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@alum.mit.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200107194436.2166846-1-nivedita@alum.mit.edu

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index f2dfd6d083ef2c6094ef5a67489176bc36ba9d28..1cc55c79d1d0dbbf7f566108f804e97ddc22d9c2 100644 (file)
--- 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