From f3212ad5b7e93c002bd2dbe552c2b0b0033317ff Mon Sep 17 00:00:00 2001 From: Donald Hunter Date: Fri, 9 Dec 2022 11:24:01 +0000 Subject: [PATCH] docs/bpf: Add documentation for BPF_MAP_TYPE_SK_STORAGE Add documentation for the BPF_MAP_TYPE_SK_STORAGE including kernel version introduced, usage and examples. Signed-off-by: Donald Hunter Acked-by: David Vernet Link: https://lore.kernel.org/r/20221209112401.69319-1-donald.hunter@gmail.com Signed-off-by: Alexei Starovoitov --- Documentation/bpf/map_sk_storage.rst | 155 +++++++++++++++++++++++++++++++++++ 1 file changed, 155 insertions(+) create mode 100644 Documentation/bpf/map_sk_storage.rst diff --git a/Documentation/bpf/map_sk_storage.rst b/Documentation/bpf/map_sk_storage.rst new file mode 100644 index 0000000..047e16c --- /dev/null +++ b/Documentation/bpf/map_sk_storage.rst @@ -0,0 +1,155 @@ +.. 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 can be retrieved using the ``bpf_sk_storage_get()`` +helper. The helper gets the storage from ``sk`` that is associated with ``map``. +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 can be deleted using the ``bpf_sk_storage_delete()`` +helper. The helper deletes the storage from ``sk`` that is identified by +``map``. 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 the socket identified by ``key`` belonging to +``map_fd`` can be added or updated using the ``bpf_map_update_elem()`` libbpf +function. ``key`` must be a pointer to a valid ``fd`` in the user space +program. The ``flags`` parameter can be used to control the update behaviour: + +- ``BPF_ANY`` will create storage for ``fd`` or update existing storage. +- ``BPF_NOEXIST`` will create storage for ``fd`` only if it did not already + exist, otherwise the call will fail with ``-EEXIST``. +- ``BPF_EXIST`` will update existing storage for ``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 the socket identified by ``key`` belonging to +``map_fd`` can be retrieved using the ``bpf_map_lookup_elem()`` libbpf +function. ``key`` must be a pointer to a valid ``fd`` in the user space +program. 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 the socket identified by ``key`` belonging to +``map_fd`` can be deleted using the ``bpf_map_delete_elem()`` libbpf +function. 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/ -- 2.7.4