rustc_library)
 
+$(obj)/build_error.o: private skip_gendwarfksyms = 1
 $(obj)/build_error.o: $(src)/build_error.rs $(obj)/compiler_builtins.o FORCE
 	+$(call if_changed_rule,rustc_library)
 
@@ -413,6 +440,7 @@ $(obj)/bindings.o: $(src)/bindings/lib.rs \
     $(obj)/bindings/bindings_helpers_generated.rs FORCE
 	+$(call if_changed_rule,rustc_library)
 
+$(obj)/uapi.o: private skip_gendwarfksyms = 1
 $(obj)/uapi.o: $(src)/uapi/lib.rs \
     $(obj)/compiler_builtins.o \
     $(obj)/uapi/uapi_generated.rs FORCE

-- 
2.47.0.105.g07ac214952-goog

On Wed, Oct 23, 2024 at 02:31:30AM +0000, Matthew Maurer wrote:
> From: Sami Tolvanen <samitolvanen@google.com>
> 
> Previously, two things stopped Rust from using MODVERSIONS:
> 1. Rust symbols are occasionally too long to be represented in the
>    original versions table
> 2. Rust types cannot be properly hashed by the existing genksyms
>    approach because:
> 	* Looking up type definitions in Rust is more complex than C
> 	* Type layout is potentially dependent on the compiler in Rust,
> 	  not just the source type declaration.
> 
> CONFIG_EXTENDED_MODVERSIONS addresses the first point, and
> CONFIG_GENDWARFKSYMS the second. If Rust wants to use MODVERSIONS, allow
> it to do so by selecting both features.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Co-developed-by: Matthew Maurer <mmaurer@google.com>
> Signed-off-by: Matthew Maurer <mmaurer@google.com>
> ---
>  init/Kconfig  |  3 ++-
>  rust/Makefile | 32 ++++++++++++++++++++++++++++++--
>  2 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/init/Kconfig b/init/Kconfig
> index 530a382ee0feb391b4717abdba3672e584a462d0..f5cce579f29b2ed89e97f8075a3bf70e32e71ad0 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -1942,7 +1942,8 @@ config RUST
>  	bool "Rust support"
>  	depends on HAVE_RUST
>  	depends on RUST_IS_AVAILABLE
> -	depends on !MODVERSIONS
> +	select EXTENDED_MODVERSIONS if MODVERSIONS
> +	depends on (GENDWARFKSYMS || !MODVERSIONS)

Nit: `depends on !MODVERSIONS || GENDWARFKSYMS` would match the
convention used below.

> +cmd_gendwarfksyms = $(if $(skip_gendwarfksyms),, \
> +	$(call rust_exports,$@,"%s\n",$$3) | \
> +	scripts/gendwarfksyms/gendwarfksyms \
> +		$(if $(KBUILD_SYMTYPES), --symtypes $(@:.o=.symtypes),) \
> +		$@ >> $(dot-target).cmd)

Note that Petr suggested adding a KBUILD_ flag for passing the
--stable parameter to gendwarfksyms, which I think would be useful,
so once I send out v5 of that series, we'll have to update this to
also use the parameter when processing Rust objects:

https://lore.kernel.org/linux-modules/8017c328-f039-46c3-b472-f203cd6e2cfe@suse.com/

Sami