linux-kernel - Re: [PATCH v2 1/3] Add the initify gcc plugin

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Message-ID: <CAGXu5j+99nc9s-ci46s3r_CU5-6O6JeHERcW3mYRtnP71871rw@mail.gmail.com>
Date:	Tue, 12 Jul 2016 18:38:47 -0400
From:	Kees Cook <keescook@...omium.org>
To:	Russell King - ARM Linux <linux@...linux.org.uk>
Cc:	Emese Revfy <re.emese@...il.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>,
	PaX Team <pageexec@...email.hu>,
	Brad Spengler <spender@...ecurity.net>,
	Michal Marek <mmarek@...e.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Masahiro Yamada <yamada.masahiro@...ionext.com>,
	linux-kbuild <linux-kbuild@...r.kernel.org>, minipli@...linux.so,
	Catalin Marinas <catalin.marinas@....com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	David Brown <david.brown@...aro.org>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jeff Layton <jlayton@...chiereds.net>,
	Arnd Bergmann <arnd@...db.de>, Sam Ravnborg <sam@...nborg.org>,
	Karsten Keil <isdn@...ux-pingi.de>
Subject: Re: [PATCH v2 1/3] Add the initify gcc plugin

On Tue, Jul 12, 2016 at 6:08 PM, Russell King - ARM Linux
<linux@...linux.org.uk> wrote:
> On Tue, Jul 12, 2016 at 03:45:56PM -0400, Kees Cook wrote:
>> On Mon, Jul 4, 2016 at 7:40 PM, Emese Revfy <re.emese@...il.com> wrote:
>> > If a function is called by __init and __exit functions as well then
>> > the plugin moves it to the __exit section. This causes false positive
>> > section mismatch errors/warnings that I don't know how to handle yet.
>>
>> Should the mismatch checker be updated to recognize this case? Without
>> the plugin, I assume these kinds of functions would only ever be
>> marked for __exit? If so, should the plugin strip the __init marking
>> and only add __exit?
>
> That sounds like a problem for architectures that still discard the
> __exit section at link time to reduce the size of the linked kernel
> image - though, obviously, if using the plugin results in a smaller
> kernel image _with_ the exit sections, then there's a net benefit
> size-wise.

Ah right, __exit is dropped for non-modular builds. So, for "both
__init and __exit" it sounds like the behavior depends on the build:

- if modular: remove __init marking (since we need it after init)
- if non-modular: remove __exit marking (since we'll never call exit)

Is this something the build itself (rather than the plugin) could
notice and fix up? Hmmm

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security