1.. SPDX-License-Identifier: GPL-2.0-only 2.. Copyright (C) 2022 Red Hat, Inc. 3 4===================== 5BPF_MAP_TYPE_LPM_TRIE 6===================== 7 8.. note:: 9 - ``BPF_MAP_TYPE_LPM_TRIE`` was introduced in kernel version 4.11 10 11``BPF_MAP_TYPE_LPM_TRIE`` provides a longest prefix match algorithm that 12can be used to match IP addresses to a stored set of prefixes. 13Internally, data is stored in an unbalanced trie of nodes that uses 14``prefixlen,data`` pairs as its keys. The ``data`` is interpreted in 15network byte order, i.e. big endian, so ``data[0]`` stores the most 16significant byte. 17 18LPM tries may be created with a maximum prefix length that is a multiple 19of 8, in the range from 8 to 2048. The key used for lookup and update 20operations is a ``struct bpf_lpm_trie_key_u8``, extended by 21``max_prefixlen/8`` bytes. 22 23- For IPv4 addresses the data length is 4 bytes 24- For IPv6 addresses the data length is 16 bytes 25 26The value type stored in the LPM trie can be any user defined type. 27 28.. note:: 29 When creating a map of type ``BPF_MAP_TYPE_LPM_TRIE`` you must set the 30 ``BPF_F_NO_PREALLOC`` flag. 31 32Usage 33===== 34 35Kernel BPF 36---------- 37 38bpf_map_lookup_elem() 39~~~~~~~~~~~~~~~~~~~~~ 40 41.. code-block:: c 42 43 void *bpf_map_lookup_elem(struct bpf_map *map, const void *key) 44 45The longest prefix entry for a given data value can be found using the 46``bpf_map_lookup_elem()`` helper. This helper returns a pointer to the 47value associated with the longest matching ``key``, or ``NULL`` if no 48entry was found. 49 50The ``key`` should have ``prefixlen`` set to ``max_prefixlen`` when 51performing longest prefix lookups. For example, when searching for the 52longest prefix match for an IPv4 address, ``prefixlen`` should be set to 53``32``. 54 55bpf_map_update_elem() 56~~~~~~~~~~~~~~~~~~~~~ 57 58.. code-block:: c 59 60 long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags) 61 62Prefix entries can be added or updated using the ``bpf_map_update_elem()`` 63helper. This helper replaces existing elements atomically. 64 65``bpf_map_update_elem()`` returns ``0`` on success, or negative error in 66case of failure. 67 68 .. note:: 69 The flags parameter must be one of BPF_ANY, BPF_NOEXIST or BPF_EXIST, 70 but the value is ignored, giving BPF_ANY semantics. 71 72bpf_map_delete_elem() 73~~~~~~~~~~~~~~~~~~~~~ 74 75.. code-block:: c 76 77 long bpf_map_delete_elem(struct bpf_map *map, const void *key) 78 79Prefix entries can be deleted using the ``bpf_map_delete_elem()`` 80helper. This helper will return 0 on success, or negative error in case 81of failure. 82 83Userspace 84--------- 85 86Access from userspace uses libbpf APIs with the same names as above, with 87the map identified by ``fd``. 88 89bpf_map_get_next_key() 90~~~~~~~~~~~~~~~~~~~~~~ 91 92.. code-block:: c 93 94 int bpf_map_get_next_key (int fd, const void *cur_key, void *next_key) 95 96A userspace program can iterate through the entries in an LPM trie using 97libbpf's ``bpf_map_get_next_key()`` function. The first key can be 98fetched by calling ``bpf_map_get_next_key()`` with ``cur_key`` set to 99``NULL``. Subsequent calls will fetch the next key that follows the 100current key. ``bpf_map_get_next_key()`` returns ``0`` on success, 101``-ENOENT`` if ``cur_key`` is the last key in the trie, or negative 102error in case of failure. 103 104``bpf_map_get_next_key()`` will iterate through the LPM trie elements 105from leftmost leaf first. This means that iteration will return more 106specific keys before less specific ones. 107 108Examples 109======== 110 111Please see ``tools/testing/selftests/bpf/test_lpm_map.c`` for examples 112of LPM trie usage from userspace. The code snippets below demonstrate 113API usage. 114 115Kernel BPF 116---------- 117 118The following BPF code snippet shows how to declare a new LPM trie for IPv4 119address prefixes: 120 121.. code-block:: c 122 123 #include <linux/bpf.h> 124 #include <bpf/bpf_helpers.h> 125 126 struct ipv4_lpm_key { 127 __u32 prefixlen; 128 __u32 data; 129 }; 130 131 struct { 132 __uint(type, BPF_MAP_TYPE_LPM_TRIE); 133 __type(key, struct ipv4_lpm_key); 134 __type(value, __u32); 135 __uint(map_flags, BPF_F_NO_PREALLOC); 136 __uint(max_entries, 255); 137 } ipv4_lpm_map SEC(".maps"); 138 139The following BPF code snippet shows how to lookup by IPv4 address: 140 141.. code-block:: c 142 143 void *lookup(__u32 ipaddr) 144 { 145 struct ipv4_lpm_key key = { 146 .prefixlen = 32, 147 .data = ipaddr 148 }; 149 150 return bpf_map_lookup_elem(&ipv4_lpm_map, &key); 151 } 152 153Userspace 154--------- 155 156The following snippet shows how to insert an IPv4 prefix entry into an 157LPM trie: 158 159.. code-block:: c 160 161 int add_prefix_entry(int lpm_fd, __u32 addr, __u32 prefixlen, struct value *value) 162 { 163 struct ipv4_lpm_key ipv4_key = { 164 .prefixlen = prefixlen, 165 .data = addr 166 }; 167 return bpf_map_update_elem(lpm_fd, &ipv4_key, value, BPF_ANY); 168 } 169 170The following snippet shows a userspace program walking through the entries 171of an LPM trie: 172 173 174.. code-block:: c 175 176 #include <bpf/libbpf.h> 177 #include <bpf/bpf.h> 178 179 void iterate_lpm_trie(int map_fd) 180 { 181 struct ipv4_lpm_key *cur_key = NULL; 182 struct ipv4_lpm_key next_key; 183 struct value value; 184 int err; 185 186 for (;;) { 187 err = bpf_map_get_next_key(map_fd, cur_key, &next_key); 188 if (err) 189 break; 190 191 bpf_map_lookup_elem(map_fd, &next_key, &value); 192 193 /* Use key and value here */ 194 195 cur_key = &next_key; 196 } 197 } 198