1.. SPDX-License-Identifier: GPL-2.0-only
2.. Copyright (C) 2022 Red Hat, Inc.
3
4=======================
5BPF_MAP_TYPE_SK_STORAGE
6=======================
7
8.. note::
9   - ``BPF_MAP_TYPE_SK_STORAGE`` was introduced in kernel version 5.2
10
11``BPF_MAP_TYPE_SK_STORAGE`` is used to provide socket-local storage for BPF
12programs. A map of type ``BPF_MAP_TYPE_SK_STORAGE`` declares the type of storage
13to be provided and acts as the handle for accessing the socket-local
14storage. The values for maps of type ``BPF_MAP_TYPE_SK_STORAGE`` are stored
15locally with each socket instead of with the map. The kernel is responsible for
16allocating storage for a socket when requested and for freeing the storage when
17either the map or the socket is deleted.
18
19.. note::
20  - The key type must be ``int`` and ``max_entries`` must be set to ``0``.
21  - The ``BPF_F_NO_PREALLOC`` flag must be used when creating a map for
22    socket-local storage.
23
24Usage
25=====
26
27Kernel BPF
28----------
29
30bpf_sk_storage_get()
31~~~~~~~~~~~~~~~~~~~~
32
33.. code-block:: c
34
35   void *bpf_sk_storage_get(struct bpf_map *map, void *sk, void *value, u64 flags)
36
37Socket-local storage for ``map`` can be retrieved from socket ``sk`` using the
38``bpf_sk_storage_get()`` helper. If the ``BPF_LOCAL_STORAGE_GET_F_CREATE``
39flag is used then ``bpf_sk_storage_get()`` will create the storage for ``sk``
40if it does not already exist. ``value`` can be used together with
41``BPF_LOCAL_STORAGE_GET_F_CREATE`` to initialize the storage value, otherwise
42it will be zero initialized. Returns a pointer to the storage on success, or
43``NULL`` in case of failure.
44
45.. note::
46   - ``sk`` is a kernel ``struct sock`` pointer for LSM or tracing programs.
47   - ``sk`` is a ``struct bpf_sock`` pointer for other program types.
48
49bpf_sk_storage_delete()
50~~~~~~~~~~~~~~~~~~~~~~~
51
52.. code-block:: c
53
54   long bpf_sk_storage_delete(struct bpf_map *map, void *sk)
55
56Socket-local storage for ``map`` can be deleted from socket ``sk`` using the
57``bpf_sk_storage_delete()`` helper. Returns ``0`` on success, or negative
58error in case of failure.
59
60User space
61----------
62
63bpf_map_update_elem()
64~~~~~~~~~~~~~~~~~~~~~
65
66.. code-block:: c
67
68   int bpf_map_update_elem(int map_fd, const void *key, const void *value, __u64 flags)
69
70Socket-local storage for map ``map_fd`` can be added or updated locally to a
71socket using the ``bpf_map_update_elem()`` libbpf function. The socket is
72identified by a `socket` ``fd`` stored in the pointer ``key``. The pointer
73``value`` has the data to be added or updated to the socket ``fd``. The type
74and size of ``value`` should be the same as the value type of the map
75definition.
76
77The ``flags`` parameter can be used to control the update behaviour:
78
79- ``BPF_ANY`` will create storage for `socket` ``fd`` or update existing storage.
80- ``BPF_NOEXIST`` will create storage for `socket` ``fd`` only if it did not
81  already exist, otherwise the call will fail with ``-EEXIST``.
82- ``BPF_EXIST`` will update existing storage for `socket` ``fd`` if it already
83  exists, otherwise the call will fail with ``-ENOENT``.
84
85Returns ``0`` on success, or negative error in case of failure.
86
87bpf_map_lookup_elem()
88~~~~~~~~~~~~~~~~~~~~~
89
90.. code-block:: c
91
92   int bpf_map_lookup_elem(int map_fd, const void *key, void *value)
93
94Socket-local storage for map ``map_fd`` can be retrieved from a socket using
95the ``bpf_map_lookup_elem()`` libbpf function. The storage is retrieved from
96the socket identified by a `socket` ``fd`` stored in the pointer
97``key``. Returns ``0`` on success, or negative error in case of failure.
98
99bpf_map_delete_elem()
100~~~~~~~~~~~~~~~~~~~~~
101
102.. code-block:: c
103
104   int bpf_map_delete_elem(int map_fd, const void *key)
105
106Socket-local storage for map ``map_fd`` can be deleted from a socket using the
107``bpf_map_delete_elem()`` libbpf function. The storage is deleted from the
108socket identified by a `socket` ``fd`` stored in the pointer ``key``. Returns
109``0`` on success, or negative error in case of failure.
110
111Examples
112========
113
114Kernel BPF
115----------
116
117This snippet shows how to declare socket-local storage in a BPF program:
118
119.. code-block:: c
120
121    struct {
122            __uint(type, BPF_MAP_TYPE_SK_STORAGE);
123            __uint(map_flags, BPF_F_NO_PREALLOC);
124            __type(key, int);
125            __type(value, struct my_storage);
126    } socket_storage SEC(".maps");
127
128This snippet shows how to retrieve socket-local storage in a BPF program:
129
130.. code-block:: c
131
132    SEC("sockops")
133    int _sockops(struct bpf_sock_ops *ctx)
134    {
135            struct my_storage *storage;
136            struct bpf_sock *sk;
137
138            sk = ctx->sk;
139            if (!sk)
140                    return 1;
141
142            storage = bpf_sk_storage_get(&socket_storage, sk, 0,
143                                         BPF_LOCAL_STORAGE_GET_F_CREATE);
144            if (!storage)
145                    return 1;
146
147            /* Use 'storage' here */
148
149            return 1;
150    }
151
152
153Please see the ``tools/testing/selftests/bpf`` directory for functional
154examples.
155
156References
157==========
158
159https://lwn.net/ml/netdev/20190426171103.61892-1-kafai@fb.com/
160