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``, 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