linux-kernel - Re: [PATCH v2 07/15] x86/lib/copy_user_64.S: Convert to ALTERNATIVE

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Date:	Thu, 5 Mar 2015 01:32:49 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Borislav Petkov <bp@...en8.de>
Cc:	X86 ML <x86@...nel.org>, Andy Lutomirski <luto@...capital.net>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v2 07/15] x86/lib/copy_user_64.S: Convert to ALTERNATIVE_2


* Borislav Petkov <bp@...en8.de> wrote:

> >  2) in the future: we could actually do a (limited) re-link of the
> >     kernel during bootup, and patch up the original copy_to_user call
> >     sites directly to one of the three variants. Alternatives patching
> >     done at the symbol level. Does current tooling allow something
> >     like this already?
> 
> Well, I have a patchset which uses relocs to patch vmlinux at build 
> time. And that was the initial approach to this but you cannot know 
> which features a CPU supports until boot time so you have to boot.
> 
> BUT(!), you can replace stuff like X86_FEATURE_ALWAYS at build time 
> already (this is the static_cpu_has_safe() stuff). I'll look into 
> that later and dust off my relocs pile.

We could also do a (limited) relink during early bootup, as part of 
the alternatives patching pass in essence: for that we need to stick 
the relocation info into a section and put that into the vmlinux.

Thanks,

	Ingo
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