linux-kernel - Re: [RFC PATCH 2/2] x86/kexec: Add data section to relocate

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Message-ID: <785f90158524228a32bc4c22bfb4425ceadb0197.camel@infradead.org>
Date: Tue, 12 Nov 2024 10:45:05 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "H. Peter Anvin" <hpa@...or.com>, kexec@...ts.infradead.org, jpoimboe
 <jpoimboe@...nel.org>, 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, "Kirill A. Shutemov"
 <kirill.shutemov@...ux.intel.com>, Kai Huang <kai.huang@...el.com>, Nikolay
 Borisov <nik.borisov@...e.com>, linux-kernel@...r.kernel.org, Simon Horman
 <horms@...nel.org>
Subject: Re: [RFC PATCH 2/2] x86/kexec: Add data section to relocate_kernel

On Tue, 2024-11-12 at 11:14 +0100, Peter Zijlstra wrote:
> On Tue, Nov 12, 2024 at 08:44:33AM +0000, David Woodhouse wrote:
> > On Fri, 2024-11-08 at 12:26 +0100, H. Peter Anvin wrote:
> > > 
> > > > --- a/arch/x86/kernel/vmlinux.lds.S
> > > > +++ b/arch/x86/kernel/vmlinux.lds.S
> > > > @@ -100,7 +100,7 @@ const_pcpu_hot = pcpu_hot;
> > > >         . = ALIGN(PAGE_SIZE);                                   \
> > > >         __relocate_kernel_start = .;                            \
> > > >         *(.text.relocate_kernel);                               \
> > > > -       *(.rodata.relocate_kernel);                             \
> > > > +       *(.data.relocate_kernel);                               \
> 
> Why are we having data in the middle of the text section?

This is the relocate_kernel() page. It's the last thing the kernel
calls on kexec.

The kernel first takes a *copy* of it and places it into the identity
mapping page tables which are set up for kexec.

The relocate_kernel() function is then called at its *original*
location in the kernel text. Before reloading %cr3, it stores some
information which is going to become unavailable into its own page
(currently using a bit of a nasty hack based on a hard-coded
KEXEC_CONTROL_CODE_MAX_SIZE because we can't just use data symbol
references).

Then it reloads %cr3 and jumps to the identity-mapped copy of itself.

Could we put .text.relocate_kernel and .data.relocate_kernel somewhere
*other* than the main kernel text segment? Probably... it use use
alternative instructions, but we could deal with that. And if we call
from machine_kexec() directly into the *copy* (having marked it
executable), maybe...?

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