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

[Ard Biesheuvel]

On Fri, 3 Jan 2025 at 11:10, David Woodhouse <dwmw2 at infradead.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?