[PATCH bpf-next v4 7/8] bpf: add sockopt documentation

[Stanislav Fomichev <sdf@google.com>]

Provide user documentation about sockopt prog type and cgroup hooks.

v2:
* use return code 2 for kernel bypass

Cc: Martin Lau <kafai@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 Documentation/bpf/index.rst               |  1 +
 Documentation/bpf/prog_cgroup_sockopt.rst | 39 +++++++++++++++++++++++
 2 files changed, 40 insertions(+)
 create mode 100644 Documentation/bpf/prog_cgroup_sockopt.rst

diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst
index d3fe4cac0c90..801a6ed3f2e5 100644
--- a/Documentation/bpf/index.rst
+++ b/Documentation/bpf/index.rst
@@ -42,6 +42,7 @@ Program types
 .. toctree::
    :maxdepth: 1
 
+   prog_cgroup_sockopt
    prog_cgroup_sysctl
    prog_flow_dissector
 
diff --git a/Documentation/bpf/prog_cgroup_sockopt.rst b/Documentation/bpf/prog_cgroup_sockopt.rst
new file mode 100644
index 000000000000..b117451ab571
--- /dev/null
+++ b/Documentation/bpf/prog_cgroup_sockopt.rst
@@ -0,0 +1,39 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+BPF_PROG_TYPE_SOCKOPT
+=====================
+
+``BPF_PROG_TYPE_SOCKOPT`` program type can be attached to two cgroup hooks:
+
+* ``BPF_CGROUP_GETSOCKOPT`` - called every time process executes ``getsockopt``
+  system call.
+* ``BPF_CGROUP_SETSOCKOPT`` - called every time process executes ``setsockopt``
+  system call.
+
+The context (``struct bpf_sockopt``) has associated socket (``sk``) and
+all input arguments: ``level``, ``optname``, ``optval`` and ``optlen``.
+
+BPF_CGROUP_SETSOCKOPT
+=====================
+
+``BPF_CGROUP_SETSOCKOPT`` has a read-only context and this hook has
+access to cgroup and socket local storage.
+
+BPF_CGROUP_GETSOCKOPT
+=====================
+
+``BPF_CGROUP_GETSOCKOPT`` has to fill in ``optval`` and adjust
+``optlen`` accordingly. Input ``optlen`` contains the maximum length
+of data that can be returned to the userspace. In other words, BPF
+program can't increase ``optlen``, it can only decrease it.
+
+Return Type
+===========
+
+* ``0`` - reject the syscall, ``EPERM`` will be returned to the userspace.
+* ``1`` - success: after returning from the BPF hook, kernel will also
+  handle this socket option.
+* ``2`` - success: after returning from the BPF hook, kernel will _not_
+  handle this socket option; control will be returned to the userspace
+  instead.
-- 
2.22.0.rc2.383.gf4fbbf30c2-goog