linux-kernel - [RFC PATCH 1/1] libstub: zboot: Use -ffunction-sections + --gc-sections

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Message-Id: <20230621114439.42851-2-alexghiti@rivosinc.com>
Date:   Wed, 21 Jun 2023 13:44:39 +0200
From:   Alexandre Ghiti <alexghiti@...osinc.com>
To:     Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
        linux-efi@...r.kernel.org
Cc:     Alexandre Ghiti <alexghiti@...osinc.com>
Subject: [RFC PATCH 1/1] libstub: zboot: Use -ffunction-sections + --gc-sections

Static linking is done at an object file (actually section) granularity,
meaning that if a function in an object has an undefined reference, the
link will fail, even though this function is not used in the resulting
ELF.

To avoid such failures, allow the linker to garbage collect unused sections
(which actually are functions).

Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>
---
 drivers/firmware/efi/libstub/Makefile       | 1 +
 drivers/firmware/efi/libstub/Makefile.zboot | 2 +-
 2 files changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile
index 3abb2b357482..6182366d00c0 100644
--- a/drivers/firmware/efi/libstub/Makefile
+++ b/drivers/firmware/efi/libstub/Makefile
@@ -40,6 +40,7 @@ KBUILD_CFLAGS			:= $(subst $(CC_FLAGS_FTRACE),,$(cflags-y)) \
 				   -ffreestanding \
 				   -fno-stack-protector \
 				   $(call cc-option,-fno-addrsig) \
+				   -ffunction-sections \
 				   -D__DISABLE_EXPORTS
 
 #
diff --git a/drivers/firmware/efi/libstub/Makefile.zboot b/drivers/firmware/efi/libstub/Makefile.zboot
index 89ef820f3b34..5bb52d262133 100644
--- a/drivers/firmware/efi/libstub/Makefile.zboot
+++ b/drivers/firmware/efi/libstub/Makefile.zboot
@@ -51,7 +51,7 @@ $(obj)/zboot-header.o: $(srctree)/drivers/firmware/efi/libstub/zboot-header.S FO
 
 ZBOOT_DEPS := $(obj)/zboot-header.o $(objtree)/drivers/firmware/efi/libstub/lib.a
 
-LDFLAGS_vmlinuz.efi.elf := -T $(srctree)/drivers/firmware/efi/libstub/zboot.lds
+LDFLAGS_vmlinuz.efi.elf := -T $(srctree)/drivers/firmware/efi/libstub/zboot.lds --gc-sections
 $(obj)/vmlinuz.efi.elf: $(obj)/vmlinuz.o $(ZBOOT_DEPS) FORCE
 	$(call if_changed,ld)
 
-- 
2.39.2