linux/Documentation/bpf/prog_cgroup_sockopt.rst

.. SPDX-License-Identifier: GPL-2.0

============================
BPF_PROG_TYPE_CGROUP_SOCKOPT
============================

``BPF_PROG_TYPE_CGROUP_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`` is triggered *before* the kernel handling of
sockopt and it has writable context: it can modify the supplied arguments
before passing them down to the kernel. This hook has access to the cgroup
and socket local storage.

If BPF program sets ``optlen`` to -1, the control will be returned
back to the userspace after all other BPF programs in the cgroup
chain finish (i.e. kernel ``setsockopt`` handling will *not* be executed).

Note, that ``optlen`` can not be increased beyond the user-supplied
value. It can only be decreased or set to -1. Any other value will
trigger ``EFAULT``.

Return Type
-----------

* ``0`` - reject the syscall, ``EPERM`` will be returned to the userspace.
* ``1`` - success, continue with next BPF program in the cgroup chain.

BPF_CGROUP_GETSOCKOPT
=====================

``BPF_CGROUP_GETSOCKOPT`` is triggered *after* the kernel handing of
sockopt. The BPF hook can observe ``optval``, ``optlen`` and ``retval``
if it's interested in whatever kernel has returned. BPF hook can override
the values above, adjust ``optlen`` and reset ``retval`` to 0. If ``optlen``
has been increased above initial ``getsockopt`` value (i.e. userspace
buffer is too small), ``EFAULT`` is returned.

This hook has access to the cgroup and socket local storage.

Note, that the only acceptable value to set to ``retval`` is 0 and the
original value that the kernel returned. Any other value will trigger
``EFAULT``.

Return Type
-----------

* ``0`` - reject the syscall, ``EPERM`` will be returned to the userspace.
* ``1`` - success: copy ``optval`` and ``optlen`` to userspace, return
  ``retval`` from the syscall (note that this can be overwritten by
  the BPF program from the parent cgroup).

Cgroup Inheritance
==================

Suppose, there is the following cgroup hierarchy where each cgroup
has ``BPF_CGROUP_GETSOCKOPT`` attached at each level with
``BPF_F_ALLOW_MULTI`` flag::

  A (root, parent)
   \
    B (child)

When the application calls ``getsockopt`` syscall from the cgroup B,
the programs are executed from the bottom up: B, A. First program
(B) sees the result of kernel's ``getsockopt``. It can optionally
adjust ``optval``, ``optlen`` and reset ``retval`` to 0. After that
control will be passed to the second (A) program which will see the
same context as B including any potential modifications.

Same for ``BPF_CGROUP_SETSOCKOPT``: if the program is attached to
A and B, the trigger order is B, then A. If B does any changes
to the input arguments (``level``, ``optname``, ``optval``, ``optlen``),
then the next program in the chain (A) will see those changes,
*not* the original input ``setsockopt`` arguments. The potentially
modified values will be then passed down to the kernel.

Large optval
============
When the ``optval`` is greater than the ``PAGE_SIZE``, the BPF program
can access only the first ``PAGE_SIZE`` of that data. So it has to options:

* Set ``optlen`` to zero, which indicates that the kernel should
  use the original buffer from the userspace. Any modifications
  done by the BPF program to the ``optval`` are ignored.
* Set ``optlen`` to the value less than ``PAGE_SIZE``, which
  indicates that the kernel should use BPF's trimmed ``optval``.

When the BPF program returns with the ``optlen`` greater than
``PAGE_SIZE``, the userspace will receive original kernel
buffers without any modifications that the BPF program might have
applied.

Example
=======

Recommended way to handle BPF programs is as follows:

.. code-block:: c

	SEC("cgroup/getsockopt")
	int getsockopt(struct bpf_sockopt *ctx)
	{
		/* Custom socket option. */
		if (ctx->level == MY_SOL && ctx->optname == MY_OPTNAME) {
			ctx->retval = 0;
			optval[0] = ...;
			ctx->optlen = 1;
			return 1;
		}

		/* Modify kernel's socket option. */
		if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) {
			ctx->retval = 0;
			optval[0] = ...;
			ctx->optlen = 1;
			return 1;
		}

		/* optval larger than PAGE_SIZE use kernel's buffer. */
		if (ctx->optlen > PAGE_SIZE)
			ctx->optlen = 0;

		return 1;
	}

	SEC("cgroup/setsockopt")
	int setsockopt(struct bpf_sockopt *ctx)
	{
		/* Custom socket option. */
		if (ctx->level == MY_SOL && ctx->optname == MY_OPTNAME) {
			/* do something */
			ctx->optlen = -1;
			return 1;
		}

		/* Modify kernel's socket option. */
		if (ctx->level == SOL_IP && ctx->optname == IP_FREEBIND) {
			optval[0] = ...;
			return 1;
		}

		/* optval larger than PAGE_SIZE use kernel's buffer. */
		if (ctx->optlen > PAGE_SIZE)
			ctx->optlen = 0;

		return 1;
	}

See ``tools/testing/selftests/bpf/progs/sockopt_sk.c`` for an example
of BPF program that handles socket options.