kbuild: reuse intermediate linker scripts in the final link steps

ld.bfd forces `--undefined X` symbols to be added to the resulting
binary even if they're never used. In contrast, ld.lld may silently
discard them if they're not referenced.

If a kernel exported symbol (EXPORT_SYMBOL*(X)) is not used
internally, it may get stripped away by ld.lld. An obvious example
is __memcat_p(), which is only used by the `stm` module. With
CONFIG_STM=m, the build fails:

  ERROR: "__memcat_p" [drivers/hwtracing/stm/stm_core.ko] undefined!

Work around this issue by reusing the intermediate linker scripts
in the final link steps.

Signed-off-by: Ilie Halip <[email protected]>

Makefile

@@ -975,18 +975,20 @@ drivers-y	:= $(patsubst %/, %/built-in.a, $(drivers-y))
 net-y		:= $(patsubst %/, %/built-in.a, $(net-y))
 libs-y1		:= $(patsubst %/, %/lib.a, $(libs-y))
 libs-y2		:= $(patsubst %/, %/built-in.a, $(filter-out %.a, $(libs-y)))
+libs-lds	:= $(strip $(patsubst %/, %/.lib-ksyms.o.lds, $(libs-y)))
 virt-y		:= $(patsubst %/, %/built-in.a, $(virt-y))
 
 # Externally visible symbols (used by link-vmlinux.sh)
 export KBUILD_VMLINUX_INIT := $(head-y) $(init-y)
 export KBUILD_VMLINUX_MAIN := $(core-y) $(libs-y2) $(drivers-y) $(net-y) $(virt-y)
 export KBUILD_VMLINUX_LIBS := $(libs-y1)
 export KBUILD_LDS          := arch/$(SRCARCH)/kernel/vmlinux.lds
+export KBUILD_EXTRA_LDS    := $(libs-lds)
 export LDFLAGS_vmlinux
 # used by scripts/package/Makefile
 export KBUILD_ALLDIRS := $(sort $(filter-out arch/%,$(vmlinux-alldirs)) arch Documentation include samples scripts tools)
 
-vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN) $(KBUILD_VMLINUX_LIBS)
+vmlinux-deps := $(KBUILD_LDS) $(KBUILD_EXTRA_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN) $(KBUILD_VMLINUX_LIBS)
 
 # Recurse until adjust_autoksyms.sh is satisfied
 PHONY += autoksyms_recursive

scripts/Makefile.build

@@ -484,7 +484,7 @@ cmd_export_list = $(OBJDUMP) -h $< | \
 	rm -f $(dummy-object);\
 	echo | $(CC) $(a_flags) -c -o $(dummy-object) -x assembler -;\
 	$(LD) $(ld_flags) -r -o $@ -T $(ksyms-lds) $(dummy-object);\
-	rm $(dummy-object) $(ksyms-lds)
+	rm $(dummy-object)
 
 $(obj)/lib-ksyms.o: $(lib-target) FORCE
 	$(call if_changed,export_list)

scripts/link-vmlinux.sh

@@ -84,9 +84,15 @@ modpost_link()
 vmlinux_link()
 {
 	local lds="${objtree}/${KBUILD_LDS}"
+	local extra_lds=""
 	local objects
 
 	if [ "${SRCARCH}" != "um" ]; then
+		for extra_ld in ${KBUILD_EXTRA_LDS}
+		do
+			extra_lds="$extra_lds -T ${objtree}/$extra_ld"
+		done
+
 		objects="--whole-archive			\
 			built-in.a				\
 			--no-whole-archive			\
@@ -96,8 +102,13 @@ vmlinux_link()
 			${1}"
 
 		${LD} ${KBUILD_LDFLAGS} ${LDFLAGS_vmlinux} -o ${2}	\
-			-T ${lds} ${objects}
+			-T ${lds} ${extra_lds} ${objects}
 	else
+		for extra_ld in ${KBUILD_EXTRA_LDS}
+		do
+			extra_lds="$extra_lds -Wl,-T,${objtree}/$extra_ld"
+		done
+
 		objects="-Wl,--whole-archive			\
 			built-in.a				\
 			-Wl,--no-whole-archive			\
@@ -108,6 +119,7 @@ vmlinux_link()
 
 		${CC} ${CFLAGS_vmlinux} -o ${2}			\
 			-Wl,-T,${lds}				\
+			${extra_lds}				\
 			${objects}				\
 			-lutil -lrt -lpthread
 		rm -f linux