[PATCH bpf-next v5 1/1] docs: BPF_MAP_TYPE_SOCK[MAP|HASH]

[PATCH bpf-next v5 1/1] docs: BPF_MAP_TYPE_SOCK[MAP|HASH]

[mtahhan]

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>

---
v5:
- Fixed typo.

v4:
- Restructure documentation.

v3:
- Call out that the user attaches the BPF programs to
  the sock[map|hash] maps explicitly.
- Rephrase the note that references the TCP and UDP
  functions that get replaced.
- Update simple example to attach verdict and parser
  progs to a map.

v2:
- Fixed typos and user space references to BPF helpers.
- Added update, lookup and delete BPF helpers.
---
---
 Documentation/bpf/map_sockmap.rst | 503 ++++++++++++++++++++++++++++++
 1 file changed, 503 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..1d122dc20933
--- /dev/null
+++ b/Documentation/bpf/map_sockmap.rst
@@ -0,0 +1,503 @@
+.. 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`` and ``BPF_MAP_TYPE_SOCKHASH`` maps can be used to
+redirect skbs between sockets or to apply policy at the socket level based on
+the result of a BPF (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()``.
+
+``BPF_MAP_TYPE_SOCKMAP`` is backed by an array that uses an integer key as the
+index to lookup a reference to a ``struct sock``. The map values are socket
+descriptors. Similarly, ``BPF_MAP_TYPE_SOCKHASH`` is a hash backed BPF map that
+holds references to sockets via their socket descriptors.
+
+.. note::
+    The value type is either __u32 or __u64; the latter (__u64) is to support
+    returning socket cookies to userspace. Returning the ``struct sock *`` that
+    the map holds to user-space is neither safe nor useful.
+
+These maps may have BPF programs attached to them, specifically a parser program
+and a verdict program. The parser program determines how much data has been
+parsed and therefore how much data needs to be queued to come to a verdict. The
+verdict program is essentially the redirect program and can return a verdict
+of ``__SK_DROP``, ``__SK_PASS``, or ``__SK_REDIRECT``.
+
+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
+    ``net/ipv4/tcp_bpf.c`` and ``net/ipv4/udp_bpf.c`` for TCP and UDP
+    functions, respectively.
+
+A sock object may be in multiple maps, but can only inherit a single
+parse or verdict program. If adding a sock object to a map would result
+in having multiple parsing programs the update will return an EBUSY error.
+
+The supported programs to attach to these maps are:
+
+.. 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 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``.
+
+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 cannot 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``.
+
+All these helpers will be described in more detail below.
+
+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 sample shows how to create a sockmap, attach a parser and
+verdict program, as well as add a socket entry.
+
+.. code-block:: c
+
+	int create_sample_sockmap(int sock, int parse_prog_fd, int verdict_prog_fd)
+	{
+		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));
+			return -1;
+		}
+
+		err = bpf_prog_attach(parse_prog_fd, map, BPF_SK_SKB_STREAM_PARSER, 0);
+		if (err){
+			fprintf(stderr, "Failed to attach_parser_prog_to_map: %s\n", strerror(errno));
+			goto out;
+		}
+
+		err = bpf_prog_attach(verdict_prog_fd, map, BPF_SK_SKB_STREAM_VERDICT, 0);
+		if (err){
+			fprintf(stderr, "Failed to attach_verdict_prog_to_map: %s\n", strerror(errno));
+			goto out;
+		}
+
+		err = bpf_map_update_elem(map, &index, &sock, BPF_NOEXIST);
+		if (err) {
+			fprintf(stderr, "Failed to update sockmap: %s\n", strerror(errno));
+			goto out;
+		}
+
+	out:
+		close(map);
+		return err;
+	}
+
+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/
+- https://lore.kernel.org/bpf/20200218171023.844439-7-jakub@cloudflare.com/
+
+.. _`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

Re: [PATCH bpf-next v5 1/1] docs: BPF_MAP_TYPE_SOCK[MAP|HASH]

[David Vernet]

On Fri, Dec 16, 2022 at 10:01:35AM +0000, mtahhan@redhat.com wrote:
> 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>

Hi Maryam,

FYI I think you're missing a few tags? You can add my Acked-by, and I
believe that John and Bagas acked and reviewed it respectively in on the
v3 [0] set as well.

Thanks,
David

[0]: https://lore.kernel.org/bpf/Y5EOC7HjtaRFAVfq@maniforge.lan/T/#m247ac7e1673731a9cf8b06d5ce027a1acbcce953

RE: [PATCH bpf-next v5 1/1] docs: BPF_MAP_TYPE_SOCK[MAP|HASH]

[Dave Thaler]

mtahhan@redhat.com wrote:
[...]
> +A sock object may be in multiple maps, but can only inherit a single 
> +parse or verdict program. If adding a sock object to a map would result 
> +in having multiple parsing programs the update will return an EBUSY error.

s/parsing programs/parser programs/
for consistency with rest of the patch.

> +.. note::
> +    For more details of the socket callbacks that get replaced please see
> +    ``net/ipv4/tcp_bpf.c`` and ``net/ipv4/udp_bpf.c`` for TCP and UDP
> +    functions, respectively.

Why this note?  The rest of the text looks to be usable cross-platform
but the note above implies that this documentation is lacking and the reader
has to consult the Linux source code.  Can more be documented
in the doc instead of just in the code?

> +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``.

s/start and end pointer/start and end pointers/
(grammar)

> +copied if necessary (i.e. if data was not linear and if start and end 

Nit: style manuals such as Chicago Manual of Style say to always put a comma
after "i.e." (and "e.g.").

> +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.

Grammar nit:
s/Lookup/Look up/
("look up" is a verb, whereas "lookup" is a noun)

-Dave

RE: [PATCH bpf-next v5 1/1] docs: BPF_MAP_TYPE_SOCK[MAP|HASH]

[Dave Thaler]

Maryam Tahhan <mtahhan@redhat.com>  writes:
>> +.. note::
>> +    For more details of the socket callbacks that get replaced please see
>> +    ``net/ipv4/tcp_bpf.c`` and ``net/ipv4/udp_bpf.c`` for TCP and UDP
>> +    functions, respectively.
>>
>> Why this note?  The rest of the text looks to be usable cross-platform
>> but the note above implies that this documentation is lacking and the reader
>> has to consult the Linux source code.  Can more be documented
>> in the doc instead of just in the code?
> 
> The note is just a pointer to where folks can find these functions easily. I see
> lots of places in documentation where we make notes like these. Those files are 
> essentially the callback implementations, unless we think we need to document
> each 
> callback here (which for me seems like overkill for the map documentation), I can
> either remove the note or make it seem more like a pointer?

I have no preference, but my intent is that eventually cross-platform map types
will be added to standard documentation and that documentation won't have
such a note but would need to incorporate anything that's really cross-platform into the documentation itself.   Since most socket calls are POSIX standard APIs,
any callbacks that can be described by referencing such standard APIs could be
listed in cross-platform docs.

Dave