linux-kernel - Re: [RFC PATCH PoC 00/11] x86: strict separation of startup code

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Message-ID: <aAp-SThmX5PcsrWU@gmail.com>
Date: Thu, 24 Apr 2025 20:09:13 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Ard Biesheuvel <ardb+git@...gle.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [RFC PATCH PoC 00/11] x86: strict separation of startup code


* Ard Biesheuvel <ardb+git@...gle.com> wrote:

> From: Ard Biesheuvel <ardb@...nel.org>
> 
> This is a proof-of-concept series that implements a strict separation
> between startup code and ordinary code, where startup code is built in a
> way that tolerates being invoked from the initial 1:1 mapping of memory.
> 
> The current approach of emitting this code into .head.text and checking
> for absolute relocations in that section is not 100% safe, and produces
> diagnostics that are sometimes difficult to interpret.
> 
> Instead, rely on symbol prefixes, similar to how this is implemented for
> the EFI stub and for the startup code in the arm64 port. This ensures
> that startup code can only call other startup code, unless a special
> symbol alias is emitted that exposes a non-startup routine to the
> startup code.

So when startup code accidentally references non-startup symbols 
outside the __pi namespace, we get a build/link error, right?

> This is somewhat intrusive, as there are many data objects that are 
> referenced both by startup code and by ordinary code, and an alias 
> needs to be emitted for each of those.

Yeah, but this should make it ultimately safe(r): every object is 
either local to the startup code, or has been 'exported' intentionally 
to the startup code.

> This ultimately allows the .head.text section to be dropped entirely, 
> as it no longer has a special significance. Instead, code that only 
> executes at boot is emitted into .init.text as it should.
> 
> This series is presented for discussion only - defconfig should build
> and run correctly, but allmodconfig will likely need the last patch
> omitted. 

No fundamental objections from me.

Thanks,

	Ingo