Re: [PATCH v3] kbuild: Fix TRIM_UNUSED_KSYMS with LTO_CLANG

From: Masahiro Yamada
Date: Sat Aug 14 2021 - 19:27:47 EST

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In reply to: Sami Tolvanen: "[PATCH v3] kbuild: Fix TRIM_UNUSED_KSYMS with LTO_CLANG"
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On Fri, Aug 13, 2021 at 6:46 AM 'Sami Tolvanen' via Clang Built Linux
<clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
>
> 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>
> Reviewed-by: Alexander Lobakin <alobakin@xxxxx>
> Tested-by: Alexander Lobakin <alobakin@xxxxx>
> ---
> Changes in v3:
> - Added missing FORCE.

All the modules are recompiled every time.

masahiro@oscar:~/workspace/linux-kbuild$ make LLVM=1 LLVM_IAS=1 -j24
DESCEND objtool
CALL scripts/atomic/check-atomics.sh
CALL scripts/checksyscalls.sh
CHK include/generated/compile.h
LTO [M] net/netfilter/nf_log_syslog.lto.o
LTO [M] net/netfilter/xt_mark.lto.o
LTO [M] net/netfilter/xt_nat.lto.o
LTO [M] net/netfilter/xt_LOG.lto.o
LTO [M] drivers/thermal/intel/x86_pkg_temp_thermal.lto.o
LTO [M] net/netfilter/xt_MASQUERADE.lto.o
LTO [M] net/netfilter/xt_addrtype.lto.o
LTO [M] fs/efivarfs/efivarfs.lto.o
LTO [M] net/ipv4/netfilter/iptable_nat.lto.o
CHK include/generated/autoksyms.h
MODPOST modules-only.symvers
GEN Module.symvers
LD [M] drivers/thermal/intel/x86_pkg_temp_thermal.ko
LD [M] fs/efivarfs/efivarfs.ko
LD [M] net/ipv4/netfilter/iptable_nat.ko
LD [M] net/netfilter/nf_log_syslog.ko
LD [M] net/netfilter/xt_LOG.ko
LD [M] net/netfilter/xt_MASQUERADE.ko
LD [M] net/netfilter/xt_addrtype.ko
LD [M] net/netfilter/xt_mark.ko
LD [M] net/netfilter/xt_nat.ko
Kernel: arch/x86/boot/bzImage is ready (#20)

> Changes in v2:
> - Fixed a couple of typos.
> - Fixed objtool arguments for .lto.o to always include --module.
>
> ---
> scripts/Makefile.build | 24 +++++++++++++++++++++++-
> scripts/Makefile.lib | 7 +++++++
> scripts/Makefile.modfinal | 21 ++-------------------
> scripts/Makefile.modpost | 22 +++-------------------
> scripts/gen_autoksyms.sh | 12 ------------
> 5 files changed, 35 insertions(+), 51 deletions(-)
>
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index 02197cb8e3a7..524701d9896b 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -271,12 +271,34 @@ $(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 $(filter-out FORCE,$^)
> +
> +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) --module \
> + $(@:.ko=$(mod-prelink-ext).o)
> +endif
> +
> +$(obj)/%.lto.o: $(obj)/%.o FORCE
> + $(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..af1c920a585c 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
>
> base-commit: f8fbb47c6e86c0b75f8df864db702c3e3f757361
> --
> 2.33.0.rc1.237.g0d66db33f3-goog
>
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--
Best Regards
Masahiro Yamada

Next message: Dave Chinner: "Re: [xfs] 6df693ed7b: aim7.jobs-per-min -15.7% regression"
Previous message: Matthew Wilcox: "Re: [syzbot] BUG: unable to handle kernel paging request in alloc_huge_page"
In reply to: Sami Tolvanen: "[PATCH v3] kbuild: Fix TRIM_UNUSED_KSYMS with LTO_CLANG"
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