linux-kernel - Re: [PATCH 8/9] x86/kexec: Cope with relocate_kernel() not being at the start of the page

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Message-ID: <CAMj1kXG=AdGHZctOtJ2A06hF056MepEwnPT4DKDMN3d4S5+-ww@mail.gmail.com>
Date: Mon, 6 Jan 2025 17:27:26 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: David Woodhouse <dwmw2@...radead.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, 6 Jan 2025 at 17:13, David Woodhouse <dwmw2@...radead.org> wrote:
>
> 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).

Ah I was looking at an older version of that file - I now see that the
preceding (%rip) references have been replaced as well.

In any case, thanks for the head's up - I'll get back to this at some
point and cc you if making any further changes here.