[RFC PATCH 4/9] x86/purgatory: Avoid absolute reference to GDT

[Ard Biesheuvel]

Hi Brian,

Thanks for taking a look.

On Wed, 24 Apr 2024 at 19:39, Brian Gerst <brgerst at gmail.com> wrote:
>
> On Wed, Apr 24, 2024 at 12:06 PM Ard Biesheuvel <ardb+git at google.com> wrote:
> >
> > From: Ard Biesheuvel <ardb at kernel.org>
> >
> > The purgatory is almost entirely position independent, without any need
> > for any relocation processing at load time except for the reference to
> > the GDT in the entry code. Generate this reference at runtime instead,
> > to remove the last R_X86_64_64 relocation from this code.
> >
> > While the GDT itself needs to be preserved in memory as long as it is
> > live, the GDT descriptor that is used to program the GDT can be
> > discarded so it can be allocated on the stack.
> >
> > Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> > ---
> >  arch/x86/purgatory/entry64.S | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/purgatory/entry64.S b/arch/x86/purgatory/entry64.S
> > index 9913877b0dbe..888661d9db9c 100644
> > --- a/arch/x86/purgatory/entry64.S
> > +++ b/arch/x86/purgatory/entry64.S
> > @@ -16,7 +16,11 @@
> >
> >  SYM_CODE_START(entry64)
> >         /* Setup a gdt that should be preserved */
> > -       lgdt gdt(%rip)
> > +       leaq    gdt(%rip), %rax
> > +       pushq   %rax
> > +       pushw   $gdt_end - gdt - 1
> > +       lgdt    (%rsp)
> > +       addq    $10, %rsp
>
> This misaligns the stack, pushing 16 bytes on the stack but only
> removing 10 (decimal).
>

pushw subtracts 2 from RSP and stores a word. So the total size stored
is 10 decimal not 16.

> >
> >         /* load the data segments */
> >         movl    $0x18, %eax     /* data segment */
> > @@ -83,8 +87,8 @@ SYM_DATA_START_LOCAL(gdt)
> >          * 0x08 unused
> >          * so use them as gdt ptr
>
> obsolete comment
>
> >          */
> > -       .word gdt_end - gdt - 1
> > -       .quad gdt
> > +       .word 0
> > +       .quad 0
> >         .word 0, 0, 0
>
> This can be condensed down to:
>         .quad 0, 0
>

This code and the comment are removed in the next patch.