linux-kernel - Re: [RFC PATCH resend] x86/boot: Drop CRC-32 checksum and the build tool that generates it

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Message-ID: <CAMj1kXGmyssEYer_Su1N+s9svNd3yNm7W4bf9DYHsJszxHoyUg@mail.gmail.com>
Date: Tue, 11 Mar 2025 18:44:09 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Vitaly Kuznetsov <vkuznets@...hat.com>, Ard Biesheuvel <ardb+git@...gle.com>, 
	linux-kernel@...r.kernel.org, x86@...nel.org, 
	Gerd Hoffmann <kraxel@...hat.com>, Juergen Gross <jgross@...e.com>, Jan Beulich <jbeulich@...e.com>, 
	Paul Durrant <paul@....org>, David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC PATCH resend] x86/boot: Drop CRC-32 checksum and the build
 tool that generates it

On Tue, 11 Mar 2025 at 18:29, H. Peter Anvin <hpa@...or.com> wrote:
>
> On March 11, 2025 10:25:15 AM PDT, Ard Biesheuvel <ardb@...nel.org> wrote:
> >On Tue, 11 Mar 2025 at 18:14, H. Peter Anvin <hpa@...or.com> wrote:
> >>
> >> >Ard Biesheuvel <ardb+git@...gle.com> writes:
> >> >
> >...
> >> >> it seems quite unlikely that this checksum is being used, so let's just
> >> >> drop it, along with the tool that generates it.
> >> >>
> >> >> Instead, use simple file concatenation and truncation to combine the two
> >> >> pieces into bzImage, and replace the checks on the size of the setup
> >> >> block with a couple of ASSERT()s in the linker script.
> >> >>
> >...
> >>
> >> Please leave the bytes in question as explicit zeroes if possible.
> >
> >Keeping the
> >
> >. = ALIGN(. + 4, 0x200);
> >
> >in arch/x86/boot/compressed/vmlinux.lds.S should be sufficient to
> >guarantee that the last 4 bytes of the file are zero, so it is quite
> >trivial to implement. However, I'm not quite sure what purpose that
> >would serve: could you elaborate?
>
> It means if someone *does* care it will be easier for them to adjust.

I.e., someone can always stick a CRC-32 into the last 4 bytes if they
wanted to? Yeah that makes sense.