[PATCH 15/28] x86/boot/compressed/32: Save the output address instead of recalculating it

Ard Biesheuvel
2020-03-08T08:10:04Z

From: Arvind Sankar <nivedita@alum.mit.edu>

In preparation for being able to decompress into a buffer starting at a
different address than startup_32, save the calculated output address
instead of recalculating it later.

We now keep track of three addresses:
%edx: startup_32 as we were loaded by bootloader
%ebx: new location of compressed kernel
%ebp: start of decompression buffer

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200303221205.4048668-2-nivedita@alum.mit.edu
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
arch/x86/boot/compressed/head_32.S | 25 ++++++++++++-------------
1 file changed, 12 insertions(+), 13 deletions(-)

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index f250fc49cd61..2a8965aec234 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -75,11 +75,11 @@ SYM_FUNC_START(startup_32)
*/
leal (BP_scratch+4)(%esi), %esp
call 1f
-1: popl %ebp
- subl $1b, %ebp
+1: popl %edx
+ subl $1b, %edx

/* Load new GDT */
- leal gdt(%ebp), %eax
+ leal gdt(%edx), %eax
movl %eax, 2(%eax)
lgdt (%eax)

@@ -92,13 +92,14 @@ SYM_FUNC_START(startup_32)
movl %eax, %ss

/*
- * %ebp contains the address we are loaded at by the boot loader and %ebx
+ * %edx contains the address we are loaded at by the boot loader and %ebx
* contains the address where we should move the kernel image temporarily
- * for safe in-place decompression.
+ * for safe in-place decompression. %ebp contains the address that the kernel
+ * will be decompressed to.
*/

#ifdef CONFIG_RELOCATABLE
- movl %ebp, %ebx
+ movl %edx, %ebx
movl BP_kernel_alignment(%esi), %eax
decl %eax
addl %eax, %ebx
@@ -110,10 +111,10 @@ SYM_FUNC_START(startup_32)
movl $LOAD_PHYSICAL_ADDR, %ebx
1:

+ movl %ebx, %ebp // Save the output address for later
/* Target address to relocate to for decompression */
- movl BP_init_size(%esi), %eax
- subl $_end, %eax
- addl %eax, %ebx
+ addl BP_init_size(%esi), %ebx
+ subl $_end, %ebx

/* Set up the stack */
leal boot_stack_end(%ebx), %esp
@@ -127,7 +128,7 @@ SYM_FUNC_START(startup_32)
* where decompression in place becomes safe.
*/
pushl %esi
- leal (_bss-4)(%ebp), %esi
+ leal (_bss-4)(%edx), %esi
leal (_bss-4)(%ebx), %edi
movl $(_bss - startup_32), %ecx
shrl $2, %ecx
@@ -197,9 +198,7 @@ SYM_FUNC_START_LOCAL_NOALIGN(.Lrelocated)
/* push arguments for extract_kernel: */
pushl $z_output_len /* decompressed length, end of relocs */

- leal _end(%ebx), %eax
- subl BP_init_size(%esi), %eax
- pushl %eax /* output address */
+ pushl %ebp /* output address */

pushl $z_input_len /* input_len */
leal input_data(%ebx), %eax
--
2.17.1