linux-kernel - [PATCH] efi/libstub: zboot: do not use $(shell ...) in cmd_copy_and

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Date: Mon, 18 Dec 2023 17:01:27 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>,
	linux-efi@...r.kernel.org
Cc: linux-kbuild@...r.kernel.org,
	Masahiro Yamada <masahiroy@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] efi/libstub: zboot: do not use $(shell ...) in cmd_copy_and_pad

You do not need to use $(shell ...) in recipe lines, as they are already
executed in a shell. An alternative solution is $$(...), which is an
escaped sequence of the shell's command substituion, $(...).

For this case, there is a reason to avoid $(shell ...).

Kbuild detects command changes by using the if_changed macro, which
compares the previous command recorded in .*.cmd with the current
command from Makefile. If they differ, Kbuild re-runs the build rule.

To diff the commands, Make must expand $(shell ...) first. It means that
hexdump is executed every time, even when nothing needs rebuilding. If
Kbuild determines that vmlinux.bin needs rebuilding, hexdump will be
executed again to evaluate the 'cmd' macro, one more time to really
build vmlinux.bin, and finally yet again to record the expanded command
into .*.cmd.

Replace $(shell ...) with $$(...) to avoid multiple, unnecessay shell
evaluations. Since Make is agnostic about the shell code, $(...), the
if_changed macro compares the string "$(hexdump -s16 -n4 ...)" verbatim,
so hexdump is run only for building vmlinux.bin.

For the same reason, $(shell ...) in EFI_ZBOOT_OBJCOPY_FLAGS should be
eliminated.

While I was here, I replaced '&&' with ';' because a command for
if_changed is executed with 'set -e'.

Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
---

 arch/arm64/boot/Makefile                    | 2 +-
 drivers/firmware/efi/libstub/Makefile.zboot | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/boot/Makefile b/arch/arm64/boot/Makefile
index 1761f5972443..a5a787371117 100644
--- a/arch/arm64/boot/Makefile
+++ b/arch/arm64/boot/Makefile
@@ -44,7 +44,7 @@ EFI_ZBOOT_BFD_TARGET	:= elf64-littleaarch64
 EFI_ZBOOT_MACH_TYPE	:= ARM64
 EFI_ZBOOT_FORWARD_CFI	:= $(CONFIG_ARM64_BTI_KERNEL)

-EFI_ZBOOT_OBJCOPY_FLAGS	= --add-symbol zboot_code_size=0x$(shell \
+EFI_ZBOOT_OBJCOPY_FLAGS	= --add-symbol zboot_code_size=0x$$( \
 				$(NM) vmlinux|grep _kernel_codesize|cut -d' ' -f1)

 include $(srctree)/drivers/firmware/efi/libstub/Makefile.zboot
diff --git a/drivers/firmware/efi/libstub/Makefile.zboot b/drivers/firmware/efi/libstub/Makefile.zboot
index 2c489627a807..65ffd0b760b2 100644
--- a/drivers/firmware/efi/libstub/Makefile.zboot
+++ b/drivers/firmware/efi/libstub/Makefile.zboot
@@ -5,8 +5,8 @@
 # EFI_ZBOOT_FORWARD_CFI

 quiet_cmd_copy_and_pad = PAD     $@
-      cmd_copy_and_pad = cp $< $@ && \
-			 truncate -s $(shell hexdump -s16 -n4 -e '"%u"' $<) $@
+      cmd_copy_and_pad = cp $< $@; \
+			 truncate -s $$(hexdump -s16 -n4 -e '"%u"' $<) $@

 # Pad the file to the size of the uncompressed image in memory, including BSS
 $(obj)/vmlinux.bin: $(obj)/$(EFI_ZBOOT_PAYLOAD) FORCE
-- 
2.40.1