Subject: Re: [PATCH net-next v2] ipv6: sr: fix IPv6 initialization failure without lwtunnels

From: David Lebrun <[email-masked]>
Date: Sun, 13 Nov 2016 20:59:21 +0100

On 11/13/2016 06:23 AM, David Miller wrote:
> This seems like such a huge mess, quite frankly.
> 
> IPV6-SR has so many strange dependencies, a weird Kconfig option that is
> simply controlling what a responsible sysadmin should be allow to do if
> he chooses anyways.
> 
> Every distribution is going to say "¯\_(ツ)_/¯" and just turn the thing
> on in their builds.

Indeed, the issue is that seg6_iptunnel.o was included in obj-y instead
of ipv6-y, triggering the bug when CONFIG_IPV6=m. Fixed with the
following modification to the patch (tested with allyesconfig and
allmodconfig):

diff --git a/net/ipv6/Makefile b/net/ipv6/Makefile
index 8979d53..a233136 100644
--- a/net/ipv6/Makefile
+++ b/net/ipv6/Makefile
@@ -53,6 +53,6 @@ obj-$(subst m,y,$(CONFIG_IPV6)) += inet6_hashtables.o

 ifneq ($(CONFIG_IPV6),)
 obj-$(CONFIG_NET_UDP_TUNNEL) += ip6_udp_tunnel.o
-obj-$(CONFIG_LWTUNNEL) += seg6_iptunnel.o
+ipv6-$(CONFIG_LWTUNNEL) += seg6_iptunnel.o
 obj-y += mcast_snoop.o
 endif

I agree with you that the way to combine the dependencies is strange,
even if they are very few. The part of the IPv6-SR patch that is enabled
by default depends on two things: IPV6 and LWTUNNEL. The problem is that
LWTUNNEL does not depend on IPV6 and is not necessarily enabled. To fix
the bug reported by Lorenzo, I propose to select one the three following
solutions:

1. Make LWTUNNEL always enabled (removing the option).
   Pros: remove an option
   Cons: add always-enabled code

2. Create an option IPV6_SEG6_LWTUNNEL, which would select LWTUNNEL and
enable the compilation of seg6_iptunnel.o.
   Pros: logically dissociate the part of IPv6-SR that depends on
LWTUNNEL from the core patch and simplifies compilation
   Cons: add an option

3. Apply the proposed patch with the fix
   Pros: do not modify options
   Cons: weird conditional compilation

What do you think ?

David