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

[David Laight <david.laight.linux@gmail.com>]

On Thu, 24 Apr 2025 18:39:13 +0200
Ard Biesheuvel <ardb@kernel.org> wrote:

> On Thu, 24 Apr 2025 at 18:29, Ingo Molnar <mingo@kernel.org> wrote:
> >
> >
> > * Ard Biesheuvel <ardb+git@google.com> wrote:
> >  
> > > From: Ard Biesheuvel <ardb@kernel.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@kernel.org>
> > > Signed-off-by: Ard Biesheuvel <ardb@kernel.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 :-)

That could be changed to use dd as well - since it defaults to truncating the
destination at the write offset.

	David