linux-kernel - Re: [PATCH] enhance usability of /proc/sys/net/ipv4/ip_local_reserved

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Message-ID: <4F5E6600.2070806@gmx.de>
Date:	Mon, 12 Mar 2012 22:09:20 +0100
From:	Helge Deller <deller@....de>
To:	Cong Wang <amwang@...hat.com>
CC:	Octavian Purdila <opurdila@...acom.com>,
	Linux Kernel Development <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Frank Danapfel <fdanapfe@...hat.com>,
	Laszlo Ersek <lersek@...hat.com>,
	Linus <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] enhance usability of /proc/sys/net/ipv4/ip_local_reserved_ports

On 03/12/2012 04:42 AM, Cong Wang wrote:
> On Sun, 2012-03-11 at 00:36 +0100, Helge Deller wrote:
>> When writing to the ip_local_reserved_ports proc file it will currently clear
>> all previously reserved ports and update the current list with the one given
>> in the input.
>>
>> This behaviour makes it's usage quite hard, for example:
>> a) The generic proc filesystem limitation of only handle up to PAGE_SIZE-1
>>     characters at maximum may not be sufficient to provide all your wished-to-
>>     be-reserved ports at once.
>
> Yes, this should be extended IMHO.

Yes, known problem and not easy to fix.

>> b) There is no easy way to disable specific given ports, you always need to
>>     give the full port list at once. This makes shell scripting hard, since
>>     you need to parse everything yourself.
>> c) There is no easy way to just add specific ports or port ranges. Again,
>>     this would be useful for shell scripts.
>>
>
> These could be calculated in user-space, although it maybe not as easy
> as you want.

Right.

>> The following patch solves this problem by simply extending the parser
>> in proc_do_large_bitmap() to accept the keywords "add" and "release" in front
>> of given ports or port ranges and to either add or drop the given ports
>> from the already existing list.
>
> This looks a little odd, because we do "magic" things with a sysctl
> file, which is supposed to be plain text file. Do we have existing
> examples?

I don't think the networking sysctl has such tunables.
Overall, ip_local_reserved_ports is the only user of the
large_bitmap function and as such you can't compare the
input/output of this file with other sysctl files which
operate on a limited number of integers/booleans/strings only.
Furthermore my patch does not remove the "plain text"
behaviour of this file. You can still echo plain ports or
port lists into it and a "cat" gives you the same output
as before. It just extends the interface to add/remove
ports more easily if you want.

One somewhat comparable proc file which comes to my mind in this
regard is /proc/scsi/scsi where you can echo commands like
"scsi report-devs 1" and which then reacts. A user of this
interface is e.g. the known rescan-scsi-bus shell script:
http://www.garloff.de/kurt/linux/rescan-scsi-bus.sh-1.25

> BTW, as David mentioned, please Cc netdev next time.

Sure. I'll send an updated patch tomorrow and will CC netdev.

Thanks,
Helge
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