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Diffstat (limited to 'Documentation/bpf/kfuncs.rst')
| -rw-r--r-- | Documentation/bpf/kfuncs.rst | 124 |
1 files changed, 47 insertions, 77 deletions
diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst index 69eccf6f98ef..3b42cfe12437 100644 --- a/Documentation/bpf/kfuncs.rst +++ b/Documentation/bpf/kfuncs.rst @@ -179,9 +179,10 @@ both are orthogonal to each other. --------------------- The KF_RELEASE flag is used to indicate that the kfunc releases the pointer -passed in to it. There can be only one referenced pointer that can be passed in. -All copies of the pointer being released are invalidated as a result of invoking -kfunc with this flag. +passed in to it. There can be only one referenced pointer that can be passed +in. All copies of the pointer being released are invalidated as a result of +invoking kfunc with this flag. KF_RELEASE kfuncs automatically receive the +protection afforded by the KF_TRUSTED_ARGS flag described below. 2.4.4 KF_KPTR_GET flag ---------------------- @@ -470,13 +471,50 @@ struct_ops callback arg. For example: struct task_struct *acquired; acquired = bpf_task_acquire(task); + if (acquired) + /* + * In a typical program you'd do something like store + * the task in a map, and the map will automatically + * release it later. Here, we release it manually. + */ + bpf_task_release(acquired); + return 0; + } + + +References acquired on ``struct task_struct *`` objects are RCU protected. +Therefore, when in an RCU read region, you can obtain a pointer to a task +embedded in a map value without having to acquire a reference: + +.. code-block:: c + + #define private(name) SEC(".data." #name) __hidden __attribute__((aligned(8))) + private(TASK) static struct task_struct *global; + + /** + * A trivial example showing how to access a task stored + * in a map using RCU. + */ + SEC("tp_btf/task_newtask") + int BPF_PROG(task_rcu_read_example, struct task_struct *task, u64 clone_flags) + { + struct task_struct *local_copy; + + bpf_rcu_read_lock(); + local_copy = global; + if (local_copy) + /* + * We could also pass local_copy to kfuncs or helper functions here, + * as we're guaranteed that local_copy will be valid until we exit + * the RCU read region below. + */ + bpf_printk("Global task %s is valid", local_copy->comm); + else + bpf_printk("No global task found"); + bpf_rcu_read_unlock(); + + /* At this point we can no longer reference local_copy. */ - /* - * In a typical program you'd do something like store - * the task in a map, and the map will automatically - * release it later. Here, we release it manually. - */ - bpf_task_release(acquired); return 0; } @@ -534,74 +572,6 @@ 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 * 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; - } - ----- - Other kfuncs available for interacting with ``struct cgroup *`` objects are bpf_cgroup_ancestor() and bpf_cgroup_from_id(), allowing callers to access the ancestor of a cgroup and find a cgroup by its ID, respectively. Both |