On Wed, 17 Jun 2020 at 15:20, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
>
> Commit
> 17054f492dfd ("efi/x86: Implement mixed mode boot without the handover protocol")
> introduced a new entry point for the EFI stub to be booted in mixed mode
> on 32-bit firmware.
>
> When entered via efi32_pe_entry, control is first transferred to
> startup_32 to setup for the switch to long mode, and then the EFI stub
> proper is entered via efi_pe_entry. efi_pe_entry is an MS ABI function,
> and the ABI requires 32 bytes of shadow stack space to be allocated by
> the caller, as well as the stack being aligned to 8 mod 16 on entry.
>
> Allocate 40 bytes on the stack before switching to 64-bit mode when
> calling efi_pe_entry to account for this.
>
> For robustness, explicitly align boot_stack_end to 16 bytes. It is
> currently implicitly aligned since .bss is cacheline-size aligned,
> head_64.o is the first object file with a .bss section, and the heap and
> boot sizes are aligned.
>
> Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Cheers
> ---
> arch/x86/boot/compressed/head_64.S | 11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index e821a7d7d5c4..97d37f0a34f5 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -213,7 +213,6 @@ SYM_FUNC_START(startup_32)
> * We place all of the values on our mini stack so lret can
> * used to perform that far jump.
> */
> - pushl $__KERNEL_CS
> leal startup_64(%ebp), %eax
> #ifdef CONFIG_EFI_MIXED
> movl efi32_boot_args(%ebp), %edi
> @@ -224,11 +223,20 @@ SYM_FUNC_START(startup_32)
> movl efi32_boot_args+8(%ebp), %edx // saved bootparams pointer
> cmpl $0, %edx
> jnz 1f
> + /*
> + * efi_pe_entry uses MS calling convention, which requires 32 bytes of
> + * shadow space on the stack even if all arguments are passed in
> + * registers. We also need an additional 8 bytes for the space that
> + * would be occupied by the return address, and this also results in
> + * the correct stack alignment for entry.
> + */
> + subl $40, %esp
> leal efi_pe_entry(%ebp), %eax
> movl %edi, %ecx // MS calling convention
> movl %esi, %edx
> 1:
> #endif
> + pushl $__KERNEL_CS
> pushl %eax
>
> /* Enter paged protected Mode, activating Long Mode */
> @@ -784,6 +792,7 @@ SYM_DATA_LOCAL(boot_heap, .fill BOOT_HEAP_SIZE, 1, 0)
>
> SYM_DATA_START_LOCAL(boot_stack)
> .fill BOOT_STACK_SIZE, 1, 0
> + .balign 16
> SYM_DATA_END_LABEL(boot_stack, SYM_L_LOCAL, boot_stack_end)
>
> /*
> --
> 2.26.2
>
