linux-kernel - Re: [GIT PULL] EFI fix

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Message-ID: <20160517090434.GA21993@codeblueprint.co.uk>
Date:	Tue, 17 May 2016 10:04:34 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>, Alex Thorlton <athorlton@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Borislav Petkov <bp@...en8.de>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [GIT PULL] EFI fix

On Mon, 16 May, at 01:05:45PM, Linus Torvalds wrote:
> 
> So that whole 8-vs-16 offset confusion depends on the frame pointer!
> If frame pointers were enabled, it will be 16. If they weren't, it
> will be 8. That patch that changes it from 8 to 16 will just move the
> bug around. Before, it was correct when frame pointers were disabled
> and buggy otherwise. Now, it's correct if frame pointers are enabled,
> and buggy otherwise.
 
Urgh, right.

We didn't always use frame pointers in efi_call(), they were
introduced in commit 779c433b8ea5c ("x86/asm/efi: Create a stack frame
in efi_call()") earlier this year to stop objtool complaining.

I admit I totally missed the part about pulling arguments off the
stack when I reviewed that patch.

> I may be missing something, but I think that commit is pure garbage.

You're correct.

> I think the right fix is to just get rid of that silly conditional
> frame pointer thing, and always use frame pointers in this stub
> function. And then we don't need that (odd) load to get the old stack
> pointer into %rax - we can just use the frame pointer.
> 
> Something like the attached completely untested patch.

Looks good to me, but I haven't tested it.

Now I'm wondering whether other users of FRAME_BEGIN/FRAME_END make
this same mistake. Coccinelle might be able to detect it perhaps.