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diff --git a/Documentation/bpf/map_bloom_filter.rst b/Documentation/bpf/map_bloom_filter.rst new file mode 100644 index 0000000000..c82487f2fe --- /dev/null +++ b/Documentation/bpf/map_bloom_filter.rst @@ -0,0 +1,174 @@ +.. SPDX-License-Identifier: GPL-2.0-only +.. Copyright (C) 2022 Red Hat, Inc. + +========================= +BPF_MAP_TYPE_BLOOM_FILTER +========================= + +.. note:: + - ``BPF_MAP_TYPE_BLOOM_FILTER`` was introduced in kernel version 5.16 + +``BPF_MAP_TYPE_BLOOM_FILTER`` provides a BPF bloom filter map. Bloom +filters are a space-efficient probabilistic data structure used to +quickly test whether an element exists in a set. In a bloom filter, +false positives are possible whereas false negatives are not. + +The bloom filter map does not have keys, only values. When the bloom +filter map is created, it must be created with a ``key_size`` of 0. The +bloom filter map supports two operations: + +- push: adding an element to the map +- peek: determining whether an element is present in the map + +BPF programs must use ``bpf_map_push_elem`` to add an element to the +bloom filter map and ``bpf_map_peek_elem`` to query the map. These +operations are exposed to userspace applications using the existing +``bpf`` syscall in the following way: + +- ``BPF_MAP_UPDATE_ELEM`` -> push +- ``BPF_MAP_LOOKUP_ELEM`` -> peek + +The ``max_entries`` size that is specified at map creation time is used +to approximate a reasonable bitmap size for the bloom filter, and is not +otherwise strictly enforced. If the user wishes to insert more entries +into the bloom filter than ``max_entries``, this may lead to a higher +false positive rate. + +The number of hashes to use for the bloom filter is configurable using +the lower 4 bits of ``map_extra`` in ``union bpf_attr`` at map creation +time. If no number is specified, the default used will be 5 hash +functions. In general, using more hashes decreases both the false +positive rate and the speed of a lookup. + +It is not possible to delete elements from a bloom filter map. A bloom +filter map may be used as an inner map. The user is responsible for +synchronising concurrent updates and lookups to ensure no false negative +lookups occur. + +Usage +===== + +Kernel BPF +---------- + +bpf_map_push_elem() +~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + long bpf_map_push_elem(struct bpf_map *map, const void *value, u64 flags) + +A ``value`` can be added to a bloom filter using the +``bpf_map_push_elem()`` helper. The ``flags`` parameter must be set to +``BPF_ANY`` when adding an entry to the bloom filter. This helper +returns ``0`` on success, or negative error in case of failure. + +bpf_map_peek_elem() +~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + long bpf_map_peek_elem(struct bpf_map *map, void *value) + +The ``bpf_map_peek_elem()`` helper is used to determine whether +``value`` is present in the bloom filter map. This helper returns ``0`` +if ``value`` is probably present in the map, or ``-ENOENT`` if ``value`` +is definitely not present in the map. + +Userspace +--------- + +bpf_map_update_elem() +~~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + int bpf_map_update_elem (int fd, const void *key, const void *value, __u64 flags) + +A userspace program can add a ``value`` to a bloom filter using libbpf's +``bpf_map_update_elem`` function. The ``key`` parameter must be set to +``NULL`` and ``flags`` must be set to ``BPF_ANY``. Returns ``0`` on +success, or negative error in case of failure. + +bpf_map_lookup_elem() +~~~~~~~~~~~~~~~~~~~~~ + +.. code-block:: c + + int bpf_map_lookup_elem (int fd, const void *key, void *value) + +A userspace program can determine the presence of ``value`` in a bloom +filter using libbpf's ``bpf_map_lookup_elem`` function. The ``key`` +parameter must be set to ``NULL``. Returns ``0`` if ``value`` is +probably present in the map, or ``-ENOENT`` if ``value`` is definitely +not present in the map. + +Examples +======== + +Kernel BPF +---------- + +This snippet shows how to declare a bloom filter in a BPF program: + +.. code-block:: c + + struct { + __uint(type, BPF_MAP_TYPE_BLOOM_FILTER); + __type(value, __u32); + __uint(max_entries, 1000); + __uint(map_extra, 3); + } bloom_filter SEC(".maps"); + +This snippet shows how to determine presence of a value in a bloom +filter in a BPF program: + +.. code-block:: c + + void *lookup(__u32 key) + { + if (bpf_map_peek_elem(&bloom_filter, &key) == 0) { + /* Verify not a false positive and fetch an associated + * value using a secondary lookup, e.g. in a hash table + */ + return bpf_map_lookup_elem(&hash_table, &key); + } + return 0; + } + +Userspace +--------- + +This snippet shows how to use libbpf to create a bloom filter map from +userspace: + +.. code-block:: c + + int create_bloom() + { + LIBBPF_OPTS(bpf_map_create_opts, opts, + .map_extra = 3); /* number of hashes */ + + return bpf_map_create(BPF_MAP_TYPE_BLOOM_FILTER, + "ipv6_bloom", /* name */ + 0, /* key size, must be zero */ + sizeof(ipv6_addr), /* value size */ + 10000, /* max entries */ + &opts); /* create options */ + } + +This snippet shows how to add an element to a bloom filter from +userspace: + +.. code-block:: c + + int add_element(struct bpf_map *bloom_map, __u32 value) + { + int bloom_fd = bpf_map__fd(bloom_map); + return bpf_map_update_elem(bloom_fd, NULL, &value, BPF_ANY); + } + +References +========== + +https://lwn.net/ml/bpf/20210831225005.2762202-1-joannekoong@fb.com/ |