linux-kernel - Re: [PATCH v3] kallsyms: add support for relative offsets in kallsyms address table

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Message-ID: <CAGXu5j+yNZRC99BonVwdawqQTBDQWMKu4gw9TTnA8nOU+fqSMg@mail.gmail.com>
Date:	Thu, 21 Jan 2016 14:55:00 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	"x86@...nel.org" <x86@...nel.org>, Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Michal Marek <mmarek@...e.cz>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Arnd Bergmann <arnd@...db.de>,
	linux-arch <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v3] kallsyms: add support for relative offsets in kallsyms
 address table

On Thu, Jan 21, 2016 at 2:50 PM, Andrew Morton
<akpm@...ux-foundation.org> wrote:
> On Thu, 21 Jan 2016 18:19:43 +0100 Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
>
>> Similar to how relative extables are implemented, it is possible to emit
>> the kallsyms table in such a way that it contains offsets relative to some
>> anchor point in the kernel image rather than absolute addresses. The benefit
>> is that such table entries are no longer subject to dynamic relocation when
>> the build time and runtime offsets of the kernel image are different. Also,
>> on 64-bit architectures, it essentially cuts the size of the address table
>> in half since offsets can typically be expressed in 32 bits.
>>
>> Since it is useful for some architectures (like x86) to retain the ability
>> to emit absolute values as well, this patch adds support for both, by
>> emitting absolute addresses as positive 32-bit values, and addresses
>> relative to the lowest encountered relative symbol as negative values, which
>> are subtracted from the runtime address of this base symbol to produce the
>> actual address.
>>
>> Support for the above is enabled by default for all architectures except
>> IA-64, whose symbols are too far apart to capture in this manner.
>
> I'm not really understanding the benefits of this.  A smaller address
> table is nice, but why is it desirable that "such table entries are no
> longer subject to dynamic relocation when the build time and runtime
> offsets of the kernel image are different"?

IIUC, this means that the relocation work done after decompression now
doesn't have to do relocation updates for all these values, which
means a smaller relocation table as well.

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security