linux-kernel - Re: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

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Message-ID: <99446363-152f-43a8-8b74-26f0d883a364@zytor.com>
Date: Tue, 1 Oct 2024 14:13:05 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org
Cc: Ard Biesheuvel <ardb@...nel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, Uros Bizjak <ubizjak@...il.com>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky
 <boris.ostrovsky@...cle.com>,
        Greg Kroah-Hartman
 <gregkh@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>, Masahiro Yamada <masahiroy@...nel.org>,
        Kees Cook <kees@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
        Keith Packard <keithp@...thp.com>,
        Justin Stitt <justinstitt@...gle.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>, linux-doc@...r.kernel.org,
        linux-pm@...r.kernel.org, kvm@...r.kernel.org,
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        rust-for-linux@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [RFC PATCH 25/28] x86: Use PIE codegen for the core kernel

On 9/25/24 08:01, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> As an intermediate step towards enabling PIE linking for the 64-bit x86
> kernel, enable PIE codegen for all objects that are linked into the
> kernel proper.
> 
> This substantially reduces the number of relocations that need to be
> processed when booting a relocatable KASLR kernel.
> 

This really seems like going completely backwards to me.

You are imposing a more restrictive code model on the kernel, optimizing 
for boot time in a way that will exert a permanent cost on the running 
kernel.

There is a *huge* difference between the kernel and user space here:

KERNEL MEMORY IS PERMANENTLY ALLOCATED, AND IS NEVER SHARED.

Dirtying user pages requires them to be unshared and dirty, which is 
undesirable. Kernel pages are *always* unshared and dirty.

> It also brings us much closer to the ordinary PIE relocation model used
> for most of user space, which is therefore much better supported and
> less likely to create problems as we increase the range of compilers and
> linkers that need to be supported.

We have been resisting *for ages* making the kernel worse to accomodate 
broken compilers. We don't "need" to support more compilers -- we need 
the compilers to support us. We have working compilers; any new compiler 
that wants to play should be expected to work correctly.

	-hpa