linux/Documentation/bpf/map_cgroup_storage.rst
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   1.. SPDX-License-Identifier: GPL-2.0-only
   2.. Copyright (C) 2020 Google LLC.
   3
   4===========================
   5BPF_MAP_TYPE_CGROUP_STORAGE
   6===========================
   7
   8The ``BPF_MAP_TYPE_CGROUP_STORAGE`` map type represents a local fix-sized
   9storage. It is only available with ``CONFIG_CGROUP_BPF``, and to programs that
  10attach to cgroups; the programs are made available by the same Kconfig. The
  11storage is identified by the cgroup the program is attached to.
  12
  13The map provide a local storage at the cgroup that the BPF program is attached
  14to. It provides a faster and simpler access than the general purpose hash
  15table, which performs a hash table lookups, and requires user to track live
  16cgroups on their own.
  17
  18This document describes the usage and semantics of the
  19``BPF_MAP_TYPE_CGROUP_STORAGE`` map type. Some of its behaviors was changed in
  20Linux 5.9 and this document will describe the differences.
  21
  22Usage
  23=====
  24
  25The map uses key of type of either ``__u64 cgroup_inode_id`` or
  26``struct bpf_cgroup_storage_key``, declared in ``linux/bpf.h``::
  27
  28    struct bpf_cgroup_storage_key {
  29            __u64 cgroup_inode_id;
  30            __u32 attach_type;
  31    };
  32
  33``cgroup_inode_id`` is the inode id of the cgroup directory.
  34``attach_type`` is the the program's attach type.
  35
  36Linux 5.9 added support for type ``__u64 cgroup_inode_id`` as the key type.
  37When this key type is used, then all attach types of the particular cgroup and
  38map will share the same storage. Otherwise, if the type is
  39``struct bpf_cgroup_storage_key``, then programs of different attach types
  40be isolated and see different storages.
  41
  42To access the storage in a program, use ``bpf_get_local_storage``::
  43
  44    void *bpf_get_local_storage(void *map, u64 flags)
  45
  46``flags`` is reserved for future use and must be 0.
  47
  48There is no implicit synchronization. Storages of ``BPF_MAP_TYPE_CGROUP_STORAGE``
  49can be accessed by multiple programs across different CPUs, and user should
  50take care of synchronization by themselves. The bpf infrastructure provides
  51``struct bpf_spin_lock`` to synchronize the storage. See
  52``tools/testing/selftests/bpf/progs/test_spin_lock.c``.
  53
  54Examples
  55========
  56
  57Usage with key type as ``struct bpf_cgroup_storage_key``::
  58
  59    #include <bpf/bpf.h>
  60
  61    struct {
  62            __uint(type, BPF_MAP_TYPE_CGROUP_STORAGE);
  63            __type(key, struct bpf_cgroup_storage_key);
  64            __type(value, __u32);
  65    } cgroup_storage SEC(".maps");
  66
  67    int program(struct __sk_buff *skb)
  68    {
  69            __u32 *ptr = bpf_get_local_storage(&cgroup_storage, 0);
  70            __sync_fetch_and_add(ptr, 1);
  71
  72            return 0;
  73    }
  74
  75Userspace accessing map declared above::
  76
  77    #include <linux/bpf.h>
  78    #include <linux/libbpf.h>
  79
  80    __u32 map_lookup(struct bpf_map *map, __u64 cgrp, enum bpf_attach_type type)
  81    {
  82            struct bpf_cgroup_storage_key = {
  83                    .cgroup_inode_id = cgrp,
  84                    .attach_type = type,
  85            };
  86            __u32 value;
  87            bpf_map_lookup_elem(bpf_map__fd(map), &key, &value);
  88            // error checking omitted
  89            return value;
  90    }
  91
  92Alternatively, using just ``__u64 cgroup_inode_id`` as key type::
  93
  94    #include <bpf/bpf.h>
  95
  96    struct {
  97            __uint(type, BPF_MAP_TYPE_CGROUP_STORAGE);
  98            __type(key, __u64);
  99            __type(value, __u32);
 100    } cgroup_storage SEC(".maps");
 101
 102    int program(struct __sk_buff *skb)
 103    {
 104            __u32 *ptr = bpf_get_local_storage(&cgroup_storage, 0);
 105            __sync_fetch_and_add(ptr, 1);
 106
 107            return 0;
 108    }
 109
 110And userspace::
 111
 112    #include <linux/bpf.h>
 113    #include <linux/libbpf.h>
 114
 115    __u32 map_lookup(struct bpf_map *map, __u64 cgrp, enum bpf_attach_type type)
 116    {
 117            __u32 value;
 118            bpf_map_lookup_elem(bpf_map__fd(map), &cgrp, &value);
 119            // error checking omitted
 120            return value;
 121    }
 122
 123Semantics
 124=========
 125
 126``BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE`` is a variant of this map type. This
 127per-CPU variant will have different memory regions for each CPU for each
 128storage. The non-per-CPU will have the same memory region for each storage.
 129
 130Prior to Linux 5.9, the lifetime of a storage is precisely per-attachment, and
 131for a single ``CGROUP_STORAGE`` map, there can be at most one program loaded
 132that uses the map. A program may be attached to multiple cgroups or have
 133multiple attach types, and each attach creates a fresh zeroed storage. The
 134storage is freed upon detach.
 135
 136There is a one-to-one association between the map of each type (per-CPU and
 137non-per-CPU) and the BPF program during load verification time. As a result,
 138each map can only be used by one BPF program and each BPF program can only use
 139one storage map of each type. Because of map can only be used by one BPF
 140program, sharing of this cgroup's storage with other BPF programs were
 141impossible.
 142
 143Since Linux 5.9, storage can be shared by multiple programs. When a program is
 144attached to a cgroup, the kernel would create a new storage only if the map
 145does not already contain an entry for the cgroup and attach type pair, or else
 146the old storage is reused for the new attachment. If the map is attach type
 147shared, then attach type is simply ignored during comparison. Storage is freed
 148only when either the map or the cgroup attached to is being freed. Detaching
 149will not directly free the storage, but it may cause the reference to the map
 150to reach zero and indirectly freeing all storage in the map.
 151
 152The map is not associated with any BPF program, thus making sharing possible.
 153However, the BPF program can still only associate with one map of each type
 154(per-CPU and non-per-CPU). A BPF program cannot use more than one
 155``BPF_MAP_TYPE_CGROUP_STORAGE`` or more than one
 156``BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE``.
 157
 158In all versions, userspace may use the the attach parameters of cgroup and
 159attach type pair in ``struct bpf_cgroup_storage_key`` as the key to the BPF map
 160APIs to read or update the storage for a given attachment. For Linux 5.9
 161attach type shared storages, only the first value in the struct, cgroup inode
 162id, is used during comparison, so userspace may just specify a ``__u64``
 163directly.
 164
 165The storage is bound at attach time. Even if the program is attached to parent
 166and triggers in child, the storage still belongs to the parent.
 167
 168Userspace cannot create a new entry in the map or delete an existing entry.
 169Program test runs always use a temporary storage.
 170