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 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 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.