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Diffstat (limited to 'Documentation/bpf/prog_cgroup_sockopt.rst')
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diff --git a/Documentation/bpf/prog_cgroup_sockopt.rst b/Documentation/bpf/prog_cgroup_sockopt.rst new file mode 100644 index 000000000..172f95720 --- /dev/null +++ b/Documentation/bpf/prog_cgroup_sockopt.rst @@ -0,0 +1,107 @@ +.. 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. |