s390/purgatory: Make sure we fail the build if purgatory has missing symbols

Since we link purgatory with -r aka we enable "incremental linking"
no checks for unresolved symbols are done while linking the purgatory.

This commit adds an extra check for unresolved symbols by calling ld
without -r before running objcopy to generate purgatory.ro.

This will help us catch missing symbols in the purgatory sooner.

Note this commit also removes --no-undefined from LDFLAGS_purgatory
as that has no effect.
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/lkml/20191212205304.191610-1-hdegoede@redhat.comTested-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

arch/s390/purgatory/.gitignore

 purgatory
+purgatory.chk
 purgatory.lds
 purgatory.ro

arch/s390/purgatory/Makefile

@@ -4,7 +4,7 @@ OBJECT_FILES_NON_STANDARD := y
 
 purgatory-y := head.o purgatory.o string.o sha256.o mem.o
 
-targets += $(purgatory-y) purgatory.lds purgatory purgatory.ro
+targets += $(purgatory-y) purgatory.lds purgatory purgatory.chk purgatory.ro
 PURGATORY_OBJS = $(addprefix $(obj)/,$(purgatory-y))
 
 $(obj)/sha256.o: $(srctree)/lib/crypto/sha256.c FORCE
@@ -26,15 +26,22 @@ KBUILD_CFLAGS += $(CLANG_FLAGS)
 KBUILD_CFLAGS += $(call cc-option,-fno-PIE)
 KBUILD_AFLAGS := $(filter-out -DCC_USING_EXPOLINE,$(KBUILD_AFLAGS))
 
-LDFLAGS_purgatory := -r --no-undefined -nostdlib -z nodefaultlib -T
+# Since we link purgatory with -r unresolved symbols are not checked, so we
+# also link a purgatory.chk binary without -r to check for unresolved symbols.
+PURGATORY_LDFLAGS := -nostdlib -z nodefaultlib
+LDFLAGS_purgatory := -r $(PURGATORY_LDFLAGS) -T
+LDFLAGS_purgatory.chk := -e purgatory_start $(PURGATORY_LDFLAGS)
 $(obj)/purgatory: $(obj)/purgatory.lds $(PURGATORY_OBJS) FORCE
 		$(call if_changed,ld)
 
+$(obj)/purgatory.chk: $(obj)/purgatory FORCE
+		$(call if_changed,ld)
+
 OBJCOPYFLAGS_purgatory.ro := -O elf64-s390
 OBJCOPYFLAGS_purgatory.ro += --remove-section='*debug*'
 OBJCOPYFLAGS_purgatory.ro += --remove-section='.comment'
 OBJCOPYFLAGS_purgatory.ro += --remove-section='.note.*'
-$(obj)/purgatory.ro: $(obj)/purgatory FORCE
+$(obj)/purgatory.ro: $(obj)/purgatory $(obj)/purgatory.chk FORCE
 		$(call if_changed,objcopy)
 
 $(obj)/kexec-purgatory.o: $(obj)/kexec-purgatory.S $(obj)/purgatory.ro FORCE