1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
|
.. 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 ``EFAULT`` errno.
Example
=======
See ``tools/testing/selftests/bpf/progs/sockopt_sk.c`` for an example
of BPF program that handles socket options.
|
