linux-kernel - Re: [tip: x86/build] x86/boot: Add back some padding for the CRC-32 checksum

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Message-ID: <20250415215427.GLZ_7Vk6t9Vg-kgAhH@fat_crate.local>
Date: Tue, 15 Apr 2025 23:54:27 +0200
From: Borislav Petkov <bp@...en8.de>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Ard Biesheuvel <ardb@...nel.org>, linux-kernel@...r.kernel.org,
	linux-tip-commits@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Ian Campbell <ijc@...lion.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org
Subject: Re: [tip: x86/build] x86/boot: Add back some padding for the CRC-32
 checksum

On Tue, Apr 15, 2025 at 10:00:11AM -0700, H. Peter Anvin wrote:
> >This was done on hpa's request - maybe he has a duration in mind for
> >this grace period?
> 
> I would prefer to leave it indefinitely, because an all zero pattern is far easier to detect than what would look like a false checksum.
> 
> So I remembered eventually who wanted it: it was a direct from flash boot loader who wanted to detect a partial flash failure before invoking the kernel, so that it can redirect to a secondary kernel.

What is a "direct from flash boot loader"?

> That would obviously not be an UEFI environment, so the signing issue is not applicable.
> 
> An all zero end field actually works for that purpose (although requires a boot loader patch), because an unprogrammed flash sector contains FFFFFFFF not 00000000.
> 
> We have kept the bzImage format backwards compatible – sometimes at considerable effort – and the cost of reasonably continuing to do so is absolutely minimal. This is an incompatible change, so at least it is appropriate to give unambiguous indication thereof.
> 
> In other words: it ain't broken, don't try to fix it. It is all downside, no upside.

Sure but look at what is there now:

                /* Add 4 bytes of extra space for the obsolete CRC-32 checksum */
                . = ALIGN(. + 4, 0x200);
                _edata = . ;

This basically screams at me: "delete me, delete me!" :-P

So I would probably put the gist of what you say above as a comment there so
that we have the rationale stated there, for future, trigger-happy
generations.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette