linux/Documentation/bpf/prog_cgroup_sysctl.rst
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   1.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
   2
   3===========================
   4BPF_PROG_TYPE_CGROUP_SYSCTL
   5===========================
   6
   7This document describes ``BPF_PROG_TYPE_CGROUP_SYSCTL`` program type that
   8provides cgroup-bpf hook for sysctl.
   9
  10The hook has to be attached to a cgroup and will be called every time a
  11process inside that cgroup tries to read from or write to sysctl knob in proc.
  12
  131. Attach type
  14**************
  15
  16``BPF_CGROUP_SYSCTL`` attach type has to be used to attach
  17``BPF_PROG_TYPE_CGROUP_SYSCTL`` program to a cgroup.
  18
  192. Context
  20**********
  21
  22``BPF_PROG_TYPE_CGROUP_SYSCTL`` provides access to the following context from
  23BPF program::
  24
  25    struct bpf_sysctl {
  26        __u32 write;
  27        __u32 file_pos;
  28    };
  29
  30* ``write`` indicates whether sysctl value is being read (``0``) or written
  31  (``1``). This field is read-only.
  32
  33* ``file_pos`` indicates file position sysctl is being accessed at, read
  34  or written. This field is read-write. Writing to the field sets the starting
  35  position in sysctl proc file ``read(2)`` will be reading from or ``write(2)``
  36  will be writing to. Writing zero to the field can be used e.g. to override
  37  whole sysctl value by ``bpf_sysctl_set_new_value()`` on ``write(2)`` even
  38  when it's called by user space on ``file_pos > 0``. Writing non-zero
  39  value to the field can be used to access part of sysctl value starting from
  40  specified ``file_pos``. Not all sysctl support access with ``file_pos !=
  41  0``, e.g. writes to numeric sysctl entries must always be at file position
  42  ``0``. See also ``kernel.sysctl_writes_strict`` sysctl.
  43
  44See `linux/bpf.h`_ for more details on how context field can be accessed.
  45
  463. Return code
  47**************
  48
  49``BPF_PROG_TYPE_CGROUP_SYSCTL`` program must return one of the following
  50return codes:
  51
  52* ``0`` means "reject access to sysctl";
  53* ``1`` means "proceed with access".
  54
  55If program returns ``0`` user space will get ``-1`` from ``read(2)`` or
  56``write(2)`` and ``errno`` will be set to ``EPERM``.
  57
  584. Helpers
  59**********
  60
  61Since sysctl knob is represented by a name and a value, sysctl specific BPF
  62helpers focus on providing access to these properties:
  63
  64* ``bpf_sysctl_get_name()`` to get sysctl name as it is visible in
  65  ``/proc/sys`` into provided by BPF program buffer;
  66
  67* ``bpf_sysctl_get_current_value()`` to get string value currently held by
  68  sysctl into provided by BPF program buffer. This helper is available on both
  69  ``read(2)`` from and ``write(2)`` to sysctl;
  70
  71* ``bpf_sysctl_get_new_value()`` to get new string value currently being
  72  written to sysctl before actual write happens. This helper can be used only
  73  on ``ctx->write == 1``;
  74
  75* ``bpf_sysctl_set_new_value()`` to override new string value currently being
  76  written to sysctl before actual write happens. Sysctl value will be
  77  overridden starting from the current ``ctx->file_pos``. If the whole value
  78  has to be overridden BPF program can set ``file_pos`` to zero before calling
  79  to the helper. This helper can be used only on ``ctx->write == 1``. New
  80  string value set by the helper is treated and verified by kernel same way as
  81  an equivalent string passed by user space.
  82
  83BPF program sees sysctl value same way as user space does in proc filesystem,
  84i.e. as a string. Since many sysctl values represent an integer or a vector
  85of integers, the following helpers can be used to get numeric value from the
  86string:
  87
  88* ``bpf_strtol()`` to convert initial part of the string to long integer
  89  similar to user space `strtol(3)`_;
  90* ``bpf_strtoul()`` to convert initial part of the string to unsigned long
  91  integer similar to user space `strtoul(3)`_;
  92
  93See `linux/bpf.h`_ for more details on helpers described here.
  94
  955. Examples
  96***********
  97
  98See `test_sysctl_prog.c`_ for an example of BPF program in C that access
  99sysctl name and value, parses string value to get vector of integers and uses
 100the result to make decision whether to allow or deny access to sysctl.
 101
 1026. Notes
 103********
 104
 105``BPF_PROG_TYPE_CGROUP_SYSCTL`` is intended to be used in **trusted** root
 106environment, for example to monitor sysctl usage or catch unreasonable values
 107an application, running as root in a separate cgroup, is trying to set.
 108
 109Since `task_dfl_cgroup(current)` is called at `sys_read` / `sys_write` time it
 110may return results different from that at `sys_open` time, i.e. process that
 111opened sysctl file in proc filesystem may differ from process that is trying
 112to read from / write to it and two such processes may run in different
 113cgroups, what means ``BPF_PROG_TYPE_CGROUP_SYSCTL`` should not be used as a
 114security mechanism to limit sysctl usage.
 115
 116As with any cgroup-bpf program additional care should be taken if an
 117application running as root in a cgroup should not be allowed to
 118detach/replace BPF program attached by administrator.
 119
 120.. Links
 121.. _linux/bpf.h: ../../include/uapi/linux/bpf.h
 122.. _strtol(3): http://man7.org/linux/man-pages/man3/strtol.3p.html
 123.. _strtoul(3): http://man7.org/linux/man-pages/man3/strtoul.3p.html
 124.. _test_sysctl_prog.c:
 125   ../../tools/testing/selftests/bpf/progs/test_sysctl_prog.c
 126