bpf, docs: DEVMAPs and XDP_REDIRECT

Add documentation for BPF_MAP_TYPE_DEVMAP and BPF_MAP_TYPE_DEVMAP_HASH
including kernel version introduced, usage and examples.

Add documentation that describes XDP_REDIRECT.

Signed-off-by: Maryam Tahhan <mtahhan@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221115144921.165483-1-mtahhan@redhat.com

1 files changed, 222 insertions, 0 deletions

diff --git a/Documentation/bpf/map_devmap.rst b/Documentation/bpf/map_devmap.rst
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+.. SPDX-License-Identifier: GPL-2.0-only
+.. Copyright (C) 2022 Red Hat, Inc.
+
+=================================================
+BPF_MAP_TYPE_DEVMAP and BPF_MAP_TYPE_DEVMAP_HASH
+=================================================
+
+.. note::
+   - ``BPF_MAP_TYPE_DEVMAP`` was introduced in kernel version 4.14
+   - ``BPF_MAP_TYPE_DEVMAP_HASH`` was introduced in kernel version 5.4
+
+``BPF_MAP_TYPE_DEVMAP`` and ``BPF_MAP_TYPE_DEVMAP_HASH`` are BPF maps primarily
+used as backend maps for the XDP BPF helper call ``bpf_redirect_map()``.
+``BPF_MAP_TYPE_DEVMAP`` is backed by an array that uses the key as
+the index to lookup a reference to a net device. While ``BPF_MAP_TYPE_DEVMAP_HASH``
+is backed by a hash table that uses a key to lookup a reference to a net device.
+The user provides either <``key``/ ``ifindex``> or <``key``/ ``struct bpf_devmap_val``>
+pairs to update the maps with new net devices.
+
+.. note::
+    - The key to a hash map doesn't have to be an ``ifindex``.
+    - While ``BPF_MAP_TYPE_DEVMAP_HASH`` allows for densely packing the net devices
+      it comes at the cost of a hash of the key when performing a look up.
+
+The setup and packet enqueue/send code is shared between the two types of
+devmap; only the lookup and insertion is different.
+
+Usage
+=====
+Kernel BPF
+----------
+.. c:function::
+     long bpf_redirect_map(struct bpf_map *map, u32 key, u64 flags)
+
+Redirect the packet to the endpoint referenced by ``map`` at index ``key``.
+For ``BPF_MAP_TYPE_DEVMAP`` and ``BPF_MAP_TYPE_DEVMAP_HASH`` this map contains
+references to net devices (for forwarding packets through other ports).
+
+The lower two bits of *flags* are used as the return code if the map lookup
+fails. This is so that the return value can be one of the XDP program return
+codes up to ``XDP_TX``, as chosen by the caller. The higher bits of ``flags``
+can be set to ``BPF_F_BROADCAST`` or ``BPF_F_EXCLUDE_INGRESS`` as defined
+below.
+
+With ``BPF_F_BROADCAST`` the packet will be broadcast to all the interfaces
+in the map, with ``BPF_F_EXCLUDE_INGRESS`` the ingress interface will be excluded
+from the broadcast.
+
+.. note::
+    - The key is ignored if BPF_F_BROADCAST is set.
+    - The broadcast feature can also be used to implement multicast forwarding:
+      simply create multiple DEVMAPs, each one corresponding to a single multicast group.
+
+This helper will return ``XDP_REDIRECT`` on success, or the value of the two
+lower bits of the ``flags`` argument if the map lookup fails.
+
+More information about redirection can be found :doc:`redirect`
+
+.. c:function::
+   void *bpf_map_lookup_elem(struct bpf_map *map, const void *key)
+
+Net device entries can be retrieved using the ``bpf_map_lookup_elem()``
+helper.
+
+Userspace
+---------
+.. note::
+    DEVMAP entries can only be updated/deleted from user space and not
+    from an eBPF program. Trying to call these functions from a kernel eBPF
+    program will result in the program failing to load and a verifier warning.
+
+.. c:function::
+   int bpf_map_update_elem(int fd, const void *key, const void *value, __u64 flags);
+
+ Net device entries can be added or updated using the ``bpf_map_update_elem()``
+ helper. This helper replaces existing elements atomically. The ``value`` parameter
+ can be ``struct bpf_devmap_val`` or a simple ``int ifindex`` for backwards
+ compatibility.
+
+ .. code-block:: c
+
+    struct bpf_devmap_val {
+        __u32 ifindex;   /* device index */
+        union {
+            int   fd;  /* prog fd on map write */
+            __u32 id;  /* prog id on map read */
+        } bpf_prog;
+    };
+
+ 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.
+
+ DEVMAPs can associate a program with a device entry by adding a ``bpf_prog.fd``
+ to ``struct bpf_devmap_val``. Programs are run after ``XDP_REDIRECT`` and have
+ access to both Rx device and Tx device. The  program associated with the ``fd``
+ must have type XDP with expected attach type ``xdp_devmap``.
+ When a program is associated with a device index, the program is run on an
+ ``XDP_REDIRECT`` and before the buffer is added to the per-cpu queue. Examples
+ of how to attach/use xdp_devmap progs can be found in the kernel selftests:
+
+ - ``tools/testing/selftests/bpf/prog_tests/xdp_devmap_attach.c``
+ - ``tools/testing/selftests/bpf/progs/test_xdp_with_devmap_helpers.c``
+
+.. c:function::
+   int bpf_map_lookup_elem(int fd, const void *key, void *value);
+
+ Net device entries can be retrieved using the ``bpf_map_lookup_elem()``
+ helper.
+
+.. c:function::
+   int bpf_map_delete_elem(int fd, const void *key);
+
+ Net device entries can be deleted using the ``bpf_map_delete_elem()``
+ helper. This helper will return 0 on success, or negative error in case of
+ failure.
+
+Examples
+========
+
+Kernel BPF
+----------
+
+The following code snippet shows how to declare a ``BPF_MAP_TYPE_DEVMAP``
+called tx_port.
+
+.. code-block:: c
+
+    struct {
+        __uint(type, BPF_MAP_TYPE_DEVMAP);
+        __type(key, __u32);
+        __type(value, __u32);
+        __uint(max_entries, 256);
+    } tx_port SEC(".maps");
+
+The following code snippet shows how to declare a ``BPF_MAP_TYPE_DEVMAP_HASH``
+called forward_map.
+
+.. code-block:: c
+
+    struct {
+        __uint(type, BPF_MAP_TYPE_DEVMAP_HASH);
+        __type(key, __u32);
+        __type(value, struct bpf_devmap_val);
+        __uint(max_entries, 32);
+    } forward_map SEC(".maps");
+
+.. note::
+
+    The value type in the DEVMAP above is a ``struct bpf_devmap_val``
+
+The following code snippet shows a simple xdp_redirect_map program. This program
+would work with a user space program that populates the devmap ``forward_map`` based
+on ingress ifindexes. The BPF program (below) is redirecting packets using the
+ingress ``ifindex`` as the ``key``.
+
+.. code-block:: c
+
+    SEC("xdp")
+    int xdp_redirect_map_func(struct xdp_md *ctx)
+    {
+        int index = ctx->ingress_ifindex;
+
+        return bpf_redirect_map(&forward_map, index, 0);
+    }
+
+The following code snippet shows a BPF program that is broadcasting packets to
+all the interfaces in the ``tx_port`` devmap.
+
+.. code-block:: c
+
+    SEC("xdp")
+    int xdp_redirect_map_func(struct xdp_md *ctx)
+    {
+        return bpf_redirect_map(&tx_port, 0, BPF_F_BROADCAST | BPF_F_EXCLUDE_INGRESS);
+    }
+
+User space
+----------
+
+The following code snippet shows how to update a devmap called ``tx_port``.
+
+.. code-block:: c
+
+    int update_devmap(int ifindex, int redirect_ifindex)
+    {
+        int ret;
+
+        ret = bpf_map_update_elem(bpf_map__fd(tx_port), &ifindex, &redirect_ifindex, 0);
+        if (ret < 0) {
+            fprintf(stderr, "Failed to update devmap_ value: %s\n",
+                strerror(errno));
+        }
+
+        return ret;
+    }
+
+The following code snippet shows how to update a hash_devmap called ``forward_map``.
+
+.. code-block:: c
+
+    int update_devmap(int ifindex, int redirect_ifindex)
+    {
+        struct bpf_devmap_val devmap_val = { .ifindex = redirect_ifindex };
+        int ret;
+
+        ret = bpf_map_update_elem(bpf_map__fd(forward_map), &ifindex, &devmap_val, 0);
+        if (ret < 0) {
+            fprintf(stderr, "Failed to update devmap_ value: %s\n",
+                strerror(errno));
+        }
+        return ret;
+    }
+
+References
+===========
+
+- https://lwn.net/Articles/728146/
+- https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/commit/?id=6f9d451ab1a33728adb72d7ff66a7b374d665176
+- https://elixir.bootlin.com/linux/latest/source/net/core/filter.c#L4106