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

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Date:	Thu, 21 Jan 2016 15:20:45 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kees Cook <keescook@...omium.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, 21 Jan 2016 14:55:00 -0800 Kees Cook <keescook@...omium.org> wrote:

> 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.

Makes sense, thanks.  I altered the changelog

: 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 so 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.
: 
: Also, the 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.