| .. 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/ |