Re: [PATCH net-next] selftests: drv-net: warn if LOCAL_V6 has DAD or not kept on link down

On Wed, 09 Sep 2026 20:17:51 +0000 netdev-bot+sashiko@kernel.org wrote:

quoted

diff --git a/tools/testing/selftests/drivers/net/README.rst b/tools/testing/selftests/drivers/net/README.rst
index c6bed9a985bc..3fe49bce4f3a 100644
--- a/tools/testing/selftests/drivers/net/README.rst
+++ b/tools/testing/selftests/drivers/net/README.rst

@@ -70,6 +70,15 @@ LOCAL_V4, LOCAL_V6, REMOTE_V4, REMOTE_V6
 
 Local and remote endpoint IP addresses.
 
+Tests reconfigure the device freely, including taking the link down, and
+expect the addresses to work immediately afterwards. IPv6 needs help::
+
+  ip -6 address add 2001:db8:1::1/64 dev eth0 nodad
+  sysctl -w net.ipv6.conf.eth0.keep_addr_on_down=1
+
+Without those the address is flushed when the link goes down, or spends
+a second unusable while waiting for DAD to finish.
+
 LOCAL_PREFIX_V6
 ~~~~~~~~~~~~~~~
 

diff --git a/tools/testing/selftests/drivers/net/lib/py/env.py b/tools/testing/selftests/drivers/net/lib/py/env.py
index 25903f580b40..6262080a8bf4 100644
--- a/tools/testing/selftests/drivers/net/lib/py/env.py
+++ b/tools/testing/selftests/drivers/net/lib/py/env.py

@@ -239,6 +239,25 @@ class NetDrvEpEnv(NetDrvEnvBase):
         if missing:
             raise Exception("Invalid environment, missing configuration:", missing,
                             "Please see tools/testing/selftests/drivers/net/README.rst")
+        if "LOCAL_V6" in self.env:
+            self._check_v6_env()  

[Severity: Medium]
Do the in-tree environment providers meet the invariant that is being
documented here?

Two cases look like they do not:

1) NetDrvEpEnv.create_local() (the netdevsim path in the same file) adds
   the addresses with nodad, but never enables keep_addr_on_down on the
   nsim devices or on 'all':

     ip(f"   addr add dev {self._ns.nsims[0].ifname} {self.nsim_v4_pfx}1/24")
     ip(f"-6 addr add dev {self._ns.nsims[0].ifname} {self.nsim_v6_pfx}1/64 nodad")
     ip(f"   link set dev {self._ns.nsims[0].ifname} up")

   Both the devconf_all and per-device defaults are 0, and
   addrconf_ifdown() drops permanent global addresses when the aggregate
   is not positive:

   net/ipv6/addrconf.c:addrconf_ifdown() {
	...
	int _keep_addr = READ_ONCE(net->ipv6.devconf_all->keep_addr_on_down);

	if (!_keep_addr)
		_keep_addr = READ_ONCE(idev->cnf.keep_addr_on_down);

	keep_addr = (_keep_addr > 0);
	...
   }

   So a test that follows the new documentation and bounces the link
   would lose the nsim LOCAL_V6 address. Note also that this path never
   reaches _check_v6_env(), since the call is only made from the branch
   that requires NETIF, so there is no warning either.

This is no different to the HW case, we expect the outside runner 
to configure the system. As proven by the fact that the WARN print
added here does not trigger in NIPA on netdevsim:

https://netdev-ctrl.bots.linux.dev/logs/vmksft/net-drv/results/816282/13-napi-id-py/stdout

2) tools/testing/selftests/net/lib/ksft_setup_loopback.sh sets up the
   env that this code validates, and adds LOCAL_V6 without nodad and
   without touching keep_addr_on_down:

     ip -netns "${SERVER_NS}" addr add $LOCAL_V6/64 dev server
     ...
     export NETIF=server

   Would every run driven by that script now print both new warnings,
   pointing at in-tree setup code rather than at a user
   misconfiguration? Should the script and create_local() be updated
   along with the documentation?

Not sure if and how that script is actually used :S