From d43198017ea39e5040a578d0dd622c7b0e939019 Mon Sep 17 00:00:00 2001 From: Yonghong Song Date: Tue, 25 Oct 2022 21:29:22 -0700 Subject: [PATCH] docs/bpf: Add documentation for new cgroup local storage Add some descriptions and examples for BPF_MAP_TYPE_CGRP_STORAGE. Also illustate the major difference between BPF_MAP_TYPE_CGRP_STORAGE and BPF_MAP_TYPE_CGROUP_STORAGE and recommend to use BPF_MAP_TYPE_CGRP_STORAGE instead of BPF_MAP_TYPE_CGROUP_STORAGE in the end. Acked-by: David Vernet Signed-off-by: Yonghong Song Link: https://lore.kernel.org/r/20221026042922.676383-1-yhs@fb.com Signed-off-by: Alexei Starovoitov --- Documentation/bpf/map_cgrp_storage.rst | 109 +++++++++++++++++++++++++ 1 file changed, 109 insertions(+) create mode 100644 Documentation/bpf/map_cgrp_storage.rst diff --git a/Documentation/bpf/map_cgrp_storage.rst b/Documentation/bpf/map_cgrp_storage.rst new file mode 100644 index 000000000000..5d3f603efffa --- /dev/null +++ b/Documentation/bpf/map_cgrp_storage.rst @@ -0,0 +1,109 @@ +.. SPDX-License-Identifier: GPL-2.0-only +.. Copyright (C) 2022 Meta Platforms, Inc. and affiliates. + +========================= +BPF_MAP_TYPE_CGRP_STORAGE +========================= + +The ``BPF_MAP_TYPE_CGRP_STORAGE`` map type represents a local fix-sized +storage for cgroups. It is only available with ``CONFIG_CGROUPS``. +The programs are made available by the same Kconfig. The +data for a particular cgroup can be retrieved by looking up the map +with that cgroup. + +This document describes the usage and semantics of the +``BPF_MAP_TYPE_CGRP_STORAGE`` map type. + +Usage +===== + +The map key must be ``sizeof(int)`` representing a cgroup fd. +To access the storage in a program, use ``bpf_cgrp_storage_get``:: + + void *bpf_cgrp_storage_get(struct bpf_map *map, struct cgroup *cgroup, void *value, u64 flags) + +``flags`` could be 0 or ``BPF_LOCAL_STORAGE_GET_F_CREATE`` which indicates that +a new local storage will be created if one does not exist. + +The local storage can be removed with ``bpf_cgrp_storage_delete``:: + + long bpf_cgrp_storage_delete(struct bpf_map *map, struct cgroup *cgroup) + +The map is available to all program types. + +Examples +======== + +A BPF program example with BPF_MAP_TYPE_CGRP_STORAGE:: + + #include + #include + #include + + struct { + __uint(type, BPF_MAP_TYPE_CGRP_STORAGE); + __uint(map_flags, BPF_F_NO_PREALLOC); + __type(key, int); + __type(value, long); + } cgrp_storage SEC(".maps"); + + SEC("tp_btf/sys_enter") + int BPF_PROG(on_enter, struct pt_regs *regs, long id) + { + struct task_struct *task = bpf_get_current_task_btf(); + long *ptr; + + ptr = bpf_cgrp_storage_get(&cgrp_storage, task->cgroups->dfl_cgrp, 0, + BPF_LOCAL_STORAGE_GET_F_CREATE); + if (ptr) + __sync_fetch_and_add(ptr, 1); + + return 0; + } + +Userspace accessing map declared above:: + + #include + #include + + __u32 map_lookup(struct bpf_map *map, int cgrp_fd) + { + __u32 *value; + value = bpf_map_lookup_elem(bpf_map__fd(map), &cgrp_fd); + if (value) + return *value; + return 0; + } + +Difference Between BPF_MAP_TYPE_CGRP_STORAGE and BPF_MAP_TYPE_CGROUP_STORAGE +============================================================================ + +The old cgroup storage map ``BPF_MAP_TYPE_CGROUP_STORAGE`` has been marked as +deprecated (renamed to ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``). The new +``BPF_MAP_TYPE_CGRP_STORAGE`` map should be used instead. The following +illusates the main difference between ``BPF_MAP_TYPE_CGRP_STORAGE`` and +``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``. + +(1). ``BPF_MAP_TYPE_CGRP_STORAGE`` can be used by all program types while + ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` is available only to cgroup program types + like BPF_CGROUP_INET_INGRESS or BPF_CGROUP_SOCK_OPS, etc. + +(2). ``BPF_MAP_TYPE_CGRP_STORAGE`` supports local storage for more than one + cgroup while ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` only supports one cgroup + which is attached by a BPF program. + +(3). ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` allocates local storage at attach time so + ``bpf_get_local_storage()`` always returns non-NULL local storage. + ``BPF_MAP_TYPE_CGRP_STORAGE`` allocates local storage at runtime so + it is possible that ``bpf_cgrp_storage_get()`` may return null local storage. + To avoid such null local storage issue, user space can do + ``bpf_map_update_elem()`` to pre-allocate local storage before a BPF program + is attached. + +(4). ``BPF_MAP_TYPE_CGRP_STORAGE`` supports deleting local storage by a BPF program + while ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` only deletes storage during + prog detach time. + +So overall, ``BPF_MAP_TYPE_CGRP_STORAGE`` supports all ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED`` +functionality and beyond. It is recommended to use ``BPF_MAP_TYPE_CGRP_STORAGE`` +instead of ``BPF_MAP_TYPE_CGROUP_STORAGE_DEPRECATED``.