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

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Date:	Mon, 7 Jan 2013 10:38:12 -0500
From:	Neil Horman <nhorman@...driver.com>
To:	Florian Fainelli <florian@...nwrt.org>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Vlad Yasevich <vyasevich@...il.com>, linux-sctp@...r.kernel.org
Subject: Re: [PATCH v2] sctp: Change defaults on cookie hmac selection

On Mon, Jan 07, 2013 at 04:15:24PM +0100, Florian Fainelli wrote:
> Le 01/07/13 15:49, Neil Horman a écrit :
> >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.
> 
> Ok for the explanation, but this still breaks an oldconfig because
> we do not actually propose the user with a default choice:
> 
>     choice[1-3?]:     Default SCTP cookie HMAC encoding
>       1. Enable optional MD5 hmac cookie generation
> (SCTP_DEFAULT_COOKIE_HMAC_MD5) (NEW)
>       2. Enable optional SHA1 hmac cookie generation
> (SCTP_DEFAULT_COOKIE_HMAC_SHA1) (NEW)
>       3. Use no hmac alg in SCTP cookie generation
> (SCTP_DEFAULT_COOKIE_HMAC_NONE) (NEW)
> 
> I do not see any difference in what I am proposed if the default
> config symbol is SCTP_DEFAULT_COOKIE_HMAC_MD5, I can still
> optionally choose SHA1 to be supported, and I do have a valid
> default config for this choice. While if I keep SCTP_COOKIE_HMAC_MD5
No, thats the problem, your old config is no longer valid with this new Kconfig
file.  Your config is telling the config utility that you want your default
Cookie hmac to be MD5, but you've explicitly told it (via your yes "" | make
oldconfig command), that you want SCTP_COOKIE_HMAC_MD5 to be disabled, so the
config utility is left with no choice to prompt you again for a default hmac,
which your command answers again by saying SCTP_DEFAULT_COOKIE_HMAC_MD5 (the
default choice of 1).  Thats your loop, you keep telling the config utility that
you both want the default hmac to be md5, and that you don't want to allow md5
to be an available hmac alg.  

Thats not a bug.  I'm sorry if your old configuration needs manual updating, but
there are no guarantees that old configurations will 'just work' in perpituity.

Neil

> as the default I have to manually enter which option I want.
> --
> Florian
> --
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