linux-kernel - Re: [PATCH] sysctl: allow embedded targets to disable sysctl

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Message-ID: <m13as3d9ae.fsf@ebiederm.dsl.xmission.com>
Date:	Fri, 08 Feb 2008 03:36:41 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Holger Schurig <hs4233@...l.mn-solutions.de>,
	Michael Opdenacker <michael-lists@...e-electrons.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysctl: allow embedded targets to disable sysctl_check.c

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Thu, 7 Feb 2008 14:38:58 +0100 Holger Schurig <hs4233@...l.mn-solutions.de>
> wrote:
>
>> Disable sysctl_check.c for embedded targets. This saves about about 11 kB
>> in .text and another 11 kB in .data on a PXA255 embedded platform.
>> 
>
> Nice improvement.  But iirc sysctl_check was overtly a temporary thing.
> Eric, was that the intention?

Well so far sysctl_check has been a remarkably effective little piece of code
in catching a great many long over looked bugs.

I do agree that the static tables are big.  My current inclination is to modify
sys_sysctl so that it does a look up in the binary tables to find the ascii
names and then sys_sysctl can lookup the information in the ascii tables.

If we do that we can completely remove ctl_name form the external sysctl data
structures, which should save us quite a bit of space and make it absolutely
impossible to add a new binary name.  And with the current ability to compile
out sys_sysctl the embedded folks would get their space savings.

I believe the only tricky bit is there are a few places in the network code
where we need to translate from ifindex to interface name.  Otherwise
the mapping is fixed.

No that isn't quite right.  Getting the binary to ascii translation for the
values is also a bit tricky.

As for the rest of the checks I don't know if they are that big.  If they
are then an option to compile them out on embedded platforms where you
know what you are doing makes sense.  At the same time sysctl has been so
badly abused in the past, and so very many bugs have been over looked
that I am extremely reluctant to disable simple sanity checks at
registration time.

If we can remove the need for sysctl users to implement the binary
interface many of those checks go completely away as the reason for their
existence would be gone.

I have seen to many absolutely horrible things in the usage of the sysctl
tables to be happy with an option that removes the sanity checks at this
point, although the patch likely makes sense from a code size perspective.

Let's see if we can find a bit of time to make those big tables completely
specific to sys_sysctl and kill ctl_name in the kernel.  Long term that is
a whole lot more maintainable, and smaller for everyone who can disable
sys_sysctl.

Eric

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