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

[David Woodhouse]

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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