8030e250d8
Extend existing doc with more details about requiring ctx->optlen = 0 for handling optval > PAGE_SIZE. Signed-off-by: Stanislav Fomichev <sdf@google.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Link: https://lore.kernel.org/bpf/20200617010416.93086-3-sdf@google.com
108 lines
4.0 KiB
ReStructuredText
108 lines
4.0 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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============================
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BPF_PROG_TYPE_CGROUP_SOCKOPT
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============================
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``BPF_PROG_TYPE_CGROUP_SOCKOPT`` program type can be attached to two
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cgroup hooks:
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* ``BPF_CGROUP_GETSOCKOPT`` - called every time process executes ``getsockopt``
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system call.
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* ``BPF_CGROUP_SETSOCKOPT`` - called every time process executes ``setsockopt``
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system call.
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The context (``struct bpf_sockopt``) has associated socket (``sk``) and
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all input arguments: ``level``, ``optname``, ``optval`` and ``optlen``.
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BPF_CGROUP_SETSOCKOPT
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=====================
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``BPF_CGROUP_SETSOCKOPT`` is triggered *before* the kernel handling of
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sockopt and it has writable context: it can modify the supplied arguments
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before passing them down to the kernel. This hook has access to the cgroup
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and socket local storage.
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If BPF program sets ``optlen`` to -1, the control will be returned
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back to the userspace after all other BPF programs in the cgroup
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chain finish (i.e. kernel ``setsockopt`` handling will *not* be executed).
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Note, that ``optlen`` can not be increased beyond the user-supplied
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value. It can only be decreased or set to -1. Any other value will
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trigger ``EFAULT``.
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Return Type
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-----------
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* ``0`` - reject the syscall, ``EPERM`` will be returned to the userspace.
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* ``1`` - success, continue with next BPF program in the cgroup chain.
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BPF_CGROUP_GETSOCKOPT
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=====================
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``BPF_CGROUP_GETSOCKOPT`` is triggered *after* the kernel handing of
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sockopt. The BPF hook can observe ``optval``, ``optlen`` and ``retval``
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if it's interested in whatever kernel has returned. BPF hook can override
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the values above, adjust ``optlen`` and reset ``retval`` to 0. If ``optlen``
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has been increased above initial ``getsockopt`` value (i.e. userspace
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buffer is too small), ``EFAULT`` is returned.
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This hook has access to the cgroup and socket local storage.
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Note, that the only acceptable value to set to ``retval`` is 0 and the
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original value that the kernel returned. Any other value will trigger
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``EFAULT``.
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Return Type
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-----------
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* ``0`` - reject the syscall, ``EPERM`` will be returned to the userspace.
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* ``1`` - success: copy ``optval`` and ``optlen`` to userspace, return
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``retval`` from the syscall (note that this can be overwritten by
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the BPF program from the parent cgroup).
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Cgroup Inheritance
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==================
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Suppose, there is the following cgroup hierarchy where each cgroup
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has ``BPF_CGROUP_GETSOCKOPT`` attached at each level with
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``BPF_F_ALLOW_MULTI`` flag::
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A (root, parent)
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\
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B (child)
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When the application calls ``getsockopt`` syscall from the cgroup B,
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the programs are executed from the bottom up: B, A. First program
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(B) sees the result of kernel's ``getsockopt``. It can optionally
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adjust ``optval``, ``optlen`` and reset ``retval`` to 0. After that
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control will be passed to the second (A) program which will see the
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same context as B including any potential modifications.
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Same for ``BPF_CGROUP_SETSOCKOPT``: if the program is attached to
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A and B, the trigger order is B, then A. If B does any changes
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to the input arguments (``level``, ``optname``, ``optval``, ``optlen``),
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then the next program in the chain (A) will see those changes,
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*not* the original input ``setsockopt`` arguments. The potentially
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modified values will be then passed down to the kernel.
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Large optval
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============
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When the ``optval`` is greater than the ``PAGE_SIZE``, the BPF program
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can access only the first ``PAGE_SIZE`` of that data. So it has to options:
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* Set ``optlen`` to zero, which indicates that the kernel should
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use the original buffer from the userspace. Any modifications
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done by the BPF program to the ``optval`` are ignored.
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* Set ``optlen`` to the value less than ``PAGE_SIZE``, which
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indicates that the kernel should use BPF's trimmed ``optval``.
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When the BPF program returns with the ``optlen`` greater than
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``PAGE_SIZE``, the userspace will receive ``EFAULT`` errno.
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Example
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=======
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See ``tools/testing/selftests/bpf/progs/sockopt_sk.c`` for an example
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of BPF program that handles socket options.
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