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 should never be.

This patch adds a bloom filter map for bpf programs.
The bloom filter map supports peek (determining whether an element
is present in the map) and push (adding an element to the map)
operations.These operations are exposed to userspace applications
through the already existing syscalls in the following way:

BPF_MAP_LOOKUP_ELEM -> peek
BPF_MAP_UPDATE_ELEM -> push

The bloom filter map does not have keys, only values. In light of
this, the bloom filter map's API matches that of queue stack maps:
user applications use BPF_MAP_LOOKUP_ELEM/BPF_MAP_UPDATE_ELEM
which correspond internally to bpf_map_peek_elem/bpf_map_push_elem,
and bpf programs must use the bpf_map_peek_elem and bpf_map_push_elem
APIs to query or add an element to the bloom filter map. When the
bloom filter map is created, it must be created with a key_size of 0.

For updates, the user will pass in the element to add to the map
as the value, with a NULL key. For lookups, the user will pass in the
element to query in the map as the value. In the verifier layer, this
requires us to modify the argument type of a bloom filter's
BPF_FUNC_map_peek_elem call to ARG_PTR_TO_MAP_VALUE; as well, in
the syscall layer, we need to copy over the user value so that in
bpf_map_peek_elem, we know which specific value to query.

A few things to please take note of:
 * If there are any concurrent lookups + updates, the user is
responsible for synchronizing this to ensure no false negative lookups
occur.
 * The number of hashes to use for the bloom filter is configurable from
userspace. If no number is specified, the default used will be 5 hash
functions. The benchmarks later in this patchset can help compare the
performance of using different number of hashes on different entry
sizes. In general, using more hashes decreases the speed of a lookup,
but increases the false positive rate of an element being detected in the
bloom filter.
 * Deleting an element in the bloom filter map is not supported.
 * The bloom filter map may be used as an inner map.
 * 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", they may do so but they should be
aware that this may lead to a higher false positive rate.

Signed-off-by: Joanne Koong <redacted>
---
 include/linux/bpf_types.h      |   1 +
 include/uapi/linux/bpf.h       |  10 ++
 kernel/bpf/Makefile            |   2 +-
 kernel/bpf/bloom_filter.c      | 205 +++++++++++++++++++++++++++++++++
 kernel/bpf/syscall.c           |  14 ++-
 kernel/bpf/verifier.c          |  19 ++-
 tools/include/uapi/linux/bpf.h |  10 ++
 7 files changed, 255 insertions(+), 6 deletions(-)
 create mode 100644 kernel/bpf/bloom_filter.c

diff --git a/include/linux/bpf_types.h b/include/linux/bpf_types.h
index 9c81724e4b98..c4424ac2fa02 100644
--- a/include/linux/bpf_types.h
+++ b/include/linux/bpf_types.h

@@ -125,6 +125,7 @@ BPF_MAP_TYPE(BPF_MAP_TYPE_STACK, stack_map_ops)
 BPF_MAP_TYPE(BPF_MAP_TYPE_STRUCT_OPS, bpf_struct_ops_map_ops)
 #endif
 BPF_MAP_TYPE(BPF_MAP_TYPE_RINGBUF, ringbuf_map_ops)
+BPF_MAP_TYPE(BPF_MAP_TYPE_BLOOM_FILTER, bloom_filter_map_ops)

 BPF_LINK_TYPE(BPF_LINK_TYPE_RAW_TRACEPOINT, raw_tracepoint)
 BPF_LINK_TYPE(BPF_LINK_TYPE_TRACING, tracing)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 791f31dd0abe..1d82860fd98e 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -906,6 +906,7 @@ enum bpf_map_type {
        BPF_MAP_TYPE_RINGBUF,
        BPF_MAP_TYPE_INODE_STORAGE,
        BPF_MAP_TYPE_TASK_STORAGE,
+       BPF_MAP_TYPE_BLOOM_FILTER,
 };

 /* Note that tracing related programs such as

@@ -1210,6 +1211,15 @@ enum {

 /* Create a map that is suitable to be an inner map with dynamic max entries */
        BPF_F_INNER_MAP         = (1U << 12),
+
+/* For bloom filter maps, the next 4 bits represent how many hashes to use.
+ * The maximum number of hash functions supported is 15. If this is not set,
+ * the default number of hash functions used will be 5.
+ */
+       BPF_F_BLOOM_FILTER_HASH_BIT_1 = (1U << 13),
+       BPF_F_BLOOM_FILTER_HASH_BIT_2 = (1U << 14),
+       BPF_F_BLOOM_FILTER_HASH_BIT_3 = (1U << 15),
+       BPF_F_BLOOM_FILTER_HASH_BIT_4 = (1U << 16),