[PATCH v2 1/2 bpf-next] bpf: Add bpf_cgroup_from_id() kfunc

[Tejun Heo]

cgroup ID is an userspace-visible 64bit value uniquely identifying a given
cgroup. As the IDs are used widely, it's useful to be able to look up the
matching cgroups. Add bpf_cgroup_from_id().

v2: Separate out selftest into its own patch as suggested by Alexei.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
---
 Documentation/bpf/kfuncs.rst | 10 +++++++---
 kernel/bpf/helpers.c         | 18 ++++++++++++++++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index ca96ef3f6896..226313747be5 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -583,13 +583,17 @@ You may also acquire a reference to a ``struct cgroup`` kptr that's already
 
 ----
 
-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.
+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
+return a cgroup kptr.
 
 .. kernel-doc:: kernel/bpf/helpers.c
    :identifiers: bpf_cgroup_ancestor
 
+.. kernel-doc:: kernel/bpf/helpers.c
+   :identifiers: bpf_cgroup_from_id
+
 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:
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 5b278a38ae58..a784be6f8bac 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2101,6 +2101,23 @@ __bpf_kfunc struct cgroup *bpf_cgroup_ancestor(struct cgroup *cgrp, int level)
 	cgroup_get(ancestor);
 	return ancestor;
 }
+
+/**
+ * bpf_cgroup_from_id - Find a cgroup from its ID. A cgroup returned by this
+ * kfunc which is not subsequently stored in a map, must be released by calling
+ * bpf_cgroup_release().
+ * @cgrp: The cgroup for which we're performing a lookup.
+ * @level: The level of ancestor to look up.
+ */
+__bpf_kfunc struct cgroup *bpf_cgroup_from_id(u64 cgid)
+{
+	struct cgroup *cgrp;
+
+	cgrp = cgroup_get_from_id(cgid);
+	if (IS_ERR(cgrp))
+		return NULL;
+	return cgrp;
+}
 #endif /* CONFIG_CGROUPS */
 
 /**
@@ -2167,6 +2184,7 @@ BTF_ID_FLAGS(func, bpf_cgroup_acquire, KF_ACQUIRE | KF_TRUSTED_ARGS)
 BTF_ID_FLAGS(func, bpf_cgroup_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_cgroup_release, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_cgroup_ancestor, KF_ACQUIRE | KF_TRUSTED_ARGS | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_cgroup_from_id, KF_ACQUIRE | KF_RET_NULL)
 #endif
 BTF_ID_FLAGS(func, bpf_task_from_pid, KF_ACQUIRE | KF_RET_NULL)
 BTF_SET8_END(generic_btf_ids)
-- 
2.39.2