networkd: preserve exact host-prefix pool addresses

Address pool allocation checks a selected prefix before address_acquire()
forces its lowest bit to one. For /32 and /128, this changes the
checked address and can return an address that is already in use.

Keep exact host-prefix addresses unchanged and only select the first
host address when the prefix leaves host bits available.

(cherry picked from commit a0a493a038)
(cherry picked from commit 27e2f6b49d)

src/network/networkd-address.c

@@ -1641,13 +1641,13 @@ static int address_acquire(Link *link, const Address *address, union in_addr_uni
         if (r == 0)
                 return -EBUSY;
 
-        /* Pick first address in range for ourselves. */
-        if (address->family == AF_INET)
-                a.in.s_addr |= htobe32(1);
-        else if (address->family == AF_INET6)
-                a.in6.s6_addr[15] |= 1;
-        else
-                assert_not_reached();
+        /* Pick first address in range for ourselves. A host prefix already identifies the exact address. */
+        if (address->prefixlen < FAMILY_ADDRESS_SIZE(address->family) * 8) {
+                if (address->family == AF_INET)
+                        a.in.s_addr |= htobe32(1);
+                else
+                        a.in6.s6_addr[15] |= 1;
+        }
 
         *ret = a;
         return 0;

test/test-network/conf/25-address-static.network

@@ -170,12 +170,19 @@ Label=subnet16
 Address=0.0.0.0/24
 Label=subnet24
 
+[Address]
+Address=0.0.0.0/32
+Label=host32
+
 [Address]
 Address=::/73
 
 [Address]
 Address=::/74
 
+[Address]
+Address=::/128
+
 [Address]
 Address=10.10.1.1/24
 # just a random label which should exist

test/test-network/systemd-networkd-tests.py

@@ -4191,25 +4191,34 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
 
         ip4_null_16 = None
         ip4_null_24 = None
+        ip4_null_32 = None
         output = check_output('ip -4 --json address show dev dummy98')
         for i in json.loads(output)[0]['addr_info']:
             if i['label'] == 'subnet16':
                 ip4_null_16 = i['local']
             elif i['label'] == 'subnet24':
                 ip4_null_24 = i['local']
+            elif i['label'] == 'host32':
+                ip4_null_32 = i
         self.assertTrue(ip4_null_16.endswith('.0.1'))
         self.assertTrue(ip4_null_24.endswith('.1'))
+        self.assertIsNotNone(ip4_null_32)
+        self.assertEqual(ip4_null_32['prefixlen'], 32)
 
         ip6_null_73 = None
         ip6_null_74 = None
+        ip6_null_128 = None
         output = check_output('ip -6 --json address show dev dummy98')
         for i in json.loads(output)[0]['addr_info']:
             if i['prefixlen'] == 73:
                 ip6_null_73 = i['local']
             elif i['prefixlen'] == 74:
                 ip6_null_74 = i['local']
+            elif i['prefixlen'] == 128 and i['local'].startswith('fd'):
+                ip6_null_128 = i
         self.assertTrue(ip6_null_73.endswith(':1'))
         self.assertTrue(ip6_null_74.endswith(':1'))
+        self.assertIsNotNone(ip6_null_128)
 
         self.verify_address_static(
             label1='label1',