linux-kernel - [PATCH] x86/boot: Work around broken busybox truncate tool

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Message-ID: <20250424101917.1552527-2-ardb+git@google.com>
Date: Thu, 24 Apr 2025 12:19:18 +0200
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-kernel@...r.kernel.org
Cc: x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>, phasta@...nel.org
Subject: [PATCH] x86/boot: Work around broken busybox truncate tool

From: Ard Biesheuvel <ardb@...nel.org>

The GNU coreutils version of truncate, which is the original, accepts a
% prefix for the -s size argument which means the file in question
should be padded to a multiple of the given size. This is currently used
to pad the setup block of bzImage to a multiple of 4k before appending
the decompressor.

busybux reimplements truncate but does not support this idiom, and
therefore fails the build since commit

  9c54baab4401 ("x86/boot: Drop CRC-32 checksum and the build tool that generates it")

Work around this by avoiding truncate altogether, and relying on dd to
perform the padding.

Reported-by: <phasta@...nel.org>
Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
---
I personally think using a busybox environment for building the kernel
is a terrible idea, and does not satisfy the build tool requirements
listed in the documentation. But apparently, it used to work and now it
doesn't, and the workaround is rather straight-forward.

IOW, I don't care whether this gets applied or not, so I will leave it
to others to make the argument.

 arch/x86/boot/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/boot/Makefile b/arch/x86/boot/Makefile
index 81f55da81967..640fcac3af74 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -59,7 +59,7 @@ KBUILD_CFLAGS	+= $(CONFIG_CC_IMPLICIT_FALLTHROUGH)
 $(obj)/bzImage: asflags-y  := $(SVGA_MODE)
 
 quiet_cmd_image = BUILD   $@
-      cmd_image = cp $< $@; truncate -s %4K $@; cat $(obj)/vmlinux.bin >>$@
+      cmd_image = (dd if=$< bs=4k conv=sync status=none; cat $(filter-out $<,$(real-prereqs))) >$@
 
 $(obj)/bzImage: $(obj)/setup.bin $(obj)/vmlinux.bin FORCE
 	$(call if_changed,image)
-- 
2.49.0.805.g082f7c87e0-goog