bpf: Document BPF_PROG_TYPE_CGROUP_SYSCTL

Add documentation for BPF_PROG_TYPE_CGROUP_SYSCTL, including general
info, attach type, context, return code, helpers, example and usage
considerations.

A separate file prog_cgroup_sysctl.rst is added to Documentation/bpf/.

In the future more program types can be documented in their own
prog_<name>.rst files.

Another way to place program type specific documentation would be to
group program types somehow (e.g. cgroup.rst for all cgroup-bpf
programs), but it may not scale well since some program types may belong
to different groups, e.g. BPF_PROG_TYPE_CGROUP_SKB can be documented
together with either cgroup-bpf programs or programs that access skb.

The new file is added to the index and verified by `make htmldocs` /
sanity-check by lynx.
Signed-off-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/index.rst

@@ -36,6 +36,15 @@ Two sets of Questions and Answers (Q&A) are maintained.
    bpf_devel_QA
 
 
+Program types
+=============
+
+.. toctree::
+   :maxdepth: 1
+
+   prog_cgroup_sysctl
+
+
 .. Links:
 .. _Documentation/networking/filter.txt: ../networking/filter.txt
 .. _man-pages: https://www.kernel.org/doc/man-pages/

Documentation/bpf/prog_cgroup_sysctl.rst

+.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+
+===========================
+BPF_PROG_TYPE_CGROUP_SYSCTL
+===========================
+
+This document describes ``BPF_PROG_TYPE_CGROUP_SYSCTL`` program type that
+provides cgroup-bpf hook for sysctl.
+
+The hook has to be attached to a cgroup and will be called every time a
+process inside that cgroup tries to read from or write to sysctl knob in proc.
+
+1. Attach type
+**************
+
+``BPF_CGROUP_SYSCTL`` attach type has to be used to attach
+``BPF_PROG_TYPE_CGROUP_SYSCTL`` program to a cgroup.
+
+2. Context
+**********
+
+``BPF_PROG_TYPE_CGROUP_SYSCTL`` provides access to the following context from
+BPF program::
+
+    struct bpf_sysctl {
+        __u32 write;
+        __u32 file_pos;
+    };
+
+* ``write`` indicates whether sysctl value is being read (``0``) or written
+  (``1``). This field is read-only.
+
+* ``file_pos`` indicates file position sysctl is being accessed at, read
+  or written. This field is read-write. Writing to the field sets the starting
+  position in sysctl proc file ``read(2)`` will be reading from or ``write(2)``
+  will be writing to. Writing zero to the field can be used e.g. to override
+  whole sysctl value by ``bpf_sysctl_set_new_value()`` on ``write(2)`` even
+  when it's called by user space on ``file_pos > 0``. Writing non-zero
+  value to the field can be used to access part of sysctl value starting from
+  specified ``file_pos``. Not all sysctl support access with ``file_pos !=
+  0``, e.g. writes to numeric sysctl entries must always be at file position
+  ``0``. See also ``kernel.sysctl_writes_strict`` sysctl.
+
+See `linux/bpf.h`_ for more details on how context field can be accessed.
+
+3. Return code
+**************
+
+``BPF_PROG_TYPE_CGROUP_SYSCTL`` program must return one of the following
+return codes:
+
+* ``0`` means "reject access to sysctl";
+* ``1`` means "proceed with access".
+
+If program returns ``0`` user space will get ``-1`` from ``read(2)`` or
+``write(2)`` and ``errno`` will be set to ``EPERM``.
+
+4. Helpers
+**********
+
+Since sysctl knob is represented by a name and a value, sysctl specific BPF
+helpers focus on providing access to these properties:
+
+* ``bpf_sysctl_get_name()`` to get sysctl name as it is visible in
+  ``/proc/sys`` into provided by BPF program buffer;
+
+* ``bpf_sysctl_get_current_value()`` to get string value currently held by
+  sysctl into provided by BPF program buffer. This helper is available on both
+  ``read(2)`` from and ``write(2)`` to sysctl;
+
+* ``bpf_sysctl_get_new_value()`` to get new string value currently being
+  written to sysctl before actual write happens. This helper can be used only
+  on ``ctx->write == 1``;
+
+* ``bpf_sysctl_set_new_value()`` to override new string value currently being
+  written to sysctl before actual write happens. Sysctl value will be
+  overridden starting from the current ``ctx->file_pos``. If the whole value
+  has to be overridden BPF program can set ``file_pos`` to zero before calling
+  to the helper. This helper can be used only on ``ctx->write == 1``. New
+  string value set by the helper is treated and verified by kernel same way as
+  an equivalent string passed by user space.
+
+BPF program sees sysctl value same way as user space does in proc filesystem,
+i.e. as a string. Since many sysctl values represent an integer or a vector
+of integers, the following helpers can be used to get numeric value from the
+string:
+
+* ``bpf_strtol()`` to convert initial part of the string to long integer
+  similar to user space `strtol(3)`_;
+* ``bpf_strtoul()`` to convert initial part of the string to unsigned long
+  integer similar to user space `strtoul(3)`_;
+
+See `linux/bpf.h`_ for more details on helpers described here.
+
+5. Examples
+***********
+
+See `test_sysctl_prog.c`_ for an example of BPF program in C that access
+sysctl name and value, parses string value to get vector of integers and uses
+the result to make decision whether to allow or deny access to sysctl.
+
+6. Notes
+********
+
+``BPF_PROG_TYPE_CGROUP_SYSCTL`` is intended to be used in **trusted** root
+environment, for example to monitor sysctl usage or catch unreasonable values
+an application, running as root in a separate cgroup, is trying to set.
+
+Since `task_dfl_cgroup(current)` is called at `sys_read` / `sys_write` time it
+may return results different from that at `sys_open` time, i.e. process that
+opened sysctl file in proc filesystem may differ from process that is trying
+to read from / write to it and two such processes may run in different
+cgroups, what means ``BPF_PROG_TYPE_CGROUP_SYSCTL`` should not be used as a
+security mechanism to limit sysctl usage.
+
+As with any cgroup-bpf program additional care should be taken if an
+application running as root in a cgroup should not be allowed to
+detach/replace BPF program attached by administrator.
+
+.. Links
+.. _linux/bpf.h: ../../include/uapi/linux/bpf.h
+.. _strtol(3): http://man7.org/linux/man-pages/man3/strtol.3p.html
+.. _strtoul(3): http://man7.org/linux/man-pages/man3/strtoul.3p.html
+.. _test_sysctl_prog.c:
+   ../../tools/testing/selftests/bpf/progs/test_sysctl_prog.c