selftests/net: Add selftest for IPv4 RTM_GETMULTICAST support

This change introduces a new selftest case to verify the functionality
of dumping IPv4 multicast addresses using the RTM_GETMULTICAST netlink
message. The test utilizes the ynl library to interact with the
netlink interface and validate that the kernel correctly reports the
joined IPv4 multicast addresses.

To run the test, execute the following command:

$ vng -v --user root --cpus 16 -- \
    make -C tools/testing/selftests TARGETS=net \
    TEST_PROGS=rtnetlink.py TEST_GEN_PROGS="" run_tests

Cc: Maciej Żenczykowski <maze@google.com>
Cc: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
Link: https://patch.msgid.link/20250207110836.2407224-2-yuyanghuang@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: Yuyang Huang

Documentation/netlink/specs/rt_addr.yaml

@@ -168,6 +168,29 @@ operations:
         reply:
           value: 20
           attributes: *ifaddr-all
+    -
+      name: getmaddrs
+      doc: Get / dump IPv4/IPv6 multicast addresses.
+      attribute-set: addr-attrs
+      fixed-header: ifaddrmsg
+      do:
+        request:
+          value: 58
+          attributes:
+            - ifa-family
+            - ifa-index
+        reply:
+          value: 58
+          attributes: &mcaddr-attrs
+            - ifa-multicast
+            - ifa-cacheinfo
+      dump:
+        request:
+          value: 58
+            - ifa-family
+        reply:
+          value: 58
+          attributes: *mcaddr-attrs
 
 mcast-groups:
   list:

tools/testing/selftests/net/Makefile

@@ -36,6 +36,7 @@ TEST_PROGS += cmsg_so_priority.sh
 TEST_PROGS += cmsg_time.sh cmsg_ipv6.sh
 TEST_PROGS += netns-name.sh
 TEST_PROGS += nl_netdev.py
+TEST_PROGS += rtnetlink.py
 TEST_PROGS += srv6_end_dt46_l3vpn_test.sh
 TEST_PROGS += srv6_end_dt4_l3vpn_test.sh
 TEST_PROGS += srv6_end_dt6_l3vpn_test.sh

tools/testing/selftests/net/lib/py/__init__.py

@@ -5,5 +5,5 @@
 from .netns import NetNS
 from .nsim import *
 from .utils import *
-from .ynl import NlError, YnlFamily, EthtoolFamily, NetdevFamily, RtnlFamily
+from .ynl import NlError, YnlFamily, EthtoolFamily, NetdevFamily, RtnlFamily, RtnlAddrFamily
 from .ynl import NetshaperFamily

tools/testing/selftests/net/lib/py/ynl.py

@@ -42,6 +42,10 @@ def __init__(self, recv_size=0):
         super().__init__((SPEC_PATH / Path('rt_link.yaml')).as_posix(),
                          schema='', recv_size=recv_size)
 
+class RtnlAddrFamily(YnlFamily):
+    def __init__(self, recv_size=0):
+        super().__init__((SPEC_PATH / Path('rt_addr.yaml')).as_posix(),
+                         schema='', recv_size=recv_size)
 
 class NetdevFamily(YnlFamily):
     def __init__(self, recv_size=0):

tools/testing/selftests/net/rtnetlink.py

@@ -0,0 +1,30 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+
+from lib.py import ksft_exit, ksft_run, ksft_ge, RtnlAddrFamily
+import socket
+
+IPV4_ALL_HOSTS_MULTICAST = b'\xe0\x00\x00\x01'
+
+def dump_mcaddr_check(rtnl: RtnlAddrFamily) -> None:
+    """
+    Verify that at least one interface has the IPv4 all-hosts multicast address.
+    At least the loopback interface should have this address.
+    """
+
+    addresses = rtnl.getmaddrs({"ifa-family": socket.AF_INET}, dump=True)
+
+    all_host_multicasts = [
+        addr for addr in addresses if addr['ifa-multicast'] == IPV4_ALL_HOSTS_MULTICAST
+    ]
+
+    ksft_ge(len(all_host_multicasts), 1,
+            "No interface found with the IPv4 all-hosts multicast address")
+
+def main() -> None:
+    rtnl = RtnlAddrFamily()
+    ksft_run([dump_mcaddr_check], args=(rtnl, ))
+    ksft_exit()
+
+if __name__ == "__main__":
+    main()