linux-kernel - Re: CTL_UNNUMBERED (Re: [PATCH] 9p: Don't use binary sysctl numbers.)

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Date:	Sat, 21 Jul 2007 16:36:03 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Alexey Dobriyan <adobriyan@...il.com>
Cc:	Latchesar Ionkov <lucho@...kov.net>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: CTL_UNNUMBERED (Re: [PATCH] 9p: Don't use binary sysctl numbers.)

Alexey Dobriyan <adobriyan@...il.com> writes:

>
> That's separate patch but CTL_UNNUMBERED must die, because it's totally
> unneeded. If you don't want sysctl(2) interface just SKIP ->ctl_name
> initialization and save one line for something useful.

As for the 9p code it doesn't seem to need or want a real binary
interface.  The 9p debug code picking of a semi-random number and not
patching it into sysctl.h like it should for a binary interface is
an implementation bug, and a maintenance problem.

Further it is a classic example of the silliness that goes on
when people actually try and add to the binary interface.

So not assigning a binary number very much looks like the right thing
to do for 9p.

I expect if the change had not happened in a mega patch to 9p that
seems to have changed everything the addition of a new user space
interface would more likely have been caught in a code review.

Now to the issue of using CTL_UNNUMBERED versus knowing that the magic
value is zero and we can just leave it uninitialized.  I don't much
care but given how often people who are not actively watching this
mess up I tend to prefer the explicit value.  It is a practical
question of how do we get the word out that we should not expand the
binary interface anymore. 

The only really practical way I can see us doing better then we are
today is to have a separate tree that maps binary numbers into ascii
strings and so we remove the ctl_name field entirely from ctl_table. 

That way people attempting to assign binary numbers using old
conventions will have code that doesn't even compile, and the
developers themselves are more likely to spot the problem.

Eric
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