[PATCH bpf-next v2 1/1] docs: BPF_MAP_TYPE_SOCK[MAP|HASH]

mtahhan
2022-12-01T15:15:00Z

From: Maryam Tahhan <mtahhan@redhat.com>

Add documentation for BPF_MAP_TYPE_SOCK[MAP|HASH]
including kernel versions introduced, usage
and examples.

Signed-off-by: Maryam Tahhan <mtahhan@redhat.com>

---
v2:
- Fixed typos and user space references to BPF helpers.
- Added update, lookup and delete BPF helpers.
---
---
Documentation/bpf/map_sockmap.rst | 482 ++++++++++++++++++++++++++++++
1 file changed, 482 insertions(+)
create mode 100644 Documentation/bpf/map_sockmap.rst

diff --git a/Documentation/bpf/map_sockmap.rst b/Documentation/bpf/map_sockmap.rst
new file mode 100644
index 000000000000..398a01e863ff
--- /dev/null
+++ b/Documentation/bpf/map_sockmap.rst
@@ -0,0 +1,482 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright Red Hat
+
+==============================================
+BPF_MAP_TYPE_SOCKMAP and BPF_MAP_TYPE_SOCKHASH
+==============================================
+
+.. note::
+ - ``BPF_MAP_TYPE_SOCKMAP`` was introduced in kernel version 4.14
+ - ``BPF_MAP_TYPE_SOCKHASH`` was introduced in kernel version 4.18
+
+``BPF_MAP_TYPE_SOCKMAP`` is backed by an array that uses an integer key as the
+index to lookup a reference to a sock struct. The map values are sockets.
+Similarly, ``BPF_MAP_TYPE_SOCKHASH`` is a hash backed BPF map that holds
+references to sockets.
+
+.. note::
+ - The value type must be __u32 or __u64.
+ - The value type of __u64 is to support socket cookies.
+
+When these maps are created BPF programs are attached to them. The list of
+allowed programs is shown below:
+
+.. code-block:: c
+
+ struct sk_psock_progs {
+ struct bpf_prog *msg_parser;
+ struct bpf_prog *stream_parser;
+ struct bpf_prog *stream_verdict;
+ struct bpf_prog *skb_verdict;
+ };
+
+.. note::
+ Users are not allowed to attach ``stream_verdict`` and ``skb_verdict``
+ programs to the same map.
+
+The parser programs determine how much data needs to be queued to come to a
+verdict. The verdict programs return a verdict ``__SK_DROP``, ``__SK_PASS``, or
+``__SK_REDIRECT``.
+
+The attach types for the map programs are:
+
+- ``msg_parser`` program - ``BPF_SK_MSG_VERDICT``.
+- ``stream_parser`` program - ``BPF_SK_SKB_STREAM_PARSER``.
+- ``stream_verdict`` program - ``BPF_SK_SKB_STREAM_VERDICT``.
+- ``skb_verdict`` program - ``BPF_SK_SKB_VERDICT``.
+
+These maps can be used to redirect skbs between sockets or to apply policy at
+the socket level based on the result of a verdict program with the help of the
+BPF helpers ``bpf_sk_redirect_map()``, ``bpf_sk_redirect_hash()``,
+``bpf_msg_redirect_map()`` and ``bpf_msg_redirect_hash()``.
+
+When a socket is inserted into one of these maps, its socket callbacks are
+replaced and a ``struct sk_psock`` is attached to it. Additionally, this
+``sk_psock`` inherits the programs that are attached to the map.
+
+.. note::
+ For more details of the socket callbacks that get replaced please see:
+
+ - TCP BPF functions: ``net/ipv4/tcp_bpf.c``
+ - UDP BPF functions: ``net/ipv4/udp_bpf.c``
+
+There are additional helpers available to use with the parser and verdict
+programs: ``bpf_msg_apply_bytes()`` and ``bpf_msg_cork_bytes()``. With
+``bpf_msg_apply_bytes()`` BPF programs can tell the infrastructure how many
+bytes the given verdict should apply to. The helper ``bpf_msg_cork_bytes()``
+handles a different case where a BPF program can not reach a verdict on a msg
+until it receives more bytes AND the program doesn't want to forward the packet
+until it is known to be good.
+
+Finally, the helpers ``bpf_msg_pull_data()`` and ``bpf_msg_push_data()`` are
+available to ``BPF_PROG_TYPE_SK_MSG`` BPF programs to pull in data and set the
+start and end pointer to given values or to add metadata to the ``struct
+sk_msg_buff *msg``.
+
+Usage
+=====
+Kernel BPF
+----------
+bpf_msg_redirect_map()
+^^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_msg_redirect_map(struct sk_msg_buff *msg, struct bpf_map *map, u32 key, u64 flags)
+
+This helper is used in programs implementing policies at the socket level. If
+the message ``msg`` is allowed to pass (i.e. if the verdict BPF program
+returns ``SK_PASS``), redirect it to the socket referenced by ``map`` (of type
+``BPF_MAP_TYPE_SOCKMAP``) at index ``key``. Both ingress and egress interfaces
+can be used for redirection. The ``BPF_F_INGRESS`` value in ``flags`` is used
+to select the ingress path otherwise the egress path is selected. This is the
+only flag supported for now.
+
+Returns ``SK_PASS`` on success, or ``SK_DROP`` on error.
+
+bpf_sk_redirect_map()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_sk_redirect_map(struct sk_buff *skb, struct bpf_map *map, u32 key u64 flags)
+
+Redirect the packet to the socket referenced by ``map`` (of type
+``BPF_MAP_TYPE_SOCKMAP``) at index ``key``. Both ingress and egress interfaces
+can be used for redirection. The ``BPF_F_INGRESS`` value in ``flags`` is used
+to select the ingress path otherwise the egress path is selected. This is the
+only flag supported for now.
+
+Returns ``SK_PASS`` on success, or ``SK_DROP`` on error.
+
+bpf_map_lookup_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
+
+socket entries of type ``struct sock *`` can be retrieved using the
+``bpf_map_lookup_elem()`` helper.
+
+bpf_sock_map_update()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_sock_map_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64 flags)
+
+Add an entry to, or update a ``map`` referencing sockets. The ``skops`` is used
+as a new value for the entry associated to ``key``. The ``flags`` argument can
+be one of the following:
+
+- ``BPF_ANY``: Create a new element or update an existing element.
+- ``BPF_NOEXIST``: Create a new element only if it did not exist.
+- ``BPF_EXIST``: Update an existing element.
+
+If the ``map`` has BPF programs (parser and verdict), those will be inherited
+by the socket being added. If the socket is already attached to BPF programs,
+this results in an error.
+
+Returns 0 on success, or a negative error in case of failure.
+
+bpf_sock_hash_update()
+^^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_sock_hash_update(struct bpf_sock_ops *skops, struct bpf_map *map, void *key, u64 flags)
+
+Add an entry to, or update a sockhash ``map`` referencing sockets. The ``skops``
+is used as a new value for the entry associated to ``key``.
+
+The ``flags`` argument can be one of the following:
+
+- ``BPF_ANY``: Create a new element or update an existing element.
+- ``BPF_NOEXIST``: Create a new element only if it did not exist.
+- ``BPF_EXIST``: Update an existing element.
+
+If the ``map`` has BPF programs (parser and verdict), those will be inherited
+by the socket being added. If the socket is already attached to BPF programs,
+this results in an error.
+
+Returns 0 on success, or a negative error in case of failure.
+
+bpf_msg_redirect_hash()
+^^^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_msg_redirect_hash(struct sk_msg_buff *msg, struct bpf_map *map, void *key, u64 flags)
+
+This helper is used in programs implementing policies at the socket level. If
+the message ``msg`` is allowed to pass (i.e. if the verdict BPF program returns
+``SK_PASS``), redirect it to the socket referenced by ``map`` (of type
+``BPF_MAP_TYPE_SOCKHASH``) using hash ``key``. Both ingress and egress
+interfaces can be used for redirection. The ``BPF_F_INGRESS`` value in
+``flags`` is used to select the ingress path otherwise the egress path is
+selected. This is the only flag supported for now.
+
+Returns ``SK_PASS`` on success, or ``SK_DROP`` on error.
+
+bpf_sk_redirect_hash()
+^^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_sk_redirect_hash(struct sk_buff *skb, struct bpf_map *map, void *key, u64 flags)
+
+This helper is used in programs implementing policies at the skb socket level.
+If the sk_buff ``skb`` is allowed to pass (i.e. if the verdict BPF program
+returns ``SK_PASS``), redirect it to the socket referenced by ``map`` (of type
+``BPF_MAP_TYPE_SOCKHASH``) using hash ``key``. Both ingress and egress
+interfaces can be used for redirection. The ``BPF_F_INGRESS`` value in
+``flags`` is used to select the ingress path otherwise the egress path is
+selected. This is the only flag supported for now.
+
+Returns ``SK_PASS`` on success, or ``SK_DROP`` on error.
+
+bpf_msg_apply_bytes()
+^^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_msg_apply_bytes(struct sk_msg_buff *msg, u32 bytes)
+
+For socket policies, apply the verdict of the BPF program to the next (number
+of ``bytes``) of message ``msg``. For example, this helper can be used in the
+following cases:
+
+- A single ``sendmsg()`` or ``sendfile()`` system call contains multiple
+ logical messages that the BPF program is supposed to read and for which it
+ should apply a verdict.
+- A BPF program only cares to read the first ``bytes`` of a ``msg``. If the
+ message has a large payload, then setting up and calling the BPF program
+ repeatedly for all bytes, even though the verdict is already known, would
+ create unnecessary overhead.
+
+Returns 0
+
+bpf_msg_cork_bytes()
+^^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_msg_cork_bytes(struct sk_msg_buff *msg, u32 bytes)
+
+For socket policies, prevent the execution of the verdict BPF program for
+message ``msg`` until the number of ``bytes`` have been accumulated.
+
+This can be used when one needs a specific number of bytes before a verdict can
+be assigned, even if the data spans multiple ``sendmsg()`` or ``sendfile()``
+calls.
+
+Returns 0
+
+bpf_msg_pull_data()
+^^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_msg_pull_data(struct sk_msg_buff *msg, u32 start, u32 end, u64 flags)
+
+For socket policies, pull in non-linear data from user space for ``msg`` and set
+pointers ``msg->data`` and ``msg->data_end`` to ``start`` and ``end`` bytes
+offsets into ``msg``, respectively.
+
+If a program of type ``BPF_PROG_TYPE_SK_MSG`` is run on a ``msg`` it can only
+parse data that the (``data``, ``data_end``) pointers have already consumed.
+For ``sendmsg()`` hooks this is likely the first scatterlist element. But for
+calls relying on the ``sendpage`` handler (e.g. ``sendfile()``) this will be
+the range (**0**, **0**) because the data is shared with user space and by
+default the objective is to avoid allowing user space to modify data while (or
+after) BPF verdict is being decided. This helper can be used to pull in data
+and to set the start and end pointer to given values. Data will be copied if
+necessary (i.e. if data was not linear and if start and end pointers do not
+point to the same chunk).
+
+A call to this helper is susceptible to change the underlying packet buffer.
+Therefore, at load time, all checks on pointers previously done by the verifier
+are invalidated and must be performed again, if the helper is used in
+combination with direct packet access.
+
+All values for ``flags`` are reserved for future usage, and must be left at
+zero.
+
+Returns 0 on success, or a negative error in case of failure.
+
+bpf_map_lookup_elem()
+^^^^^^^^^^^^^^^^^^^^^
+
+.. code-block:: c
+
+ void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
+
+Lookup a socket entry in the sockmap or sockhash map.
+
+Returns the socket entry associated to ``key``, or NULL if no entry was found.
+
+bpf_map_update_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_map_update_elem(struct bpf_map *map, const void *key, const void *value, u64 flags)
+
+Add or update a socket entry in a sockmap or sockhash.
+
+The flags argument can be one of the following:
+
+- BPF_ANY: Create a new element or update an existing element.
+- BPF_NOEXIST: Create a new element only if it did not exist.
+- BPF_EXIST: Update an existing element.
+
+Returns 0 on success, or a negative error in case of failure.
+
+bpf_map_delete_elem()
+^^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ long bpf_map_delete_elem(struct bpf_map *map, const void *key)
+
+Delete a socket entry from a sockmap or a sockhash.
+
+Returns 0 on success, or a negative error in case of failure.
+
+User space
+----------
+bpf_map_update_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ int bpf_map_update_elem(int fd, const void *key, const void *value, __u64 flags)
+
+Sockmap entries can be added or updated using the ``bpf_map_update_elem()``
+function. The ``key`` parameter is the index value of the sockmap array. And the
+``value`` parameter is the FD value of that socket.
+
+Under the hood, the sockmap update function uses the socket FD value to
+retrieve the associated socket and its attached psock.
+
+The flags argument can be one of the following:
+
+- BPF_ANY: Create a new element or update an existing element.
+- BPF_NOEXIST: Create a new element only if it did not exist.
+- BPF_EXIST: Update an existing element.
+
+bpf_map_lookup_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ int bpf_map_lookup_elem(int fd, const void *key, void *value)
+
+Sockmap entries can be retrieved using the ``bpf_map_lookup_elem()`` function.
+
+.. note::
+ The entry returned is a socket cookie rather than a socket itself.
+
+bpf_map_delete_elem()
+^^^^^^^^^^^^^^^^^^^^^
+.. code-block:: c
+
+ int bpf_map_delete_elem(int fd, const void *key)
+
+Sockmap entries can be deleted using the ``bpf_map_delete_elem()``
+function.
+
+Returns 0 on success, or negative error in case of failure.
+
+Examples
+========
+
+Kernel BPF
+----------
+Several examples of the use of sockmap APIs can be found in:
+
+- `tools/testing/selftests/bpf/progs/test_sockmap_kern.h`_
+- `tools/testing/selftests/bpf/progs/sockmap_parse_prog.c`_
+- `tools/testing/selftests/bpf/progs/sockmap_verdict_prog.c`_
+- `tools/testing/selftests/bpf/progs/test_sockmap_listen.c`_
+- `tools/testing/selftests/bpf/progs/test_sockmap_update.c`_
+
+The following code snippet shows how to declare a sockmap.
+
+.. code-block:: c
+
+ struct {
+ __uint(type, BPF_MAP_TYPE_SOCKMAP);
+ __uint(max_entries, 1);
+ __type(key, __u32);
+ __type(value, __u64);
+ } sock_map_rx SEC(".maps");
+
+The following code snippet shows a sample parser program.
+
+.. code-block:: c
+
+ SEC("sk_skb/stream_parser")
+ int bpf_prog_parser(struct __sk_buff *skb)
+ {
+ return skb->len;
+ }
+
+The following code snippet shows a simple verdict program that interacts with a
+sockmap to redirect traffic to another socket based on the local port.
+
+.. code-block:: c
+
+ SEC("sk_skb/stream_verdict")
+ int bpf_prog_verdict(struct __sk_buff *skb)
+ {
+ __u32 lport = skb->local_port;
+ __u32 idx = 0;
+
+ if (lport == 10000)
+ return bpf_sk_redirect_map(skb, &sock_map_rx, idx, 0);
+
+ return SK_PASS;
+ }
+
+The following code snippet shows how to declare a sockhash map.
+
+.. code-block:: c
+
+ struct socket_key {
+ __u32 src_ip;
+ __u32 dst_ip;
+ __u32 src_port;
+ __u32 dst_port;
+ };
+
+ struct {
+ __uint(type, BPF_MAP_TYPE_SOCKHASH);
+ __uint(max_entries, 1);
+ __type(key, struct socket_key);
+ __type(value, __u64);
+ } sock_hash_rx SEC(".maps");
+
+The following code snippet shows a simple verdict program that interacts with a
+sockhash to redirect traffic to another socket based on a hash of some of the
+skb parameters.
+
+.. code-block:: c
+
+ static inline
+ void extract_socket_key(struct __sk_buff *skb, struct socket_key *key)
+ {
+ key->src_ip = skb->remote_ip4;
+ key->dst_ip = skb->local_ip4;
+ key->src_port = skb->remote_port >> 16;
+ key->dst_port = (bpf_htonl(skb->local_port)) >> 16;
+ }
+
+ SEC("sk_skb/stream_verdict")
+ int bpf_prog_verdict(struct __sk_buff *skb)
+ {
+ struct socket_key key;
+
+ extract_socket_key(skb, &key);
+
+ return bpf_sk_redirect_hash(skb, &sock_hash_rx, &key, 0);
+ }
+
+User space
+----------
+Several examples of the use of sockmap APIs can be found in:
+
+- `tools/testing/selftests/bpf/prog_tests/sockmap_basic.c`_
+- `tools/testing/selftests/bpf/test_sockmap.c`_
+- `tools/testing/selftests/bpf/test_maps.c`_
+
+The following code snippet shows how to create a sockmap and add a socket
+entry:
+
+.. code-block:: c
+
+ /* Create a map and populate it with one socket*/
+ static void create_sample_sockmap(int s)
+ {
+ const int index = 0;
+ int map, err;
+
+ map = bpf_map_create(BPF_MAP_TYPE_SOCKMAP, NULL, sizeof(int), sizeof(int), 1, NULL);
+ if (map < 0) {
+ fprintf(stderr, "Failed to create sockmap: %s\n", strerror(errno));
+ goto out;
+ }
+
+ err = bpf_map_update_elem(map, &index, &s, BPF_NOEXIST);
+ if (err) {
+ fprintf(stderr, "Failed to update sockmap: %s\n", strerror(errno));
+ goto out;
+ }
+
+ out:
+ close(map);
+ }
+
+References
+===========
+
+- https://github.com/jrfastab/linux-kernel-xdp/commit/c89fd73cb9d2d7f3c716c3e00836f07b1aeb261f
+- https://lwn.net/Articles/731133/
+- http://vger.kernel.org/lpc_net2018_talks/ktls_bpf_paper.pdf
+- https://lwn.net/Articles/748628/
+
+.. _`tools/testing/selftests/bpf/progs/test_sockmap_kern.h`: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/progs/test_sockmap_kern.h
+.. _`tools/testing/selftests/bpf/progs/sockmap_parse_prog.c`: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/progs/sockmap_parse_prog.c
+.. _`tools/testing/selftests/bpf/progs/sockmap_verdict_prog.c`: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/progs/sockmap_verdict_prog.c
+.. _`tools/testing/selftests/bpf/prog_tests/sockmap_basic.c`: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/prog_tests/sockmap_basic.c
+.. _`tools/testing/selftests/bpf/test_sockmap.c`: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/test_sockmap.c
+.. _`tools/testing/selftests/bpf/test_maps.c`: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/test_maps.c
+.. _`tools/testing/selftests/bpf/progs/test_sockmap_listen.c`: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/progs/test_sockmap_listen.c
+.. _`tools/testing/selftests/bpf/progs/test_sockmap_update.c`: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/progs/test_sockmap_update.c
--
2.34.1