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Message-ID: <9e876a7c73574bc29a769998d87acd7c5a0f72cc.camel@infradead.org>
Date: Mon, 06 Jan 2025 16:13:22 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Ard Biesheuvel <ardb@...nel.org>
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>, Eric Biederman
<ebiederm@...ssion.com>, Sourabh Jain <sourabhjain@...ux.ibm.com>, Hari
Bathini <hbathini@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>,
Thomas Zimmermann <tzimmermann@...e.de>, Andrew Morton
<akpm@...ux-foundation.org>, Baoquan He <bhe@...hat.com>, Yuntao Wang
<ytcoode@...il.com>, David Kaplan <david.kaplan@....com>, Tao Liu
<ltao@...hat.com>, "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
Kai Huang <kai.huang@...el.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
Breno Leitao <leitao@...ian.org>, Wei Yang <richard.weiyang@...il.com>,
Rong Xu <xur@...gle.com>, Thomas Weißschuh
<thomas.weissschuh@...utronix.de>, linux-kernel@...r.kernel.org,
kexec@...ts.infradead.org, Simon Horman <horms@...nel.org>, Dave Young
<dyoung@...hat.com>, Peter Zijlstra <peterz@...radead.org>, bsz@...zon.de,
nathan@...nel.org
Subject: Re: [PATCH 8/9] x86/kexec: Cope with relocate_kernel() not being at
the start of the page
On Mon, 2025-01-06 at 17:09 +0100, Ard Biesheuvel wrote:
>
> I suppose you're referring to
>
> #ifdef CONFIG_KEXEC_JUMP
> /* Saved in save_processor_state. */
> movq $saved_context, %rax
> lgdt saved_context_gdt_desc(%rax)
> #endif
>
> Any reason not to simply use
>
> lgdt saved_context+saved_context_gdt_desc(%rip)
>
> here?
Because the %rip isn't what you (and the linker) think it is.
This code is copied into a control page which is allocated as part of
the kexec kimage. It can only access things within that same page via
%rip. (Which is not as much of a restriction as it sounds, as for most
of its execution the rest of the kernel isn't even present in the page
tables).
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