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 <donald.hunter@gmail.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221209112401.69319-1-donald.hunter@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/map_sk_storage.rst

+.. 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/