1 .. SPDX-License-Identifier: GPL-2.0-only
2 .. Copyright (C) 2020 Google LLC.
3
4 ===========================
5 BPF_MAP_TYPE_CGROUP_STORAGE
6 ===========================
7
8 The ``BPF_MAP_TYPE_CGROUP_STORAGE`` map type represents a local fix-sized
9 storage. It is only available with ``CONFIG_CGROUP_BPF``, and to programs that
10 attach to cgroups; the programs are made available by the same Kconfig. The
11 storage is identified by the cgroup the program is attached to.
12
13 The map provide a local storage at the cgroup that the BPF program is attached
14 to. It provides a faster and simpler access than the general purpose hash
15 table, which performs a hash table lookups, and requires user to track live
16 cgroups on their own.
17
18 This document describes the usage and semantics of the
19 ``BPF_MAP_TYPE_CGROUP_STORAGE`` map type. Some of its behaviors was changed in
20 Linux 5.9 and this document will describe the differences.
21
22 Usage
23 =====
24
25 The 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 program's attach type.
35
36 Linux 5.9 added support for type ``__u64 cgroup_inode_id`` as the key type.
37 When this key type is used, then all attach types of the particular cgroup and
38 map will share the same storage. Otherwise, if the type is
39 ``struct bpf_cgroup_storage_key``, then programs of different attach types
40 be isolated and see different storages.
41
42 To 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
48 There is no implicit synchronization. Storages of ``BPF_MAP_TYPE_CGROUP_STORAGE``
49 can be accessed by multiple programs across different CPUs, and user should
50 take 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
54 Examples
55 ========
56
57 Usage 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
75 Userspace 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
92 Alternatively, 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
110 And 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
123 Semantics
124 =========
125
126 ``BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE`` is a variant of this map type. This
127 per-CPU variant will have different memory regions for each CPU for each
128 storage. The non-per-CPU will have the same memory region for each storage.
129
130 Prior to Linux 5.9, the lifetime of a storage is precisely per-attachment, and
131 for a single ``CGROUP_STORAGE`` map, there can be at most one program loaded
132 that uses the map. A program may be attached to multiple cgroups or have
133 multiple attach types, and each attach creates a fresh zeroed storage. The
134 storage is freed upon detach.
135
136 There is a one-to-one association between the map of each type (per-CPU and
137 non-per-CPU) and the BPF program during load verification time. As a result,
138 each map can only be used by one BPF program and each BPF program can only use
139 one storage map of each type. Because of map can only be used by one BPF
140 program, sharing of this cgroup's storage with other BPF programs were
141 impossible.
142
143 Since Linux 5.9, storage can be shared by multiple programs. When a program is
144 attached to a cgroup, the kernel would create a new storage only if the map
145 does not already contain an entry for the cgroup and attach type pair, or else
146 the old storage is reused for the new attachment. If the map is attach type
147 shared, then attach type is simply ignored during comparison. Storage is freed
148 only when either the map or the cgroup attached to is being freed. Detaching
149 will not directly free the storage, but it may cause the reference to the map
150 to reach zero and indirectly freeing all storage in the map.
151
152 The map is not associated with any BPF program, thus making sharing possible.
153 However, 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
158 In all versions, userspace may use the attach parameters of cgroup and
159 attach type pair in ``struct bpf_cgroup_storage_key`` as the key to the BPF map
160 APIs to read or update the storage for a given attachment. For Linux 5.9
161 attach type shared storages, only the first value in the struct, cgroup inode
162 id, is used during comparison, so userspace may just specify a ``__u64``
163 directly.
164
165 The storage is bound at attach time. Even if the program is attached to parent
166 and triggers in child, the storage still belongs to the parent.
167
168 Userspace cannot create a new entry in the map or delete an existing entry.
169 Program test runs always use a temporary storage.
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