From 850ded46c64299f04d15e39caaba21963fb966f8 Mon Sep 17 00:00:00 2001
From: Sami Tolvanen <samitolvanen@google.com>
Date: Mon, 16 Aug 2021 11:05:19 -0700
Subject: kbuild: Fix TRIM_UNUSED_KSYMS with LTO_CLANG

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@google.com>
Reviewed-by: Alexander Lobakin <alobakin@pm.me>
Tested-by: Alexander Lobakin <alobakin@pm.me>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
---
 scripts/Makefile.build | 27 ++++++++++++++++++++++++++-
 1 file changed, 26 insertions(+), 1 deletion(-)

(limited to 'scripts/Makefile.build')

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 02197cb8e3a7..ea549579bfb7 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -88,6 +88,10 @@ endif
 
 targets-for-modules := $(patsubst %.o, %.mod, $(filter %.o, $(obj-m)))
 
+ifdef CONFIG_LTO_CLANG
+targets-for-modules += $(patsubst %.o, %.lto.o, $(filter %.o, $(obj-m)))
+endif
+
 ifdef need-modorder
 targets-for-modules += $(obj)/modules.order
 endif
@@ -271,12 +275,33 @@ $(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 $@
+endif
+
+$(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   $@
-- 
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