[PATCH bpf-next v3] docs/bpf: Add documentation for BPF_MAP_TYPE_SK_STORAGE

[Donald Hunter <donald.hunter@gmail.com>]

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>
---
v2 -> v3:
- Fix void * return, reported by Yonghong Song
- Add tracing programs to API note, reported by Yonghong Song
v1 -> v2:
- Fix bpf_sk_storage_* function signatures, reported by Yonghong Song
- Fix NULL return on failure, reported by Yonghong Song

 Documentation/bpf/map_sk_storage.rst | 142 +++++++++++++++++++++++++++
 1 file changed, 142 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 000000000000..955b287bb7de
--- /dev/null
+++ b/Documentation/bpf/map_sk_storage.rst
@@ -0,0 +1,142 @@
+.. 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 from a BPF program. The key type must be ``int``
+and ``max_entries`` must be set to ``0``.
+
+The ``BPF_F_NO_PREALLOC`` must be used when creating a map for socket-local storage. 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.
+
+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 identified by ``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 with type identified by ``map_fd`` for the socket identified by ``key`` 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
+- ``BPF_EXIST`` will update existing storage for ``fd``
+
+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 */
+    }
+
+References
+==========
+
+https://lwn.net/ml/netdev/20190426171103.61892-1-kafai@fb.com/
-- 
2.38.1