linux-kernel - Re: [PATCH 2/2] kallsyms: handle special absolute symbols

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Date:	Thu, 6 Mar 2014 21:42:41 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrew Honig <ahonig@...gle.com>,
	Michal Marek <mmarek@...e.cz>,
	"x86@...nel.org" <x86@...nel.org>, Tejun Heo <tj@...nel.org>,
	Tony Luck <tony.luck@...el.com>,
	Fenghua Yu <fenghua.yu@...el.com>, linux-ia64@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] kallsyms: handle special absolute symbols

On Thu, Mar 6, 2014 at 7:25 PM, Rusty Russell <rusty@...tcorp.com.au> wrote:
> Kees Cook <keescook@...omium.org> writes:
>> This forces the entire per_cpu range to be reported as absolute without
>> losing their linker symbol types. Without this, the variables are
>> incorrectly shown as relocated under kASLR.
>
> I like these patches, thanks!

Oh good! Glad this is getting closer. :)

> This one's a bit broken, since the zero-based __per_cpu_start/end thing
> is an x86-64-ism.  You really do want them relocated on other
> platforms, so I think you'll need do make this conditional via
> a --per-cpu-absolute flag to kallsyms (which x86-64 would set).

Ah, hm. Can this maybe just be dynamically detected (e.g. if
__per_cpu_start == 0?), I'd hate to have another arch run into this
glitch when we could "notice" it and deal with it instead.

> Dumb Q: why don't we actually present these symbols as absolute in
> /proc/kallsyms?  Seems like it would be clearer...

You mean set "sym[0] = 'A'" instead of the force_absolute thing I
added? It seemed like I shouldn't mess with existing information, and
as you say, they're not absolute on all platforms.

-Kees

-- 
Kees Cook
Chrome OS Security
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