1.. SPDX-License-Identifier: GPL-2.0
2
3===================================
4Running BPF programs from userspace
5===================================
6
7This document describes the ``BPF_PROG_RUN`` facility for running BPF programs
8from userspace.
9
10.. contents::
11    :local:
12    :depth: 2
13
14
15Overview
16--------
17
18The ``BPF_PROG_RUN`` command can be used through the ``bpf()`` syscall to
19execute a BPF program in the kernel and return the results to userspace. This
20can be used to unit test BPF programs against user-supplied context objects, and
21as way to explicitly execute programs in the kernel for their side effects. The
22command was previously named ``BPF_PROG_TEST_RUN``, and both constants continue
23to be defined in the UAPI header, aliased to the same value.
24
25The ``BPF_PROG_RUN`` command can be used to execute BPF programs of the
26following types:
27
28- ``BPF_PROG_TYPE_SOCKET_FILTER``
29- ``BPF_PROG_TYPE_SCHED_CLS``
30- ``BPF_PROG_TYPE_SCHED_ACT``
31- ``BPF_PROG_TYPE_XDP``
32- ``BPF_PROG_TYPE_SK_LOOKUP``
33- ``BPF_PROG_TYPE_CGROUP_SKB``
34- ``BPF_PROG_TYPE_LWT_IN``
35- ``BPF_PROG_TYPE_LWT_OUT``
36- ``BPF_PROG_TYPE_LWT_XMIT``
37- ``BPF_PROG_TYPE_LWT_SEG6LOCAL``
38- ``BPF_PROG_TYPE_FLOW_DISSECTOR``
39- ``BPF_PROG_TYPE_STRUCT_OPS``
40- ``BPF_PROG_TYPE_RAW_TRACEPOINT``
41- ``BPF_PROG_TYPE_SYSCALL``
42
43When using the ``BPF_PROG_RUN`` command, userspace supplies an input context
44object and (for program types operating on network packets) a buffer containing
45the packet data that the BPF program will operate on. The kernel will then
46execute the program and return the results to userspace. Note that programs will
47not have any side effects while being run in this mode; in particular, packets
48will not actually be redirected or dropped, the program return code will just be
49returned to userspace. A separate mode for live execution of XDP programs is
50provided, documented separately below.
51
52Running XDP programs in "live frame mode"
53-----------------------------------------
54
55The ``BPF_PROG_RUN`` command has a separate mode for running live XDP programs,
56which can be used to execute XDP programs in a way where packets will actually
57be processed by the kernel after the execution of the XDP program as if they
58arrived on a physical interface. This mode is activated by setting the
59``BPF_F_TEST_XDP_LIVE_FRAMES`` flag when supplying an XDP program to
60``BPF_PROG_RUN``.
61
62The live packet mode is optimised for high performance execution of the supplied
63XDP program many times (suitable for, e.g., running as a traffic generator),
64which means the semantics are not quite as straight-forward as the regular test
65run mode. Specifically:
66
67- When executing an XDP program in live frame mode, the result of the execution
68  will not be returned to userspace; instead, the kernel will perform the
69  operation indicated by the program's return code (drop the packet, redirect
70  it, etc). For this reason, setting the ``data_out`` or ``ctx_out`` attributes
71  in the syscall parameters when running in this mode will be rejected. In
72  addition, not all failures will be reported back to userspace directly;
73  specifically, only fatal errors in setup or during execution (like memory
74  allocation errors) will halt execution and return an error. If an error occurs
75  in packet processing, like a failure to redirect to a given interface,
76  execution will continue with the next repetition; these errors can be detected
77  via the same trace points as for regular XDP programs.
78
79- Userspace can supply an ifindex as part of the context object, just like in
80  the regular (non-live) mode. The XDP program will be executed as though the
81  packet arrived on this interface; i.e., the ``ingress_ifindex`` of the context
82  object will point to that interface. Furthermore, if the XDP program returns
83  ``XDP_PASS``, the packet will be injected into the kernel networking stack as
84  though it arrived on that ifindex, and if it returns ``XDP_TX``, the packet
85  will be transmitted *out* of that same interface. Do note, though, that
86  because the program execution is not happening in driver context, an
87  ``XDP_TX`` is actually turned into the same action as an ``XDP_REDIRECT`` to
88  that same interface (i.e., it will only work if the driver has support for the
89  ``ndo_xdp_xmit`` driver op).
90
91- When running the program with multiple repetitions, the execution will happen
92  in batches. The batch size defaults to 64 packets (which is same as the
93  maximum NAPI receive batch size), but can be specified by userspace through
94  the ``batch_size`` parameter, up to a maximum of 256 packets. For each batch,
95  the kernel executes the XDP program repeatedly, each invocation getting a
96  separate copy of the packet data. For each repetition, if the program drops
97  the packet, the data page is immediately recycled (see below). Otherwise, the
98  packet is buffered until the end of the batch, at which point all packets
99  buffered this way during the batch are transmitted at once.
100
101- When setting up the test run, the kernel will initialise a pool of memory
102  pages of the same size as the batch size. Each memory page will be initialised
103  with the initial packet data supplied by userspace at ``BPF_PROG_RUN``
104  invocation. When possible, the pages will be recycled on future program
105  invocations, to improve performance. Pages will generally be recycled a full
106  batch at a time, except when a packet is dropped (by return code or because
107  of, say, a redirection error), in which case that page will be recycled
108  immediately. If a packet ends up being passed to the regular networking stack
109  (because the XDP program returns ``XDP_PASS``, or because it ends up being
110  redirected to an interface that injects it into the stack), the page will be
111  released and a new one will be allocated when the pool is empty.
112
113  When recycling, the page content is not rewritten; only the packet boundary
114  pointers (``data``, ``data_end`` and ``data_meta``) in the context object will
115  be reset to the original values. This means that if a program rewrites the
116  packet contents, it has to be prepared to see either the original content or
117  the modified version on subsequent invocations.
118