kbuild: remove stale *.symversions

cmd_update_lto_symversions merges all the existing *.symversions, but
some of them might be stale.

If the last EXPORT_SYMBOL is removed from a C file, the *.symversions
file is not deleted or updated. It contains stale CRCs, but still they
will be used for linking the vmlinux or modules.

It is not a big deal when the EXPORT_SYMBOL is really removed. However,
when the EXPORT_SYMBOL is moved to another file, the same __crc_<symbol>
will appear twice in the merged *.symversions, possibly with different
CRCs if the function argument is changed at the same time. It would
confuse module versioning.

If no EXPORT_SYMBOL is found, let's remove *.symversions explicitly.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>

scripts/Makefile.build

@@ -177,6 +177,8 @@ cmd_modversions_c =								\
 	if $(NM) $@ 2>/dev/null | grep -q __ksymtab; then			\
 		$(call cmd_gensymtypes_c,$(KBUILD_SYMTYPES),$(@:.o=.symtypes))	\
 		    > $@.symversions;						\
+	else									\
+		rm -f $@.symversions;						\
 	fi;
 else
 cmd_modversions_c =								\