On Wed, Apr 24, 2024 at 12:06 PM Ard Biesheuvel <ardb+git@xxxxxxxxxx> wrote:
>
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> ---
> 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).
>
> /* 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
>
> /* 0x10 4GB flat code segment */
> --
> 2.44.0.769.g3c40516874-goog
Brian Gerst
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