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: <CAMj1kXEXPTdm=oA7+WPPBcCX69x_N_Y8woaaNwDFh0QAZNjf5g@mail.gmail.com>
Date: Mon, 6 Jan 2025 17:09:12 +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 Fri, 3 Jan 2025 at 11:10, David Woodhouse <dwmw2@...radead.org> wrote:
>
> On Tue, 2024-12-17 at 10:25 +0100, Ard Biesheuvel wrote:
> >
> > > > You can still avoid the absolute relocations though, ...
> > > ...
> > > > > +       addq    $identity_mapped, %rsi
> > > > > +       subq    $__relocate_kernel_start, %rsi
> > > >
> > > > ... if you turn this into
> > > >
> > > > 0:     addq    $identity_mapped - 0b, %rsi
> > > >        subq    $__relocate_kernel_start - 0b, %rsi
> > >
> > > Is there any benefit to doing so? Are absolute relocations problematic?
> >
> > Every absolute relocation produces an entry in the relocation table
> > that needs to be applied at every boot when KASLR is in effect. Beyond
> > that, it doesn't matter.
> >
> > I've looked into PIC codegen/PIE linking for the core kernel, which is
> > why this caught my eye. If that effort ever advances, I'll need to
> > revisit this code as well and apply the change I suggested.
>
> OK, since it looks like I'll be reposting this series once I'm back at
> a keyboard for real, I've done that in my tree.
>

Thanks

> There's one more absolute relocation, for saved_context just before
> returning to the kernel from the 'virtual_mapped' code. That's only
> reloading the GDT, and we could probably do that from the C code in
> machine_kexec().

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?