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 <redacted>
Tested-by: Alexander Lobakin <redacted>
---
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..778dabea3a89 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 $^
+
+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
+       $(call if_changed,cc_lto_link_modules)