Re: [PATCH 1/5] x86/boot/compressed/32: Save the output address instead of recalculating it

From: Ard Biesheuvel <ardb@kernel.org>
To: Arvind Sankar <nivedita@alum.mit.edu>
Cc: linux-efi <linux-efi@vger.kernel.org>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/5] x86/boot/compressed/32: Save the output address instead of recalculating it
Date: Tue, 3 Mar 2020 20:10:46 +0100	[thread overview]
Message-ID: <CAKv+Gu87ssM3g+Hc9cPJPy5oFkgFCtueovwxmLK-gTKzUbSLNQ@mail.gmail.com> (raw)
In-Reply-To: <20200301230537.2247550-2-nivedita@alum.mit.edu>

On Mon, 2 Mar 2020 at 00:05, Arvind Sankar <nivedita@alum.mit.edu> wrote:
>
> In preparation for being able to decompress starting at a different
> address than startup_32, save the calculated output address instead of
> recalculating it later.
>

Could you expand this a bit? Are you talking about *running* the
decompressor code at another offset? Or about the space it uses. I
think I know but I'd like to be sure :-)

> 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>
> ---
>  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 46bbe7ab4adf..894182500606 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
> @@ -196,9 +197,7 @@ 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
> --
> 2.24.1
>