Re: [PATCH 11/13] kbuild: always postpone CRC links for module versioning

From: Kees Cook <keescook@chromium.org>
To: Masahiro Yamada <masahiroy@kernel.org>
Cc: linux-kbuild@vger.kernel.org,
	Sami Tolvanen <samitolvanen@google.com>,
	linux-kernel@vger.kernel.org,
	Michal Marek <michal.lkml@markovi.net>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	clang-built-linux@googlegroups.com
Subject: Re: [PATCH 11/13] kbuild: always postpone CRC links for module versioning
Date: Wed, 18 Aug 2021 23:43:33 -0700	[thread overview]
Message-ID: <202108182342.F24979957@keescook> (raw)
In-Reply-To: <20210819005744.644908-12-masahiroy@kernel.org>

On Thu, Aug 19, 2021 at 09:57:42AM +0900, Masahiro Yamada wrote:
> When CONFIG_MODVERSIONS=y, the CRCs of EXPORT_SYMBOL are linked into
> *.o files in-place.
> 
> It is impossible for Clang LTO because *.o files are not ELF, but LLVM
> bitcode. The CRCs are stored in separate *.symversions files, and then
> supplied to the modpost link.
> 
> Let's do so for CONFIG_LTO_CLANG=n is possible, and unify the module
> versioning code.

I worry about this also now being "unparallelized", but it is a nice
cleanup.

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> 
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---
> 
>  scripts/Makefile.build  | 32 ++++++--------------------------
>  scripts/link-vmlinux.sh | 22 ++++++++++++++--------
>  2 files changed, 20 insertions(+), 34 deletions(-)
> 
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 957addea830b..874e84a1f3fc 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -158,17 +158,12 @@ quiet_cmd_cc_o_c = CC $(quiet_modtag)  $@
>  ifdef CONFIG_MODVERSIONS
>  # When module versioning is enabled the following steps are executed:
>  # o compile a <file>.o from <file>.c
> -# o if <file>.o doesn't contain a __ksymtab version, i.e. does
> -#   not export symbols, it's done.
> +# o if <file>.o doesn't contain __ksymtab* symbols, i.e. does
> +#   not export symbols, create an empty *.symversions
>  # o otherwise, we calculate symbol versions using the good old
>  #   genksyms on the preprocessed source and postprocess them in a way
>  #   that they are usable as a linker script
> -# o generate .tmp_<file>.o from <file>.o using the linker to
> -#   replace the unresolved symbols __crc_exported_symbol with
> -#   the actual value of the checksum generated by genksyms
> -# o remove .tmp_<file>.o to <file>.o
>  
> -ifdef CONFIG_LTO_CLANG
>  # Generate .o.symversions files for each .o with exported symbols, and link these
>  # to the kernel and/or modules at the end.
>  cmd_modversions_c =								\
> @@ -178,18 +173,6 @@ cmd_modversions_c =								\
>  	else									\
>  		rm -f $@.symversions;						\
>  	fi;
> -else
> -cmd_modversions_c =								\
> -	if $(OBJDUMP) -h $@ | grep -q __ksymtab; then				\
> -		$(call cmd_gensymtypes_c,$(KBUILD_SYMTYPES),$(@:.o=.symtypes))	\
> -		    > $(@D)/.tmp_$(@F:.o=.ver);					\
> -										\
> -		$(LD) $(KBUILD_LDFLAGS) -r -o $(@D)/.tmp_$(@F) $@ 		\
> -			-T $(@D)/.tmp_$(@F:.o=.ver);				\
> -		mv -f $(@D)/.tmp_$(@F) $@;					\
> -		rm -f $(@D)/.tmp_$(@F:.o=.ver);					\
> -	fi
> -endif
>  endif
>  
>  ifdef CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
> @@ -348,12 +331,9 @@ ifdef CONFIG_ASM_MODVERSIONS
>  cmd_modversions_S =								\
>  	if $(OBJDUMP) -h $@ | grep -q __ksymtab; then				\
>  		$(call cmd_gensymtypes_S,$(KBUILD_SYMTYPES),$(@:.o=.symtypes))	\
> -		    > $(@D)/.tmp_$(@F:.o=.ver);					\
> -										\
> -		$(LD) $(KBUILD_LDFLAGS) -r -o $(@D)/.tmp_$(@F) $@ 		\
> -			-T $(@D)/.tmp_$(@F:.o=.ver);				\
> -		mv -f $(@D)/.tmp_$(@F) $@;					\
> -		rm -f $(@D)/.tmp_$(@F:.o=.ver);					\
> +		    > $@.symversions;						\
> +	else									\
> +		rm -rf $@.symversions;						\
>  	fi
>  endif
>  
> @@ -424,7 +404,7 @@ $(obj)/lib.a: $(lib-y) FORCE
>  # Rule to prelink modules
>  #
>  
> -ifeq ($(CONFIG_LTO_CLANG) $(CONFIG_MODVERSIONS),y y)
> +ifdef CONFIG_MODVERSIONS
>  
>  cmd_merge_symver =					\
>  	rm -f $@;					\
> diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
> index 17976609c2d8..66ced6ff8e65 100755
> --- a/scripts/link-vmlinux.sh
> +++ b/scripts/link-vmlinux.sh
> @@ -59,8 +59,7 @@ append_symversion()
>  	fi
>  }
>  
> -# If CONFIG_LTO_CLANG is selected, collect generated symbol versions into
> -# .tmp_symversions.lds
> +# Collect generated symbol versions into .tmp_symversions.lds
>  gen_symversions()
>  {
>  	info GEN .tmp_symversions.lds
> @@ -94,14 +93,13 @@ modpost_link()
>  		${KBUILD_VMLINUX_LIBS}				\
>  		--end-group"
>  
> +	if [ -n "${CONFIG_MODVERSIONS}" ]; then
> +		lds="${lds} -T .tmp_symversions.lds"
> +	fi
> +
>  	if [ -n "${CONFIG_LTO_CLANG}" ]; then
>  		gen_initcalls
> -		lds="-T .tmp_initcalls.lds"
> -
> -		if [ -n "${CONFIG_MODVERSIONS}" ]; then
> -			gen_symversions
> -			lds="${lds} -T .tmp_symversions.lds"
> -		fi
> +		lds="${lds} -T .tmp_initcalls.lds"
>  
>  		# This might take a while, so indicate that we're doing
>  		# an LTO link
> @@ -198,6 +196,10 @@ vmlinux_link()
>  
>  	ldflags="${ldflags} ${wl}--script=${objtree}/${KBUILD_LDS}"
>  
> +	if [ -n "${CONFIG_MODVERSIONS}" ]; then
> +		ldflags="${ldflags} ${wl}--script=.tmp_symversions.lds"
> +	fi
> +
>  	# The kallsyms linking does not need debug symbols included.
>  	if [ "$output" != "${output#.tmp_vmlinux.kallsyms}" ] ; then
>  		ldflags="${ldflags} ${wl}--strip-debug"
> @@ -351,6 +353,10 @@ fi;
>  # final build of init/
>  ${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init need-builtin=1
>  
> +if [ -n "${CONFIG_MODVERSIONS}" ]; then
> +	gen_symversions
> +fi
> +
>  #link vmlinux.o
>  modpost_link vmlinux.o
>  objtool_link vmlinux.o
> -- 
> 2.30.2
> 

-- 
Kees Cook