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>

4 files changed, 36 insertions, 1 deletions

diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile
index 73ee88d6b043..e2f03211f9b3 100644
--- a/tools/testing/selftests/net/Makefile
+++ b/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
diff --git a/tools/testing/selftests/net/lib/py/__init__.py b/tools/testing/selftests/net/lib/py/__init__.py
index 54d8f5eba810..729457859316 100644
--- a/tools/testing/selftests/net/lib/py/__init__.py
+++ b/tools/testing/selftests/net/lib/py/__init__.py
@@ -5,5 +5,5 @@ from .ksft import *
 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
diff --git a/tools/testing/selftests/net/lib/py/ynl.py b/tools/testing/selftests/net/lib/py/ynl.py
index ad1e36baee2a..8986c584cb37 100644
--- a/tools/testing/selftests/net/lib/py/ynl.py
+++ b/tools/testing/selftests/net/lib/py/ynl.py
@@ -42,6 +42,10 @@ class RtnlFamily(YnlFamily):
         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):
diff --git a/tools/testing/selftests/net/rtnetlink.py b/tools/testing/selftests/net/rtnetlink.py
new file mode 100755
index 000000000000..80950888800b
--- /dev/null
+++ b/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()