========================================= BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK

.. note::

BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK were introduced in kernel version 4.20

BPF_MAP_TYPE_QUEUE provides FIFO storage and BPF_MAP_TYPE_STACK provides LIFO storage for BPF programs. These maps support peek, pop and push operations that are exposed to BPF programs through the respective helpers. These operations are exposed to userspace applications using the existing bpf syscall in the following way:

BPF_MAP_LOOKUP_ELEM -> peek
BPF_MAP_LOOKUP_AND_DELETE_ELEM -> pop
BPF_MAP_UPDATE_ELEM -> push

BPF_MAP_TYPE_QUEUE and BPF_MAP_TYPE_STACK do not support BPF_F_NO_PREALLOC.

Usage

Kernel BPF

bpf_map_push_elem()


.. code-block:: c

   long bpf_map_push_elem(struct bpf_map *map, const void *value, u64 flags)

An element ``value`` can be added to a queue or stack using the
``bpf_map_push_elem`` helper. The ``flags`` parameter must be set to
``BPF_ANY`` or ``BPF_EXIST``. If ``flags`` is set to ``BPF_EXIST`` then,
when the queue or stack is full, the oldest element will be removed to
make room for ``value`` to be added. 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)

This helper fetches an element value from a queue or stack without removing it. Returns 0 on success, or negative error in case of failure.

bpf_map_pop_elem()


.. code-block:: c

   long bpf_map_pop_elem(struct bpf_map *map, void *value)

This helper removes an element into ``value`` from a queue or
stack. Returns ``0`` on success, or negative error in case of failure.


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 push value onto a queue or stack using libbpf's bpf_map_update_elem function. The key parameter must be set to NULL and flags must be set to BPF_ANY or BPF_EXIST, with the same semantics as the bpf_map_push_elem kernel helper. 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 peek at the ``value`` at the head of a queue or stack
using the libbpf ``bpf_map_lookup_elem`` function. The ``key`` parameter must be
set to ``NULL``.  Returns ``0`` on success, or negative error in case of
failure.

bpf_map_lookup_and_delete_elem()

.. code-block:: c

int bpf_map_lookup_and_delete_elem (int fd, const void *key, void *value)

A userspace program can pop a value from the head of a queue or stack using the libbpf bpf_map_lookup_and_delete_elem function. The key parameter must be set to NULL. Returns 0 on success, or negative error in case of failure.

Examples

Kernel BPF

This snippet shows how to declare a queue in a BPF program:

.. code-block:: c

struct {
        __uint(type, BPF_MAP_TYPE_QUEUE);
        __type(value, __u32);
        __uint(max_entries, 10);
} queue SEC(".maps");

Userspace

This snippet shows how to use libbpf's low-level API to create a queue from userspace:

.. code-block:: c

int create_queue()
{
        return bpf_map_create(BPF_MAP_TYPE_QUEUE,
                              "sample_queue", /* name */
                              0,              /* key size, must be zero */
                              sizeof(__u32),  /* value size */
                              10,             /* max entries */
                              NULL);          /* create options */
}

References

https://lwn.net/ml/netdev/153986858555.9127.14517764371945179514.stgit@kernel/