[RFC PATCH 1/2] efi: libstub: preserve .debug sections after absolute relocation check

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

The build commands for the ARM and arm64 EFI stubs strips the .debug
sections and other sections that my legally contain absolute relocations,
in order to inspect the remaining sections for the presence of such
relocations.

This leaves us without debugging symbols in the stub for no good reason,
given that these sections are omitted from the kernel binary, and that
these relocations are thus only interpreted by the debugger.

So if the relocation check succeeds, invoke objcopy again, but this time,
leave the .debug sections in place. Note that these sections may refer
to ksymtab/kcrctab contents, so leave those in place as well.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 drivers/firmware/efi/libstub/Makefile | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index c06945160a41..66584f173123 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -60,7 +60,7 @@ CFLAGS_arm64-stub.o 		:= -DTEXT_OFFSET=$(TEXT_OFFSET)
 extra-$(CONFIG_EFI_ARMSTUB)	:= $(lib-y)
 lib-$(CONFIG_EFI_ARMSTUB)	:= $(patsubst %.o,%.stub.o,$(lib-y))
 
-STUBCOPY_FLAGS-y		:= -R .debug* -R *ksymtab* -R *kcrctab*
+STUBCOPY_RM			:= -R .debug* -R *ksymtab* -R *kcrctab*
 STUBCOPY_FLAGS-$(CONFIG_ARM64)	+= --prefix-alloc-sections=.init \
 				   --prefix-symbols=__efistub_
 STUBCOPY_RELOC-$(CONFIG_ARM64)	:= R_AARCH64_ABS
@@ -68,11 +68,20 @@ STUBCOPY_RELOC-$(CONFIG_ARM64)	:= R_AARCH64_ABS
 $(obj)/%.stub.o: $(obj)/%.o FORCE
 	$(call if_changed,stubcopy)
 
+#
+# This calls objcopy twice: the first time it includes STUBCOPY_RM, and inspects
+# the result to ensure that the actual code itself does not contain any absolute
+# references. If this succeeds, the objcopy is performed a second time, but this
+# time the .debug and other sections are retained, given that we know that the
+# absolute relocations they may contain are harmless.
+#
 quiet_cmd_stubcopy = STUBCPY $@
-      cmd_stubcopy = if $(OBJCOPY) $(STUBCOPY_FLAGS-y) $< $@; then	\
-		     $(OBJDUMP) -r $@ | grep $(STUBCOPY_RELOC-y)	\
-		     && (echo >&2 "$@: absolute symbol references not allowed in the EFI stub"; \
-			 rm -f $@; /bin/false); else /bin/false; fi
+      cmd_stubcopy = if $(OBJCOPY) $(STUBCOPY_FLAGS-y) $(STUBCOPY_RM) $< $@; \
+		     then if $(OBJDUMP) -r $@ | grep $(STUBCOPY_RELOC-y);    \
+		     then (echo >&2 "$@: absolute symbol references not allowed in the EFI stub"; \
+			 rm -f $@; /bin/false); 			     \
+		     else $(OBJCOPY) $(STUBCOPY_FLAGS-y) $< $@; fi	     \
+		     else /bin/false; fi
 
 #
 # ARM discards the .data section because it disallows r/w data in the
-- 
2.7.4