linux-kernel - Re: [REGRESSION] "efi: efistub: Convert into static library" and preparation patches

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Date:	Thu, 4 Sep 2014 08:47:57 +0200
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Matt Fleming <matt@...sole-pimps.org>,
	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	Ulf Winkelvos <ulf@...kelvos.de>,
	Matt Fleming <matt.fleming@...el.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	Seth Forshee <seth.forshee@...onical.com>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [REGRESSION] "efi: efistub: Convert into static library" and
 preparation patches

On 3 September 2014 23:47, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> Any reason we can't reuse the existing GOT fixup code in the early x86
>> boot code? We're not executing it before the EFI boot stub atm, which is
>> the reason Maarten is hitting these difficulties.
>>
>> Maarten, does the following help?
>>
>> If not, Ard please go ahead with option #2 above. Overkill yes, but I've
>> done the single __attribute__() hacks in other projects and someone
>> (usually me) always eventually forgets to tag some instance.
>>
>
> I think we really have two options: either fix up the GOT (which may be
> a null operation, if the GOT is empty) or we add a compile-time check
> that the GOT is empty, lest we will keep having these problems.
>
> Since the GOT fixup loop is only a few instructions, it doesn't seem to
> be all that problematic to just do it -- but make sure we don't end up
> running it twice on any code path!
>

So how about we:
- add ASSERT(_got == _egot, "GOT entries not supported in
boot/compressed") to the linker script
- #define __nogotentry __attribute__((visibility(hidden))) somewhere
in compiler.h or wherever else it belongs
- add the __nogotentry qualifiers to each extern that requires it,
which is currently:
early_serial_base
efi_early
free_mem_end_ptr
free_mem_ptr
real_mode
- get rid of the fixup code in assembly

Any idea what the oldest GCC is we should support?

-- 
Ard.
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