linux-kernel - Re: [PATCH 2/2] Fix efi

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Date:	Thu, 12 May 2016 08:48:35 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Alex Thorlton <athorlton@....com>
Cc:	linux-kernel@...r.kernel.org, Dimitri Sivanich <sivanich@....com>,
	Russ Anderson <rja@....com>, Mike Travis <travis@....com>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Borislav Petkov <bp@...e.de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-efi@...r.kernel.org
Subject: Re: [PATCH 2/2] Fix efi_call


* Alex Thorlton <athorlton@....com> 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@....com>
> Cc: Dimitri Sivanich <sivanich@....com>
> Cc: Russ Anderson <rja@....com>
> Cc: Mike Travis <travis@....com>
> Cc: Matt Fleming <matt@...eblueprint.co.uk>
> Cc: Borislav Petkov <bp@...e.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: x86@...nel.org
> Cc: linux-efi@...r.kernel.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