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 <void@xxxxxxxxxxxxx>
---
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..1478862702fc 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 struct cgroup kptr.
* 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.
--
2.38.1
