[PATCH] efi/x86: Drop the special GDT for the EFI thunk

[PATCH] efi/x86: Drop the special GDT for the EFI thunk

[Arvind Sankar]

Instead of using efi_gdt64 to switch back to 64-bit mode and then
switching to the real boot-time GDT, just switch to the boot-time GDT
directly. The two GDT's are identical other than efi_gdt64 not including
the 32-bit code segment.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
---
 arch/x86/boot/compressed/efi_thunk_64.S | 19 +++----------------
 1 file changed, 3 insertions(+), 16 deletions(-)

diff --git a/arch/x86/boot/compressed/efi_thunk_64.S b/arch/x86/boot/compressed/efi_thunk_64.S
index 2b2049259619..c4bb0f9363f5 100644
--- a/arch/x86/boot/compressed/efi_thunk_64.S
+++ b/arch/x86/boot/compressed/efi_thunk_64.S
@@ -28,8 +28,6 @@ SYM_FUNC_START(__efi64_thunk)
 	push	%rbx
 
 	leaq	1f(%rip), %rbp
-	leaq	efi_gdt64(%rip), %rbx
-	movl	%ebx, 2(%rbx)		/* Fixup the gdt base address */
 
 	movl	%ds, %eax
 	push	%rax
@@ -48,7 +46,8 @@ SYM_FUNC_START(__efi64_thunk)
 	movl	%r8d, 0xc(%rsp)
 	movl	%r9d, 0x10(%rsp)
 
-	sgdt	0x14(%rsp)
+	leaq	0x14(%rsp), %rbx
+	sgdt	(%rbx)
 
 	/*
 	 * Switch to gdt with 32-bit segments. This is the firmware GDT
@@ -68,8 +67,7 @@ SYM_FUNC_START(__efi64_thunk)
 	pushq	%rax
 	lretq
 
-1:	lgdt	0x14(%rsp)
-	addq	$32, %rsp
+1:	addq	$32, %rsp
 	movq	%rdi, %rax
 
 	pop	%rbx
@@ -175,14 +173,3 @@ SYM_DATA_END(efi32_boot_cs)
 SYM_DATA_START(efi32_boot_ds)
 	.word	0
 SYM_DATA_END(efi32_boot_ds)
-
-SYM_DATA_START(efi_gdt64)
-	.word	efi_gdt64_end - efi_gdt64
-	.long	0			/* Filled out by user */
-	.word	0
-	.quad	0x0000000000000000	/* NULL descriptor */
-	.quad	0x00af9a000000ffff	/* __KERNEL_CS */
-	.quad	0x00cf92000000ffff	/* __KERNEL_DS */
-	.quad	0x0080890000000000	/* TS descriptor */
-	.quad   0x0000000000000000	/* TS continued */
-SYM_DATA_END_LABEL(efi_gdt64, SYM_L_LOCAL, efi_gdt64_end)
-- 
2.26.2

Re: [PATCH] efi/x86: Drop the special GDT for the EFI thunk

[Ard Biesheuvel]

On Sun, 24 May 2020 at 00:15, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> Instead of using efi_gdt64 to switch back to 64-bit mode and then
> switching to the real boot-time GDT, just switch to the boot-time GDT
> directly. The two GDT's are identical other than efi_gdt64 not including
> the 32-bit code segment.
>
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>

Queued, thanks.

> ---
>  arch/x86/boot/compressed/efi_thunk_64.S | 19 +++----------------
>  1 file changed, 3 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/boot/compressed/efi_thunk_64.S b/arch/x86/boot/compressed/efi_thunk_64.S
> index 2b2049259619..c4bb0f9363f5 100644
> --- a/arch/x86/boot/compressed/efi_thunk_64.S
> +++ b/arch/x86/boot/compressed/efi_thunk_64.S
> @@ -28,8 +28,6 @@ SYM_FUNC_START(__efi64_thunk)
>         push    %rbx
>
>         leaq    1f(%rip), %rbp
> -       leaq    efi_gdt64(%rip), %rbx
> -       movl    %ebx, 2(%rbx)           /* Fixup the gdt base address */
>
>         movl    %ds, %eax
>         push    %rax
> @@ -48,7 +46,8 @@ SYM_FUNC_START(__efi64_thunk)
>         movl    %r8d, 0xc(%rsp)
>         movl    %r9d, 0x10(%rsp)
>
> -       sgdt    0x14(%rsp)
> +       leaq    0x14(%rsp), %rbx
> +       sgdt    (%rbx)
>
>         /*
>          * Switch to gdt with 32-bit segments. This is the firmware GDT
> @@ -68,8 +67,7 @@ SYM_FUNC_START(__efi64_thunk)
>         pushq   %rax
>         lretq
>
> -1:     lgdt    0x14(%rsp)
> -       addq    $32, %rsp
> +1:     addq    $32, %rsp
>         movq    %rdi, %rax
>
>         pop     %rbx
> @@ -175,14 +173,3 @@ SYM_DATA_END(efi32_boot_cs)
>  SYM_DATA_START(efi32_boot_ds)
>         .word   0
>  SYM_DATA_END(efi32_boot_ds)
> -
> -SYM_DATA_START(efi_gdt64)
> -       .word   efi_gdt64_end - efi_gdt64
> -       .long   0                       /* Filled out by user */
> -       .word   0
> -       .quad   0x0000000000000000      /* NULL descriptor */
> -       .quad   0x00af9a000000ffff      /* __KERNEL_CS */
> -       .quad   0x00cf92000000ffff      /* __KERNEL_DS */
> -       .quad   0x0080890000000000      /* TS descriptor */
> -       .quad   0x0000000000000000      /* TS continued */
> -SYM_DATA_END_LABEL(efi_gdt64, SYM_L_LOCAL, efi_gdt64_end)
> --
> 2.26.2
>