Map type BPF_MAP_TYPE_BLOOM_FILTER
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.
Usage
The primary use-case for bloom filters is to optimize query performance for expensive lookups into data structures. To do so you would for every lookup first query the bloom filter, if it returns false, then there is no need to do a more expensive lookup in the actual data structure since we know with 100% confidence that the key will not be present. If the returns is true however, we still need to do the lookup since the key may or may not be present.
Due to its implementation a bloom filter does not take up much memory compared to other structures and has a constant time complexity (O(1)).
Implementation
A bloom filter is a bit-field (array of bits). When querying the bloom filter we take our key and hash it so the result is an index into the bit-field. The result the value of the bit, 1 or 0. When writing a value we use the same hash and set the bit to 1. This deliberately causes hash collisions. So when the result of a query is 0, you know 100% that the key was never written to the bloom filter. But when the result is a 1, that may be because the key was written or due to hash collision.
In practice we use multiple hash methods for example 5, and see if all 5 indices are 1 to decrease false positives without increasing the size of the bitfield.
Limitations
Bloom filters need to be sized proportionally to the amount of keys to be inserted. The usefulness of bloom filters diminish the more saturated they become.
Keys can't be deleted from bloom filters, and thus are unsuited for scenarios where keys are frequently removed from a data structure. (bloom filters can, however, be reset and repopulated from the data structure, but this is expensive).
Attributes
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 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.
Syscall commands
The following syscall commands work with this map type:
Note
The BPF_MAP_LOOKUP_ELEM syscall command acts as peek, and BPF_MAP_UPDATE_ELEM as push.
Helper functions
The bloom filter map supports two operations:
- bpf_map_peek_elem: adding an element to the map
- bpf_map_push_elem: determining whether an element is present in the map
Flags
BPF_F_NUMA_NODE
When set, the numa_node attribute is respected during map creation.
BPF_F_RDONLY
Setting this flag will make it so the map can only be read via the syscall interface, but not written to.
For details please check the generic description.
BPF_F_WRONLY
Setting this flag will make it so the map can only be written to via the syscall interface, but not read from.
BPF_F_ZERO_SEED
Setting this flag will initialize the hash table with a seed of 0.
The hashing algorithm used by the hash table is seeded with a random number by default. This seeding is meant as a mitigation against Denial of Service attacks which could exploit the predictability of hashing implementations.
This random seed makes hash map operations inherently random in access time. This flag was introduced to make performance evaluation more consistent.
Warning
It is not recommended to use this flag in production due to the vulnerability to Denial of Service attacks.
Info
Only users with the CAP_SYS_ADMIN capability can use this flag, CAP_BPF is not enough due to the security risk associated with the flag.
BPF_F_RDONLY_PROG
Setting this flag will make it so the map can only be read via helper functions, but not written to.
For details please check the generic description.
BPF_F_WRONLY_PROG
Setting this flag will make it so the map can only be written to via helper functions, but not read from.
For details please check the generic description.