.. 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 primarilyused 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 asthe 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 ofdevmap; only the lookup and insertion is different. Usage=====Kernel BPF----------bpf_redirect_map()^^^^^^^^^^^^^^^^^^.. code-block:: c     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 containsreferences to net devices (for forwarding packets through other ports). The lower two bits of *flags* are used as the return code if the map lookupfails. This is so that the return value can be one of the XDP program returncodes 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 definedbelow. With ``BPF_F_BROADCAST`` the packet will be broadcast to all the interfacesin the map, with ``BPF_F_EXCLUDE_INGRESS`` the ingress interface will be excludedfrom 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 twolower bits of the ``flags`` argument if the map lookup fails. More information about redirection can be found :doc:`redirect` bpf_map_lookup_elem()^^^^^^^^^^^^^^^^^^^^^.. code-block:: c    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. User space----------.. 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. bpf_map_update_elem()^^^^^^^^^^^^^^^^^^^^^.. code-block:: c    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`` parametercan be ``struct bpf_devmap_val`` or a simple ``int ifindex`` for backwardscompatibility.  .. 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 haveaccess 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. Examplesof 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`` bpf_map_lookup_elem()^^^^^^^^^^^^^^^^^^^^^.. code-block:: 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. bpf_map_delete_elem()^^^^^^^^^^^^^^^^^^^^^.. code-block:: c .. 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 offailure. 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 programwould work with a user space program that populates the devmap ``forward_map`` basedon ingress ifindexes. The BPF program (below) is redirecting packets using theingress ``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 toall 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