[PATCH 4/4] efi: On x86-64, align the stack to a 16-byte boundary as required by ABI

[Josh Triplett <josh@joshtriplett.org>]

The x86-64 ABI specification requires a 16-byte-aligned stack.  In some
cases, GCC emits code that assumes this alignment, which crashes if not
aligned.  The EFI firmware is also entitled to assume that stack
alignment without checking, and some firmware does make that assumption.
---

ChangeLog entry:

2013-11-13  Josh Triplett  <josh@joshtriplett.org>

	* grub-core/kern/x86_64/efi/startup.S (_start): Align the stack to a
	  16-byte boundary, as required by the x86-64 ABI, before calling
	  grub_main.  In some cases, GCC emits code that assumes this
	  alignment, which crashes if not aligned.  The EFI firmware is also
	  entitled to assume that stack alignment without checking.

 grub-core/kern/x86_64/efi/startup.S | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/grub-core/kern/x86_64/efi/startup.S b/grub-core/kern/x86_64/efi/startup.S
index f86f019..94bd6ae 100644
--- a/grub-core/kern/x86_64/efi/startup.S
+++ b/grub-core/kern/x86_64/efi/startup.S
@@ -29,7 +29,11 @@ start:
 _start:
 	movq	%rcx, EXT_C(grub_efi_image_handle)(%rip)
 	movq	%rdx, EXT_C(grub_efi_system_table)(%rip)
-
+	mov	%rsp, %rax
+	subq	$8, %rsp
+	and	$~0xf, %rsp
+	mov	%rax, (%rsp)
 	call	EXT_C(grub_main)
+	mov	(%rsp), %rsp
 	ret
 
-- 
1.8.4.3