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I have removed repeated `the` inside the documentation Signed-off-by: Akhil Raj <lf32.dev@gmail.com> Link: https://lore.kernel.org/r/20220827145359.32599-1-lf32.dev@gmail.com Signed-off-by: Jonathan Corbet <corbet@lwn.net>
169 lines
6.3 KiB
ReStructuredText
169 lines
6.3 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0-only
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.. Copyright (C) 2020 Google LLC.
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===========================
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BPF_MAP_TYPE_CGROUP_STORAGE
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===========================
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The ``BPF_MAP_TYPE_CGROUP_STORAGE`` map type represents a local fix-sized
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storage. It is only available with ``CONFIG_CGROUP_BPF``, and to programs that
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attach to cgroups; the programs are made available by the same Kconfig. The
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storage is identified by the cgroup the program is attached to.
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The map provide a local storage at the cgroup that the BPF program is attached
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to. It provides a faster and simpler access than the general purpose hash
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table, which performs a hash table lookups, and requires user to track live
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cgroups on their own.
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This document describes the usage and semantics of the
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``BPF_MAP_TYPE_CGROUP_STORAGE`` map type. Some of its behaviors was changed in
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Linux 5.9 and this document will describe the differences.
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Usage
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=====
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The map uses key of type of either ``__u64 cgroup_inode_id`` or
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``struct bpf_cgroup_storage_key``, declared in ``linux/bpf.h``::
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struct bpf_cgroup_storage_key {
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__u64 cgroup_inode_id;
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__u32 attach_type;
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};
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``cgroup_inode_id`` is the inode id of the cgroup directory.
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``attach_type`` is the program's attach type.
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Linux 5.9 added support for type ``__u64 cgroup_inode_id`` as the key type.
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When this key type is used, then all attach types of the particular cgroup and
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map will share the same storage. Otherwise, if the type is
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``struct bpf_cgroup_storage_key``, then programs of different attach types
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be isolated and see different storages.
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To access the storage in a program, use ``bpf_get_local_storage``::
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void *bpf_get_local_storage(void *map, u64 flags)
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``flags`` is reserved for future use and must be 0.
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There is no implicit synchronization. Storages of ``BPF_MAP_TYPE_CGROUP_STORAGE``
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can be accessed by multiple programs across different CPUs, and user should
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take care of synchronization by themselves. The bpf infrastructure provides
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``struct bpf_spin_lock`` to synchronize the storage. See
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``tools/testing/selftests/bpf/progs/test_spin_lock.c``.
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Examples
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========
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Usage with key type as ``struct bpf_cgroup_storage_key``::
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#include <bpf/bpf.h>
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struct {
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__uint(type, BPF_MAP_TYPE_CGROUP_STORAGE);
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__type(key, struct bpf_cgroup_storage_key);
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__type(value, __u32);
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} cgroup_storage SEC(".maps");
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int program(struct __sk_buff *skb)
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{
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__u32 *ptr = bpf_get_local_storage(&cgroup_storage, 0);
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__sync_fetch_and_add(ptr, 1);
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return 0;
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}
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Userspace accessing map declared above::
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#include <linux/bpf.h>
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#include <linux/libbpf.h>
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__u32 map_lookup(struct bpf_map *map, __u64 cgrp, enum bpf_attach_type type)
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{
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struct bpf_cgroup_storage_key = {
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.cgroup_inode_id = cgrp,
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.attach_type = type,
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};
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__u32 value;
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bpf_map_lookup_elem(bpf_map__fd(map), &key, &value);
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// error checking omitted
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return value;
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}
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Alternatively, using just ``__u64 cgroup_inode_id`` as key type::
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#include <bpf/bpf.h>
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struct {
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__uint(type, BPF_MAP_TYPE_CGROUP_STORAGE);
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__type(key, __u64);
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__type(value, __u32);
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} cgroup_storage SEC(".maps");
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int program(struct __sk_buff *skb)
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{
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__u32 *ptr = bpf_get_local_storage(&cgroup_storage, 0);
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__sync_fetch_and_add(ptr, 1);
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return 0;
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}
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And userspace::
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#include <linux/bpf.h>
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#include <linux/libbpf.h>
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__u32 map_lookup(struct bpf_map *map, __u64 cgrp, enum bpf_attach_type type)
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{
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__u32 value;
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bpf_map_lookup_elem(bpf_map__fd(map), &cgrp, &value);
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// error checking omitted
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return value;
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}
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Semantics
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=========
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``BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE`` is a variant of this map type. This
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per-CPU variant will have different memory regions for each CPU for each
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storage. The non-per-CPU will have the same memory region for each storage.
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Prior to Linux 5.9, the lifetime of a storage is precisely per-attachment, and
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for a single ``CGROUP_STORAGE`` map, there can be at most one program loaded
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that uses the map. A program may be attached to multiple cgroups or have
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multiple attach types, and each attach creates a fresh zeroed storage. The
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storage is freed upon detach.
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There is a one-to-one association between the map of each type (per-CPU and
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non-per-CPU) and the BPF program during load verification time. As a result,
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each map can only be used by one BPF program and each BPF program can only use
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one storage map of each type. Because of map can only be used by one BPF
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program, sharing of this cgroup's storage with other BPF programs were
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impossible.
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Since Linux 5.9, storage can be shared by multiple programs. When a program is
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attached to a cgroup, the kernel would create a new storage only if the map
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does not already contain an entry for the cgroup and attach type pair, or else
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the old storage is reused for the new attachment. If the map is attach type
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shared, then attach type is simply ignored during comparison. Storage is freed
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only when either the map or the cgroup attached to is being freed. Detaching
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will not directly free the storage, but it may cause the reference to the map
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to reach zero and indirectly freeing all storage in the map.
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The map is not associated with any BPF program, thus making sharing possible.
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However, the BPF program can still only associate with one map of each type
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(per-CPU and non-per-CPU). A BPF program cannot use more than one
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``BPF_MAP_TYPE_CGROUP_STORAGE`` or more than one
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``BPF_MAP_TYPE_PERCPU_CGROUP_STORAGE``.
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In all versions, userspace may use the attach parameters of cgroup and
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attach type pair in ``struct bpf_cgroup_storage_key`` as the key to the BPF map
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APIs to read or update the storage for a given attachment. For Linux 5.9
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attach type shared storages, only the first value in the struct, cgroup inode
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id, is used during comparison, so userspace may just specify a ``__u64``
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directly.
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The storage is bound at attach time. Even if the program is attached to parent
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and triggers in child, the storage still belongs to the parent.
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Userspace cannot create a new entry in the map or delete an existing entry.
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Program test runs always use a temporary storage.
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