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

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Message-ID: <CAMj1kXH7Vi8ruqohvRrPM+Dk2vPtyaMUkNRzi0Ak77aq6Q_peg@mail.gmail.com>
Date: Thu, 24 Apr 2025 18:39:13 +0200
From: Ard Biesheuvel <ardb@...nel.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org, x86@...nel.org, 
	phasta@...nel.org, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] x86/boot: Work around broken busybox truncate tool

On Thu, 24 Apr 2025 at 18:29, Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Ard Biesheuvel <ardb+git@...gle.com> wrote:
>
> > 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.
>
> >  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))) >$@
>
> So the workaround isn't too terrible, and since someone did trigger the
> bug, debugged it and reported it to us, it costs us very little to
> apply the workaround and (re-)enable someone's Linux build environment.
>

Indeed.

> Also there's almost no existing usage of 'truncate' within the kernel
> build system. Found one only:
>
>    drivers/firmware/efi/libstub/Makefile.zboot:                  truncate -s $$(hexdump -s16 -n4 -e '"%u"' $<) $@
>

Yeah, and it was me who added that one too :-)