kbuild: fix UNUSED_KSYMS_WHITELIST for Clang LTO

Commit fbe078d397 ("kbuild: lto: add a default list of used symbols")
does not work as expected if the .config file has already specified
CONFIG_UNUSED_KSYMS_WHITELIST="my/own/white/list" before enabling
CONFIG_LTO_CLANG.

So, the user-supplied whitelist and LTO-specific white list must be
independent of each other.

I refactored the shell script so CONFIG_MODVERSIONS and CONFIG_CLANG_LTO
handle whitelists in the same way.

Fixes: fbe078d397 ("kbuild: lto: add a default list of used symbols")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>

init/Kconfig

@@ -2283,7 +2283,6 @@ config TRIM_UNUSED_KSYMS
 config UNUSED_KSYMS_WHITELIST
 	string "Whitelist of symbols to keep in ksymtab"
 	depends on TRIM_UNUSED_KSYMS
-	default "scripts/lto-used-symbollist.txt" if LTO_CLANG
 	help
 	  By default, all unused exported symbols will be un-exported from the
 	  build when TRIM_UNUSED_KSYMS is selected.

scripts/gen_autoksyms.sh

@@ -19,7 +19,26 @@ esac
 # We need access to CONFIG_ symbols
 . include/config/auto.conf
 
-ksym_wl=/dev/null
+needed_symbols=
+
+# Special case for modversions (see modpost.c)
+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 "$CONFIG_UNUSED_KSYMS_WHITELIST" ]; then
 	# Use 'eval' to expand the whitelist path and check if it is relative
 	eval ksym_wl="$CONFIG_UNUSED_KSYMS_WHITELIST"
@@ -40,16 +59,14 @@ cat > "$output_file" << EOT
 EOT
 
 [ -f modules.order ] && modlist=modules.order || modlist=/dev/null
-sed 's/ko$/mod/' $modlist |
-xargs -n1 sed -n -e '2{s/ /\n/g;/^$/!p;}' -- |
-cat - "$ksym_wl" |
+
+{
+	sed 's/ko$/mod/' $modlist | xargs -n1 sed -n -e '2p'
+	echo "$needed_symbols"
+	[ -n "$ksym_wl" ] && cat "$ksym_wl"
+} | sed -e 's/ /\n/g' | sed -n -e '/^$/!p' |
 # Remove the dot prefix for ppc64; symbol names with a dot (.) hold entry
 # point addresses.
 sed -e 's/^\.//' |
 sort -u |
 sed -e 's/\(.*\)/#define __KSYM_\1 1/' >> "$output_file"
-
-# Special case for modversions (see modpost.c)
-if [ -n "$CONFIG_MODVERSIONS" ]; then
-	echo "#define __KSYM_module_layout 1" >> "$output_file"
-fi

scripts/lto-used-symbollist.txt

@@ -1,6 +0,0 @@
-memcpy
-memmove
-memset
-_mcount
-__stack_chk_fail
-__stack_chk_guard