Re: [PATCH net v2] Revert "ipv6: preserve insertion order for same-scope addresses"

[Chris Adams <linux@cmadams.net>]

Once upon a time, Stefano Brivio <sbrivio@redhat.com> said:
> On Fri, 29 May 2026 13:23:57 +0200
> Fernando Fernandez Mancera <fmancera@suse.de> wrote:
> 
> > Chris Adams reported that preserving insertion order for same-scope
> > addresses is causing SSH connections to be dropped after stopping a VM
> > while running NetworkManager.
> > 
> > NetworkManager caches the IPv6 address configuration, when a RA arrives,
> > it determines the list of addresses to configure and checks if the
> > addresses are already in the right order in the kernel. If they aren't,
> > NetworkManager removes and re-adds them to achieve the desired order.
> > 
> > As the order changes, NetworkManager is confused and reconfigures the
> > addresses on every update. In addition, this would also affect to cloud
> > tooling that relies on IPv6 addresses order to identify primary and
> > secondaries addresses.
> 
> By the way, I'm still looking into this part, trying to find
> "problematic" examples.
> 
> And I couldn't find any, yet, because it looks like there's always a
> _single_ IPv6 address being used as a secondary for a primary IPv4
> address.

I hit two cases that triggered the behavior:

- after an overnight suspend/resume cycle (not sure if there's a minimum
  time suspended)

- with a bridge NIC, after stopping a VM with a NIC on the bridge (which
  removed an fe80:: link-local address); this is the case I used to
  bisect since it was easy to script

In both cases, the next IPv6 router-advertisement received was what
triggered NetworkManager to replace the current privacy addresses.  This
could be 2-3 minutes after the trigger.

> > This reverts commit cb3de96eea66f5e4a580086c6a1be46e765f97f4.
> > 
> > Fixes: cb3de96eea66 ("ipv6: preserve insertion order for same-scope addresses")
> > Reported-by: Chris Adams <linux@cmadams.net>
> > Closes: https://lore.kernel.org/netdev/20260521135310.GC977@cmadams.net/
> > Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
> > ---
> > v2: updated commit description to make it more accurate
> > ---
> >  net/ipv6/addrconf.c                  | 2 +-
> >  tools/testing/selftests/net/ioam6.sh | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
> > index 5476b6536eb7..bb84a78b80f6 100644
> > --- a/net/ipv6/addrconf.c
> > +++ b/net/ipv6/addrconf.c
> > @@ -1013,7 +1013,7 @@ ipv6_link_dev_addr(struct inet6_dev *idev, struct inet6_ifaddr *ifp)
> >  	list_for_each(p, &idev->addr_list) {
> >  		struct inet6_ifaddr *ifa
> >  			= list_entry(p, struct inet6_ifaddr, if_list);
> > -		if (ifp_scope > ipv6_addr_src_scope(&ifa->addr))
> > +		if (ifp_scope >= ipv6_addr_src_scope(&ifa->addr))
> >  			break;
> >  	}
> >  
> > diff --git a/tools/testing/selftests/net/ioam6.sh b/tools/testing/selftests/net/ioam6.sh
> > index b2b99889942f..845c26dd01a9 100755
> > --- a/tools/testing/selftests/net/ioam6.sh
> > +++ b/tools/testing/selftests/net/ioam6.sh
> > @@ -273,8 +273,8 @@ setup()
> >    ip -netns $ioam_node_beta link set ioam-veth-betaR name veth1 &>/dev/null
> >    ip -netns $ioam_node_gamma link set ioam-veth-gamma name veth0 &>/dev/null
> >  
> > -  ip -netns $ioam_node_alpha addr add 2001:db8:1::2/64 dev veth0 &>/dev/null
> >    ip -netns $ioam_node_alpha addr add 2001:db8:1::50/64 dev veth0 &>/dev/null
> > +  ip -netns $ioam_node_alpha addr add 2001:db8:1::2/64 dev veth0 &>/dev/null
> >    ip -netns $ioam_node_alpha link set veth0 up &>/dev/null
> >    ip -netns $ioam_node_alpha link set lo up &>/dev/null
> >    ip -netns $ioam_node_alpha route add 2001:db8:2::/64 \
> 
> -- 
> Stefano

-- 
Chris Adams <linux@cmadams.net>