Re: [PATCH 2/2] Fix efi_call

Ingo Molnar
2016-05-12T06:48:35Z

* Alex Thorlton <athorlton-sJ/iWh9BUns@public.gmane.org> wrote:

> The efi_call assembly code has a slight error that prevents us from
> using arguments 7 and higher, which will be passed in on the stack.
>
> mov (%rsp), %rax
> mov 8(%rax), %rax
> ...
> mov %rax, 40(%rsp)
>
> This code goes and grabs the return address for the current stack frame,
> and puts it on the stack, next the 5th argument for the EFI runtime
> call. Considering the fact that having the return address in that
> position on the stack makes no sense, I'm guessing that the intent of
> this code was actually to grab an argument off the stack frame for this
> call and place it into the frame for the next one.
>
> The small change to that offset (i.e. 8(%rax) to 16(%rax)) ensures that
> we grab the 7th argument off the stack, and pass it as the 6th argument
> to the EFI runtime function that we're about to call. This change gets
> our EFI runtime calls that need to pass more than 6 arguments working
> again.

I suppose the SGI/UV code is the only one using 7 arguments or more? Might make
sense to point that out in the changelog.

>
> Signed-off-by: Alex Thorlton <athorlton-sJ/iWh9BUns@public.gmane.org>
> Cc: Dimitri Sivanich <sivanich-sJ/iWh9BUns@public.gmane.org>
> Cc: Russ Anderson <rja-sJ/iWh9BUns@public.gmane.org>
> Cc: Mike Travis <travis-sJ/iWh9BUns@public.gmane.org>
> Cc: Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org>
> Cc: Borislav Petkov <bp-l3A5Bk7waGM@public.gmane.org>
> Cc: Thomas Gleixner <tglx-hfZtesqFncYOwBW4kG4KsQ@public.gmane.org>
> Cc: Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
> Cc: "H. Peter Anvin" <hpa-YMNOUZJC4hwAvxtiuMwx3w@public.gmane.org>
> Cc: x86-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org
> Cc: linux-efi-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
> ---
> arch/x86/platform/efi/efi_stub_64.S | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/platform/efi/efi_stub_64.S b/arch/x86/platform/efi/efi_stub_64.S
> index 92723ae..62938ff 100644
> --- a/arch/x86/platform/efi/efi_stub_64.S
> +++ b/arch/x86/platform/efi/efi_stub_64.S
> @@ -43,7 +43,7 @@ ENTRY(efi_call)
> FRAME_BEGIN
> SAVE_XMM
> mov (%rsp), %rax
> - mov 8(%rax), %rax
> + mov 16(%rax), %rax
> subq $48, %rsp
> mov %r9, 32(%rsp)
> mov %rax, 40(%rsp)

Just curious, how did you find this bug? It's a pretty obscure one, of the
'developer tears out hairs from frustruation' type ...

Thanks,

Ingo