[tip: x86/asm] x86/boot: Simplify calculation of output address

[tip-bot2 for Arvind Sankar]

The following commit has been merged into the x86/asm branch of tip:

Commit-ID:     183ef7adf4ed638ac0fb0c3c9a71fc00e8512b61
Gitweb:        https://git.kernel.org/tip/183ef7adf4ed638ac0fb0c3c9a71fc00e8512b61
Author:        Arvind Sankar <nivedita@xxxxxxxxxxxx>
AuthorDate:    Tue, 07 Jan 2020 14:44:34 -05:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Thu, 23 Jan 2020 11:58:43 +01:00

x86/boot: Simplify calculation of output address

Condense the calculation of decompressed kernel start a little.

Committer notes:

before:

ebp = ebx - (init_size - _end)

after:

eax = (ebx + _end) - init_size

where in both ebx contains the temporary address the kernel is moved to
for in-place decompression.

The before and after difference in register state is %eax and %ebp
but that is immaterial because the compressed image is not built with
-mregparm, i.e., all arguments of the following extract_kernel() call
are passed on the stack.

Signed-off-by: Arvind Sankar <nivedita@xxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lkml.kernel.org/r/20200107194436.2166846-1-nivedita@xxxxxxxxxxxx
---
 arch/x86/boot/compressed/head_32.S | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index f2dfd6d..1cc55c7 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -240,11 +240,9 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
 				/* push arguments for extract_kernel: */
 	pushl	$z_output_len	/* decompressed length, end of relocs */
 
-	movl    BP_init_size(%esi), %eax
-	subl    $_end, %eax
-	movl    %ebx, %ebp
-	subl    %eax, %ebp
-	pushl	%ebp		/* output address */
+	leal	_end(%ebx), %eax
+	subl    BP_init_size(%esi), %eax
+	pushl	%eax		/* output address */
 
 	pushl	$z_input_len	/* input_len */
 	leal	input_data(%ebx), %eax