Re: [PATCH] kbuild: Fix TRIM_UNUSED_KSYMS with LTO_CLANG

[Alexander Lobakin]

From:   Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Date:   Fri, 16 Jul 2021 13:45:45 -0700

> With CONFIG_LTO_CLANG, we currently link modules into native
> code just before modpost, which means with TRIM_UNUSED_KSYMS
> enabled, we still look at the LLVM bitcode in the .o files when
> generating the list of used symbols. As the bitcode doesn't
> yet have calls to compiler intrinsics and llvm-nm doesn't see
> function references that only exist in function-level inline
> assembly, we currently need a whitelist for TRIM_UNUSED_KSYMS to
> work with LTO.
>
> This change moves module LTO linking to happen earlier, and
> thus avoids the issue with LLVM bitcode and TRIM_UNUSED_KSYMS
> entirely, allowing us to also drop the whitelist from
> gen_autoksyms.sh.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1369
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> ---
>  scripts/Makefile.build    | 25 ++++++++++++++++++++++++-
>  scripts/Makefile.lib      |  7 +++++++
>  scripts/Makefile.modfinal | 21 ++-------------------
>  scripts/Makefile.modpost  | 22 +++-------------------
>  scripts/gen_autoksyms.sh  | 12 ------------
>  5 files changed, 36 insertions(+), 51 deletions(-)
>
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 10b2f2380d6f..80e0fa810870 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -202,6 +202,7 @@ sub_cmd_record_mcount =					\
>  	if [ $(@) != "scripts/mod/empty.o" ]; then	\
>  		$(objtree)/scripts/recordmcount $(RECORDMCOUNT_FLAGS) "$(@)";	\
>  	fi;
> +/

Seems like a leftover or a random typo here.

>  recordmcount_source := $(srctree)/scripts/recordmcount.c \
>  		    $(srctree)/scripts/recordmcount.h
>  else
> @@ -271,12 +272,34 @@ $(obj)/%.o: $(src)/%.c $(recordmcount_source) $$(objtool_dep) FORCE
>  	$(call if_changed_rule,cc_o_c)
>  	$(call cmd,force_checksrc)
>
> +ifdef CONFIG_LTO_CLANG
> +# Module .o files may contain LLVM bitcode, compile them into native code
> +# before ELF processing
> +quiet_cmd_cc_lto_link_modules = LTO [M] $@
> +cmd_cc_lto_link_modules =						\
> +	$(LD) $(ld_flags) -r -o $@					\
> +		$(shell [ -s $(@:.lto.o=.o.symversions) ] &&		\
> +			echo -T $(@:.lto.o=.o.symversions))		\
> +		--whole-archive $^
> +
> +ifdef CONFIG_STACK_VALIDATION
> +# objtool was skipped for LLVM bitcode, run it now that we have compiled
> +# modules into native code
> +cmd_cc_lto_link_modules += ;						\
> +	$(objtree)/tools/objtool/objtool $(objtool_args)		\

Now $(part-of-module) inside $(objtool_args) doesn't get expanded
properly, because previously it was being called on x.ko, and now
it's being called on x.lto.o. $(basename $@) returns "x.lto" instead
of "x", and Make doesn't find "x.lto.o" in $(real-objs-m).

An example of objtool args dump:

  LTO [M] fs/btrfs/btrfs.lto.o
Call: ./tools/objtool/objtool orc generate --no-fp --no-unreachable --retpoline --uaccess fs/btrfs/btrfs.lto.o
fs/btrfs/btrfs.lto.o: warning: objtool: static_call: can't find static_call_key symbol: __SCK__might_resched

As can be seen, objtools command line no longer contains "--module".
And this warning about "can't find static_call_key" can appear only
in case of !module -> no -m|--module param was given.

As a result, modules get broken and the kernel panics after loading
initramfs.

> +		$(@:.ko=$(mod-prelink-ext).o)
> +endif
> +
> +$(obj)/%.lto.o: $(obj)/%.o
> +	$(call if_changed,cc_lto_link_modules)
> +endif
> +
>  cmd_mod = { \
>  	echo $(if $($*-objs)$($*-y)$($*-m), $(addprefix $(obj)/, $($*-objs) $($*-y) $($*-m)), $(@:.mod=.o)); \
>  	$(undefined_syms) echo; \
>  	} > $@
>
> -$(obj)/%.mod: $(obj)/%.o FORCE
> +$(obj)/%.mod: $(obj)/%$(mod-prelink-ext).o FORCE
>  	$(call if_changed,mod)
>
>  quiet_cmd_cc_lst_c = MKLST   $@
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index 10950559b223..ee985366dddf 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -225,6 +225,13 @@ dtc_cpp_flags  = -Wp,-MMD,$(depfile).pre.tmp -nostdinc                    \
>  		 $(addprefix -I,$(DTC_INCLUDE))                          \
>  		 -undef -D__DTS__
>
> +ifeq ($(CONFIG_LTO_CLANG),y)
> +# With CONFIG_LTO_CLANG, .o files in modules might be LLVM bitcode, so we
> +# need to run # LTO to compile them into native code (.lto.o) before further
> +# processing.
> +mod-prelink-ext := .lto
> +endif
> +
>  # Objtool arguments are also needed for modfinal with LTO, so we define
>  # then here to avoid duplication.
>  objtool_args =								\
> diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal
> index 5e9b8057fb24..ff805777431c 100644
> --- a/scripts/Makefile.modfinal
> +++ b/scripts/Makefile.modfinal
> @@ -9,7 +9,7 @@ __modfinal:
>  include include/config/auto.conf
>  include $(srctree)/scripts/Kbuild.include
>
> -# for c_flags and objtool_args
> +# for c_flags and mod-prelink-ext
>  include $(srctree)/scripts/Makefile.lib
>
>  # find all modules listed in modules.order
> @@ -30,23 +30,6 @@ quiet_cmd_cc_o_c = CC [M]  $@
>
>  ARCH_POSTLINK := $(wildcard $(srctree)/arch/$(SRCARCH)/Makefile.postlink)
>
> -ifdef CONFIG_LTO_CLANG
> -# With CONFIG_LTO_CLANG, reuse the object file we compiled for modpost to
> -# avoid a second slow LTO link
> -prelink-ext := .lto
> -
> -# ELF processing was skipped earlier because we didn't have native code,
> -# so let's now process the prelinked binary before we link the module.
> -
> -ifdef CONFIG_STACK_VALIDATION
> -cmd_ld_ko_o +=								\
> -	$(objtree)/tools/objtool/objtool $(objtool_args)		\
> -		$(@:.ko=$(prelink-ext).o);
> -
> -endif # CONFIG_STACK_VALIDATION
> -
> -endif # CONFIG_LTO_CLANG
> -
>  quiet_cmd_ld_ko_o = LD [M]  $@
>        cmd_ld_ko_o +=							\
>  	$(LD) -r $(KBUILD_LDFLAGS)					\
> @@ -72,7 +55,7 @@ if_changed_except = $(if $(call newer_prereqs_except,$(2))$(cmd-check),      \
>
>
>  # Re-generate module BTFs if either module's .ko or vmlinux changed
> -$(modules): %.ko: %$(prelink-ext).o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
> +$(modules): %.ko: %$(mod-prelink-ext).o %.mod.o scripts/module.lds $(if $(KBUILD_BUILTIN),vmlinux) FORCE
>  	+$(call if_changed_except,ld_ko_o,vmlinux)
>  ifdef CONFIG_DEBUG_INFO_BTF_MODULES
>  	+$(if $(newer-prereqs),$(call cmd,btf_ko))
> diff --git a/scripts/Makefile.modpost b/scripts/Makefile.modpost
> index c383ba33d837..eef56d629799 100644
> --- a/scripts/Makefile.modpost
> +++ b/scripts/Makefile.modpost
> @@ -41,7 +41,7 @@ __modpost:
>  include include/config/auto.conf
>  include $(srctree)/scripts/Kbuild.include
>
> -# for ld_flags
> +# for mod-prelink-ext
>  include $(srctree)/scripts/Makefile.lib
>
>  MODPOST = scripts/mod/modpost								\
> @@ -118,22 +118,6 @@ $(input-symdump):
>  	@echo >&2 '         Modules may not have dependencies or modversions.'
>  	@echo >&2 '         You may get many unresolved symbol warnings.'
>
> -ifdef CONFIG_LTO_CLANG
> -# With CONFIG_LTO_CLANG, .o files might be LLVM bitcode, so we need to run
> -# LTO to compile them into native code before running modpost
> -prelink-ext := .lto
> -
> -quiet_cmd_cc_lto_link_modules = LTO [M] $@
> -cmd_cc_lto_link_modules =						\
> -	$(LD) $(ld_flags) -r -o $@					\
> -		$(shell [ -s $(@:.lto.o=.o.symversions) ] &&		\
> -			echo -T $(@:.lto.o=.o.symversions))		\
> -		--whole-archive $^
> -
> -%.lto.o: %.o
> -	$(call if_changed,cc_lto_link_modules)
> -endif
> -
>  modules := $(sort $(shell cat $(MODORDER)))
>
>  # KBUILD_MODPOST_WARN can be set to avoid error out in case of undefined symbols
> @@ -144,9 +128,9 @@ endif
>  # Read out modules.order to pass in modpost.
>  # Otherwise, allmodconfig would fail with "Argument list too long".
>  quiet_cmd_modpost = MODPOST $@
> -      cmd_modpost = sed 's/\.ko$$/$(prelink-ext)\.o/' $< | $(MODPOST) -T -
> +      cmd_modpost = sed 's/\.ko$$/$(mod-prelink-ext)\.o/' $< | $(MODPOST) -T -
>
> -$(output-symdump): $(MODORDER) $(input-symdump) $(modules:.ko=$(prelink-ext).o) FORCE
> +$(output-symdump): $(MODORDER) $(input-symdump) $(modules:.ko=$(mod-prelink-ext).o) FORCE
>  	$(call if_changed,modpost)
>
>  targets += $(output-symdump)
> diff --git a/scripts/gen_autoksyms.sh b/scripts/gen_autoksyms.sh
> index da320151e7c3..6ed0d225c8b1 100755
> --- a/scripts/gen_autoksyms.sh
> +++ b/scripts/gen_autoksyms.sh
> @@ -26,18 +26,6 @@ if [ -n "$CONFIG_MODVERSIONS" ]; then
>  	needed_symbols="$needed_symbols module_layout"
>  fi
>
> -# With CONFIG_LTO_CLANG, LLVM bitcode has not yet been compiled into a binary
> -# when the .mod files are generated, which means they don't yet contain
> -# references to certain symbols that will be present in the final binaries.
> -if [ -n "$CONFIG_LTO_CLANG" ]; then
> -	# intrinsic functions
> -	needed_symbols="$needed_symbols memcpy memmove memset"
> -	# ftrace
> -	needed_symbols="$needed_symbols _mcount"
> -	# stack protector symbols
> -	needed_symbols="$needed_symbols __stack_chk_fail __stack_chk_guard"
> -fi
> -
>  ksym_wl=
>  if [ -n "$CONFIG_UNUSED_KSYMS_WHITELIST" ]; then
>  	# Use 'eval' to expand the whitelist path and check if it is relative
> --
> 2.32.0.402.g57bb445576-goog
>
>

Thanks,
Al