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

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Message-ID: <20160124002134.192eb470@cotter.ozlabs.ibm.com>
Date:	Sun, 24 Jan 2016 00:21:34 +1100
From:	Balbir Singh <bsingharora@...il.com>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Marek <mmarek@...e.cz>,
	"linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Kees Cook <keescook@...omium.org>,
	Arnd Bergmann <arnd@...db.de>,
	"x86@...nel.org" <x86@...nel.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-arch <linux-arch@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v3] kallsyms: add support for relative offsets in
 kallsyms address table

On Fri, 22 Jan 2016 22:54:44 +0100
Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:

> 
> """
> kallsyms: add support for relative offsets in kallsyms address table
> 
> 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.
> 
> On 64-bit architectures, it cuts the size of the kallsyms address table in
> half, since offsets between kernel symbols can typically be expressed in 32
> bits. This saves several hundreds of kilobytes of permanent .rodata on
> average. In addition, the kallsyms address table is no longer subject to
> dynamic relocation when CONFIG_RELOCATABLE is in effect, so the relocation
> work done after decompression now doesn't have to do relocation updates for
> all these values. This saves up to 24 bytes (i.e., the size of a ELF64 RELA
> relocation table entry) per value, which easily adds up to a couple of
> megabytes of uncompressed __init data on ppc64 or arm64. Even if these
> relocation entries typically compress well, the combined size reduction of
> 2.8 MB uncompressed for a ppc64_defconfig build (of which 2.4 MB is __init
> data) results in a ~500 KB space saving in the compressed image.
> 
> 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.

snip

I still don't get the 2GB limitaiton, because of the 32 bit address
does it imply that modules load with -2GB to +2GB of the kernel base
address of the kallsyms address table?

Balbir Singh.