Documentation/bpf: Document CGROUP_STORAGE map type

   bpf_lsm

Map types

=========

.. toctree::

   :maxdepth: 1

   map_cgroup_storage

Testing and debugging BPF

=========================

.. SPDX-License-Identifier: GPL-2.0-only

.. Copyright (C) 2020 Google LLC.

===========================

BPF_MAP_TYPE_CGROUP_STORAGE

===========================

The ``BPF_MAP_TYPE_CGROUP_STORAGE`` map type represents a local fix-sized

storage. It is only available with ``CONFIG_CGROUP_BPF``, and to programs that

attach to cgroups; the programs are made available by the same Kconfig. The

storage is identified by the cgroup the program is attached to.

The map provide a local storage at the cgroup that the BPF program is attached

to. It provides a faster and simpler access than the general purpose hash

table, which performs a hash table lookups, and requires user to track live

cgroups on their own.

This document describes the usage and semantics of the

``BPF_MAP_TYPE_CGROUP_STORAGE`` map type. Some of its behaviors was changed in

Linux 5.9 and this document will describe the differences.

Usage

=====

The map uses key of type of either ``__u64 cgroup_inode_id`` or

``struct bpf_cgroup_storage_key``, declared in ``linux/bpf.h``::

    struct bpf_cgroup_storage_key {

            __u64 cgroup_inode_id;

            __u32 attach_type;

    };

``cgroup_inode_id`` is the inode id of the cgroup directory.

``attach_type`` is the the program's attach type.

Linux 5.9 added support for type ``__u64 cgroup_inode_id`` as the key type.

When this key type is used, then all attach types of the particular cgroup and

map will share the same storage. Otherwise, if the type is

``struct bpf_cgroup_storage_key``, then programs of different attach types

be isolated and see different storages.

To access the storage in a program, use ``bpf_get_local_storage``::

    void *bpf_get_local_storage(void *map, u64 flags)

``flags`` is reserved for future use and must be 0.

There is no implicit synchronization. Storages of ``BPF_MAP_TYPE_CGROUP_STORAGE``

can be accessed by multiple programs across different CPUs, and user should

take care of synchronization by themselves. The bpf infrastructure provides

``struct bpf_spin_lock`` to synchronize the storage. See

``tools/testing/selftests/bpf/progs/test_spin_lock.c``.

Examples

========

Usage with key type as ``struct bpf_cgroup_storage_key``::

    #include <bpf/bpf.h>

    struct {

            __uint(type, BPF_MAP_TYPE_CGROUP_STORAGE);

            __type(key, struct bpf_cgroup_storage_key);

            __type(value, __u32);

    } cgroup_storage SEC(".maps");

    int program(struct __sk_buff *skb)

    {

            __u32 *ptr = bpf_get_local_storage(&cgroup_storage, 0);

            __sync_fetch_and_add(ptr, 1);

            return 0;

    }

Userspace accessing map declared above::

    #include <linux/bpf.h>

    #include <linux/libbpf.h>

    __u32 map_lookup(struct bpf_map *map, __u64 cgrp, enum bpf_attach_type type)

    {

            struct bpf_cgroup_storage_key = {

                    .cgroup_inode_id = cgrp,

                    .attach_type = type,

            };

            __u32 value;

            bpf_map_lookup_elem(bpf_map__fd(map), &key, &value);

            // error checking omitted

            return value;

    }

Alternatively, using just ``__u64 cgroup_inode_id`` as key type::

    #include <bpf/bpf.h>

    struct {

            __uint(type, BPF_MAP_TYPE_CGROUP_STORAGE);

            __type(key, __u64);

            __type(value, __u32);

    } cgroup_storage SEC(".maps");

    int program(struct __sk_buff *skb)

    {

            __u32 *ptr = bpf_get_local_storage(&cgroup_storage, 0);

            __sync_fetch_and_add(ptr, 1);

            return 0;

    }

And userspace::

    #include <linux/bpf.h>

    #include <linux/libbpf.h>

    __u32 map_lookup(struct bpf_map *map, __u64 cgrp, enum bpf_attach_type type)

    {

            __u32 value;

            bpf_map_lookup_elem(bpf_map__fd(map), &cgrp, &value);

            // error checking omitted

            return value;

    }

Semantics

=========

``BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE`` is a variant of this map type. This

per-CPU variant will have different memory regions for each CPU for each

storage. The non-per-CPU will have the same memory region for each storage.

Prior to Linux 5.9, the lifetime of a storage is precisely per-attachment, and

for a single ``CGROUP_STORAGE`` map, there can be at most one program loaded

that uses the map. A program may be attached to multiple cgroups or have

multiple attach types, and each attach creates a fresh zeroed storage. The

storage is freed upon detach.

There is a one-to-one association between the map of each type (per-CPU and

non-per-CPU) and the BPF program during load verification time. As a result,

each map can only be used by one BPF program and each BPF program can only use

one storage map of each type. Because of map can only be used by one BPF

program, sharing of this cgroup's storage with other BPF programs were

impossible.

Since Linux 5.9, storage can be shared by multiple programs. When a program is

attached to a cgroup, the kernel would create a new storage only if the map

does not already contain an entry for the cgroup and attach type pair, or else

the old storage is reused for the new attachment. If the map is attach type

shared, then attach type is simply ignored during comparison. Storage is freed

only when either the map or the cgroup attached to is being freed. Detaching

will not directly free the storage, but it may cause the reference to the map

to reach zero and indirectly freeing all storage in the map.

The map is not associated with any BPF program, thus making sharing possible.

However, the BPF program can still only associate with one map of each type

(per-CPU and non-per-CPU). A BPF program cannot use more than one

``BPF_MAP_TYPE_CGROUP_STORAGE`` or more than one

``BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE``.

In all versions, userspace may use the the attach parameters of cgroup and

attach type pair in ``struct bpf_cgroup_storage_key`` as the key to the BPF map

APIs to read or update the storage for a given attachment. For Linux 5.9

attach type shared storages, only the first value in the struct, cgroup inode

id, is used during comparison, so userspace may just specify a ``__u64``

directly.

The storage is bound at attach time. Even if the program is attached to parent

and triggers in child, the storage still belongs to the parent.

Userspace cannot create a new entry in the map or delete an existing entry.

Program test runs always use a temporary storage.