From 36aa10ffd6480b93e32611411be4a8fc49804aba Mon Sep 17 00:00:00 2001 From: David Vernet Date: Wed, 7 Dec 2022 14:49:11 -0600 Subject: [PATCH] bpf/docs: Document struct cgroup * kfuncs bpf_cgroup_acquire(), bpf_cgroup_release(), bpf_cgroup_kptr_get(), and bpf_cgroup_ancestor(), are kfuncs that were recently added to kernel/bpf/helpers.c. These are "core" kfuncs in that they're available for use in any tracepoint or struct_ops BPF program. Though they have no ABI stability guarantees, we should still document them. This patch adds a struct cgroup * subsection to the Core kfuncs section which describes each of these kfuncs. Signed-off-by: David Vernet Link: https://lore.kernel.org/r/20221207204911.873646-3-void@manifault.com Signed-off-by: Alexei Starovoitov --- Documentation/bpf/kfuncs.rst | 115 +++++++++++++++++++++++++++++++++++ kernel/bpf/helpers.c | 2 +- 2 files changed, 116 insertions(+), 1 deletion(-) diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst index 24ed109afc98..9fd7fb539f85 100644 --- a/Documentation/bpf/kfuncs.rst +++ b/Documentation/bpf/kfuncs.rst @@ -305,3 +305,118 @@ Here is an example of it being used: bpf_task_release(lookup); return 0; } + +3.2 struct cgroup * kfuncs +-------------------------- + +``struct cgroup *`` objects also have acquire and release functions: + +.. kernel-doc:: kernel/bpf/helpers.c + :identifiers: bpf_cgroup_acquire bpf_cgroup_release + +These kfuncs are used in exactly the same manner as bpf_task_acquire() and +bpf_task_release() respectively, so we won't provide examples for them. + +---- + +You may also acquire a reference to a ``struct cgroup`` kptr that's already +stored in a map using bpf_cgroup_kptr_get(): + +.. kernel-doc:: kernel/bpf/helpers.c + :identifiers: bpf_cgroup_kptr_get + +Here's an example of how it can be used: + +.. code-block:: c + + /* struct containing the struct task_struct kptr which is actually stored in the map. */ + struct __cgroups_kfunc_map_value { + struct cgroup __kptr_ref * cgroup; + }; + + /* The map containing struct __cgroups_kfunc_map_value entries. */ + struct { + __uint(type, BPF_MAP_TYPE_HASH); + __type(key, int); + __type(value, struct __cgroups_kfunc_map_value); + __uint(max_entries, 1); + } __cgroups_kfunc_map SEC(".maps"); + + /* ... */ + + /** + * A simple example tracepoint program showing how a + * struct cgroup kptr that is stored in a map can + * be acquired using the bpf_cgroup_kptr_get() kfunc. + */ + SEC("tp_btf/cgroup_mkdir") + int BPF_PROG(cgroup_kptr_get_example, struct cgroup *cgrp, const char *path) + { + struct cgroup *kptr; + struct __cgroups_kfunc_map_value *v; + s32 id = cgrp->self.id; + + /* Assume a cgroup kptr was previously stored in the map. */ + v = bpf_map_lookup_elem(&__cgroups_kfunc_map, &id); + if (!v) + return -ENOENT; + + /* Acquire a reference to the cgroup kptr that's already stored in the map. */ + kptr = bpf_cgroup_kptr_get(&v->cgroup); + if (!kptr) + /* If no cgroup was present in the map, it's because + * we're racing with another CPU that removed it with + * bpf_kptr_xchg() between the bpf_map_lookup_elem() + * above, and our call to bpf_cgroup_kptr_get(). + * bpf_cgroup_kptr_get() internally safely handles this + * race, and will return NULL if the task is no longer + * present in the map by the time we invoke the kfunc. + */ + return -EBUSY; + + /* Free the reference we just took above. Note that the + * original struct cgroup kptr is still in the map. It will + * be freed either at a later time if another context deletes + * it from the map, or automatically by the BPF subsystem if + * it's still present when the map is destroyed. + */ + bpf_cgroup_release(kptr); + + return 0; + } + +---- + +Another kfunc available for interacting with ``struct cgroup *`` objects is +bpf_cgroup_ancestor(). This allows callers to access the ancestor of a cgroup, +and return it as a cgroup kptr. + +.. kernel-doc:: kernel/bpf/helpers.c + :identifiers: bpf_cgroup_ancestor + +Eventually, BPF should be updated to allow this to happen with a normal memory +load in the program itself. This is currently not possible without more work in +the verifier. bpf_cgroup_ancestor() can be used as follows: + +.. code-block:: c + + /** + * Simple tracepoint example that illustrates how a cgroup's + * ancestor can be accessed using bpf_cgroup_ancestor(). + */ + SEC("tp_btf/cgroup_mkdir") + int BPF_PROG(cgrp_ancestor_example, struct cgroup *cgrp, const char *path) + { + struct cgroup *parent; + + /* The parent cgroup resides at the level before the current cgroup's level. */ + parent = bpf_cgroup_ancestor(cgrp, cgrp->level - 1); + if (!parent) + return -ENOENT; + + bpf_printk("Parent id is %d", parent->self.id); + + /* Return the parent cgroup that was acquired above. */ + bpf_cgroup_release(parent); + return 0; + } diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c index 1e4bf466b08f..6ed5875b97a3 100644 --- a/kernel/bpf/helpers.c +++ b/kernel/bpf/helpers.c @@ -1960,7 +1960,7 @@ struct cgroup *bpf_cgroup_kptr_get(struct cgroup **cgrpp) } /** - * bpf_cgroup_release - Release the reference acquired on a struct cgroup *. + * bpf_cgroup_release - Release the reference acquired on a cgroup. * If this kfunc is invoked in an RCU read region, the cgroup is guaranteed to * not be freed until the current grace period has ended, even if its refcount * drops to 0.