linux-kernel - Re: [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in clang

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Message-ID: <CAGdbjmK3oTE=3PizXrvux2XT1GLMPp=Fto-2ySfSf_Em4Tjnsw@mail.gmail.com>
Date: Wed, 10 Jan 2024 09:28:21 -0800
From: Kevin Loughlin <kevinloughlin@...gle.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Kees Cook <keescook@...omium.org>, 
	"Masami Hiramatsu (Google)" <mhiramat@...nel.org>, Ze Gao <zegao2021@...il.com>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Pengfei Xu <pengfei.xu@...el.com>, 
	Brijesh Singh <brijesh.singh@....com>, Michael Roth <michael.roth@....com>, 
	Ashish Kalra <ashish.kalra@....com>, Tom Lendacky <thomas.lendacky@....com>, 
	Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
	linux-coco@...ts.linux.dev, Adam Dunlap <acdunlap@...gle.com>, 
	Peter Gonda <pgonda@...gle.com>, Jacob Xu <jacobhxu@...gle.com>, 
	Sidharth Telang <sidtelang@...gle.com>
Subject: Re: [RFC PATCH] x86/sev: x86/sev: enforce PC-relative addressing in clang

On Wed, Jan 10, 2024 at 5:37 AM Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> Similar issues was fixed before with fixup_pointer() tricks. Have you
> tried looking this direction.
>
> Relevant thread starting with:
>
> https://lore.kernel.org/all/20210920192341.maue7db4lcbdn46x@box.shutemov.name

Yes, I considered using `fixup_pointer()` as well, and I agree that it
remains a valid option to solve this problem. I just now mentioned the
primary downside to using `fixup_pointer()` in my response to Andi
(https://lore.kernel.org/lkml/CAGdbjmJcQeLWroBkbRC-cFQ6OuMHONsGLzpUrrA5k3uc8Rzkmw@mail.gmail.com/).
Namely, we would have to pass around `physaddr` from `__startup64()`
to any functions wanting to use `fixup_pointer()`). However, we should
be able to get around needing `physaddr` by using something like the
`GET_RIP_RELATIVE_PTR()` macro that I proposed in the same reply
(forthcoming in v2 of this RFC PATCH).