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diff --git a/Documentation/bpf/map_sk_storage.rst b/Documentation/bpf/map_sk_storage.rst new file mode 100644 index 0000000000..4e9d23ab9e --- /dev/null +++ b/Documentation/bpf/map_sk_storage.rst @@ -0,0 +1,159 @@ +.. SPDX-License-Identifier: GPL-2.0-only +.. Copyright (C) 2022 Red Hat, Inc. + +======================= +BPF_MAP_TYPE_SK_STORAGE +======================= + +.. note:: + - ``BPF_MAP_TYPE_SK_STORAGE`` was introduced in kernel version 5.2 + +``BPF_MAP_TYPE_SK_STORAGE`` is used to provide socket-local storage for BPF +programs. A map of type ``BPF_MAP_TYPE_SK_STORAGE`` declares the type of storage +to be provided and acts as the handle for accessing the socket-local +storage. The values for maps of type ``BPF_MAP_TYPE_SK_STORAGE`` are stored +locally with each socket instead of with the map. The kernel is responsible for +allocating storage for a socket when requested and for freeing the storage when +either the map or the socket is deleted. + +.. note:: + - The key type must be ``int`` and ``max_entries`` must be set to ``0``. + - The ``BPF_F_NO_PREALLOC`` flag must be used when creating a map for + socket-local storage. + +Usage +===== + +Kernel BPF +---------- + +bpf_sk_storage_get() +~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + void *bpf_sk_storage_get(struct bpf_map *map, void *sk, void *value, u64 flags) + +Socket-local storage for ``map`` can be retrieved from socket ``sk`` using the +``bpf_sk_storage_get()`` helper. If the ``BPF_LOCAL_STORAGE_GET_F_CREATE`` +flag is used then ``bpf_sk_storage_get()`` will create the storage for ``sk`` +if it does not already exist. ``value`` can be used together with +``BPF_LOCAL_STORAGE_GET_F_CREATE`` to initialize the storage value, otherwise +it will be zero initialized. Returns a pointer to the storage on success, or +``NULL`` in case of failure. + +.. note:: + - ``sk`` is a kernel ``struct sock`` pointer for LSM or tracing programs. + - ``sk`` is a ``struct bpf_sock`` pointer for other program types. + +bpf_sk_storage_delete() +~~~~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + long bpf_sk_storage_delete(struct bpf_map *map, void *sk) + +Socket-local storage for ``map`` can be deleted from socket ``sk`` using the +``bpf_sk_storage_delete()`` helper. Returns ``0`` on success, or negative +error in case of failure. + +User space +---------- + +bpf_map_update_elem() +~~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + int bpf_map_update_elem(int map_fd, const void *key, const void *value, __u64 flags) + +Socket-local storage for map ``map_fd`` can be added or updated locally to a +socket using the ``bpf_map_update_elem()`` libbpf function. The socket is +identified by a `socket` ``fd`` stored in the pointer ``key``. The pointer +``value`` has the data to be added or updated to the socket ``fd``. The type +and size of ``value`` should be the same as the value type of the map +definition. + +The ``flags`` parameter can be used to control the update behaviour: + +- ``BPF_ANY`` will create storage for `socket` ``fd`` or update existing storage. +- ``BPF_NOEXIST`` will create storage for `socket` ``fd`` only if it did not + already exist, otherwise the call will fail with ``-EEXIST``. +- ``BPF_EXIST`` will update existing storage for `socket` ``fd`` if it already + exists, otherwise the call will fail with ``-ENOENT``. + +Returns ``0`` on success, or negative error in case of failure. + +bpf_map_lookup_elem() +~~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + int bpf_map_lookup_elem(int map_fd, const void *key, void *value) + +Socket-local storage for map ``map_fd`` can be retrieved from a socket using +the ``bpf_map_lookup_elem()`` libbpf function. The storage is retrieved from +the socket identified by a `socket` ``fd`` stored in the pointer +``key``. Returns ``0`` on success, or negative error in case of failure. + +bpf_map_delete_elem() +~~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + int bpf_map_delete_elem(int map_fd, const void *key) + +Socket-local storage for map ``map_fd`` can be deleted from a socket using the +``bpf_map_delete_elem()`` libbpf function. The storage is deleted from the +socket identified by a `socket` ``fd`` stored in the pointer ``key``. Returns +``0`` on success, or negative error in case of failure. + +Examples +======== + +Kernel BPF +---------- + +This snippet shows how to declare socket-local storage in a BPF program: + +.. code-block:: c + + struct { + __uint(type, BPF_MAP_TYPE_SK_STORAGE); + __uint(map_flags, BPF_F_NO_PREALLOC); + __type(key, int); + __type(value, struct my_storage); + } socket_storage SEC(".maps"); + +This snippet shows how to retrieve socket-local storage in a BPF program: + +.. code-block:: c + + SEC("sockops") + int _sockops(struct bpf_sock_ops *ctx) + { + struct my_storage *storage; + struct bpf_sock *sk; + + sk = ctx->sk; + if (!sk) + return 1; + + storage = bpf_sk_storage_get(&socket_storage, sk, 0, + BPF_LOCAL_STORAGE_GET_F_CREATE); + if (!storage) + return 1; + + /* Use 'storage' here */ + + return 1; + } + + +Please see the ``tools/testing/selftests/bpf`` directory for functional +examples. + +References +========== + +https://lwn.net/ml/netdev/20190426171103.61892-1-kafai@fb.com/ |