[tip: x86/urgent] x86/boot: Work around broken busybox 'truncate' tool

["tip-bot2 for Ard Biesheuvel" <tip-bot2@linutronix.de>]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     032ce1ea9442e140a80e41078b5431d4c0fa2893
Gitweb:        https://git.kernel.org/tip/032ce1ea9442e140a80e41078b5431d4c0fa2893
Author:        Ard Biesheuvel <ardb@kernel.org>
AuthorDate:    Thu, 24 Apr 2025 12:19:18 +02:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Thu, 24 Apr 2025 18:23:27 +02:00

x86/boot: Work around broken busybox 'truncate' tool

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.

busybox 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")

Since very little build code within the kernel depends on the 'truncate'
utility, work around this incompatibility by avoiding truncate altogether,
and relying on dd to perform the padding.

Fixes: 9c54baab4401 ("x86/boot: Drop CRC-32 checksum and the build tool that generates it")
Reported-by: <phasta@kernel.org>
Tested-by: Philipp Stanner <phasta@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250424101917.1552527-2-ardb+git@google.com
---
 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 81f55da..640fcac 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)