selftests: drv-net: resolve remote interface name

Find out and record in env the name of the interface which remote host
will use for the IP address provided via config.

Interface name is useful for mausezahn and for setting up tunnels.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20250218225426.77726-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/drivers/net/lib/py/env.py

@@ -154,6 +154,9 @@ def __init__(self, src_path, nsim_test=None):
         self.ifname = self.dev['ifname']
         self.ifindex = self.dev['ifindex']
 
+        # resolve remote interface name
+        self.remote_ifname = self.resolve_remote_ifc()
+
         self._required_cmd = {}
 
     def create_local(self):
@@ -200,6 +203,18 @@ def _check_env(self):
             raise Exception("Invalid environment, missing configuration:", missing,
                             "Please see tools/testing/selftests/drivers/net/README.rst")
 
+    def resolve_remote_ifc(self):
+        v4 = v6 = None
+        if self.remote_v4:
+            v4 = ip("addr show to " + self.remote_v4, json=True, host=self.remote)
+        if self.remote_v6:
+            v6 = ip("addr show to " + self.remote_v6, json=True, host=self.remote)
+        if v4 and v6 and v4[0]["ifname"] != v6[0]["ifname"]:
+            raise Exception("Can't resolve remote interface name, v4 and v6 don't match")
+        if (v4 and len(v4) > 1) or (v6 and len(v6) > 1):
+            raise Exception("Can't resolve remote interface name, multiple interfaces match")
+        return v6[0]["ifname"] if v6 else v4[0]["ifname"]
+
     def __enter__(self):
         return self