netdev - Re: [PATCH v2] sctp: Change defaults on cookie hmac selection

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Date:	Mon, 7 Jan 2013 10:46:25 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	Florian Fainelli <florian@...nwrt.org>, netdev@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-sctp@...r.kernel.org
Subject: Re: [PATCH v2] sctp: Change defaults on cookie hmac selection

On Mon, Jan 07, 2013 at 10:32:01AM -0500, Vlad Yasevich wrote:
> On 01/07/2013 09:49 AM, Neil Horman wrote:
> >On Mon, Jan 07, 2013 at 02:25:39PM +0100, Florian Fainelli wrote:
> >>Hello Neil,
> >>
> >>Le 12/15/12 02:22, Neil Horman a écrit :
> >>>Recently I posted commit 3c68198e75 which made selection of the cookie hmac
> >>>algorithm selectable.  This is all well and good, but Linus noted that it
> >>>changes the default config:
> >>>http://marc.info/?l=linux-netdev&m=135536629004808&w=2
> >>>
> >>>I've modified the sctp Kconfig file to reflect the recommended way of making
> >>>this choice, using the thermal driver example specified, and brought the
> >>>defaults back into line with the way they were prior to my origional patch
> >>>
> >>>Also, on Linus' suggestion, re-adding ability to select default 'none' hmac
> >>>algorithm, so we don't needlessly bloat the kernel by forcing a non-none
> >>>default.  This also led me to note that we won't honor the default none
> >>>condition properly because of how sctp_net_init is encoded.  Fix that up as
> >>>well.
> >>>
> >>>Tested by myself (allbeit fairly quickly).  All configuration combinations seems
> >>>to work soundly.
> >>>
> >>>Signed-off-by: Neil Horman <nhorman@...driver.com>
> >>>CC: David Miller <davem@...emloft.net>
> >>>CC: Linus Torvalds <torvalds@...ux-foundation.org>
> >>>CC: Vlad Yasevich <vyasevich@...il.com>
> >>>CC: linux-sctp@...r.kernel.org
> >>>---
> >>>  net/sctp/Kconfig    | 27 +++++++++++++++++++++++++--
> >>>  net/sctp/protocol.c |  4 ++--
> >>>  2 files changed, 27 insertions(+), 4 deletions(-)
> >>>
> >>>diff --git a/net/sctp/Kconfig b/net/sctp/Kconfig
> >>>index a9edd2e..c262106 100644
> >>>--- a/net/sctp/Kconfig
> >>>+++ b/net/sctp/Kconfig
> >>>@@ -66,12 +66,36 @@ config SCTP_DBG_OBJCNT
> >>>  	  'cat /proc/net/sctp/sctp_dbg_objcnt'
> >>>
> >>>  	  If unsure, say N
> >>>+choice
> >>>+	prompt "Default SCTP cookie HMAC encoding"
> >>>+	default SCTP_COOKIE_HMAC_MD5
> >>
> >>Should not this be SCTP_DEFAULT_COOKIE_HMAC_MD5? I just tried to
> >>update to 3.8-rc2, and I usually build my kernel-headers with:
> >>
> >>yes '' | ARCH=foo make oldconfig
> >>
> >>and this just kept asking me for this config symbol because none
> >>could be provided.
> >>--
> >>Florian
> >>
> >
> >No, the config mechanism is setup to offer the user the ability to choose a
> >default cookie hmac, alg, then optionally select any other hmac algs you would
> >like to be made available (in the event you want to change the default at run
> >time).  When you select the default, it eables (via the select directive), the
> >corresponding SCTP_COOKIE_HMAC_* config option, which is used in the build, and
> >then prompts for the remaining values.
> >
> 
> Neil
> 
> Actually, I think it should be as Florian suggests.  The default
> value of the choice should be one of the values defined as part of
> the choice (the SCTP_DEFAULT_COOKIE_*).  Turning on appropriate
> default would turn on appropriate cookie config
> (SCTP_COOKIE_HMAC_*).
> 
I absolutely disagree.

> Would that save all the config trouble?
> 
Yes, it would fix it as Florian has noted, but at the cost of silently modifying
what the default hmac config vaule is.  If you've expressly disabled
SCTP_COOKIE_HMAC_MD5, and then blindly take the default choice in the
SCTP_DEFAULT_COOKIE selection option (the default default as it were), using the
approach your suggesting, then that will silently enable SCTP_COOKIE_HMAC_MD5
again, which may not be expected by users.  If you expressly have a config
option disabled in an old configuration, we should leave it there.

We're doing the right thing now, IMO.  When presented with a conflictly set
of configuration options, the config utilty is (repeatedly) prompting us to
resolve them.  That seems like a much more reasonable approach to this, than
silently changing pre-existing options so people can do the equivalent of just
blindly pressing enter through the config process (which is all yes "" | make
oldconfig is).

This is a momentary hiccup, corrected by taking 30 seconds to make a manual
config change (or by taking a second to understand what the config utility is
tell us by prompting for a default choice repeatedly).  Theres nothing to fix
here.

Neil

> -vlad
> 
> >Neil
> >
> 
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