selftests/bpf: Add selftests for bpf_cgroup_ancestor() kfunc

bpf_cgroup_ancestor() allows BPF programs to access the ancestor of a
struct cgroup *. This patch adds selftests that validate its expected
behavior.

Signed-off-by: David Vernet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/prog_tests/cgrp_kfunc.c

···
 	"test_cgrp_acquire_leave_in_map",
 	"test_cgrp_xchg_release",
 	"test_cgrp_get_release",
+	"test_cgrp_get_ancestors",
 };
 
 static struct {

tools/testing/selftests/bpf/progs/cgrp_kfunc_common.h

···
 struct cgroup *bpf_cgroup_acquire(struct cgroup *p) __ksym;
 struct cgroup *bpf_cgroup_kptr_get(struct cgroup **pp) __ksym;
 void bpf_cgroup_release(struct cgroup *p) __ksym;
+struct cgroup *bpf_cgroup_ancestor(struct cgroup *cgrp, int level) __ksym;
 
 static inline struct __cgrps_kfunc_map_value *cgrps_kfunc_map_value_lookup(struct cgroup *cgrp)
 {

tools/testing/selftests/bpf/progs/cgrp_kfunc_success.c

···
 
 	return 0;
 }
+
+SEC("tp_btf/cgroup_mkdir")
+int BPF_PROG(test_cgrp_get_ancestors, struct cgroup *cgrp, const char *path)
+{
+	struct cgroup *self, *ancestor1, *invalid;
+
+	if (!is_test_kfunc_task())
+		return 0;
+
+	self = bpf_cgroup_ancestor(cgrp, cgrp->level);
+	if (!self) {
+		err = 1;
+		return 0;
+	}
+
+	if (self->self.id != cgrp->self.id) {
+		bpf_cgroup_release(self);
+		err = 2;
+		return 0;
+	}
+	bpf_cgroup_release(self);
+
+	ancestor1 = bpf_cgroup_ancestor(cgrp, cgrp->level - 1);
+	if (!ancestor1) {
+		err = 3;
+		return 0;
+	}
+	bpf_cgroup_release(ancestor1);
+
+	invalid = bpf_cgroup_ancestor(cgrp, 10000);
+	if (invalid) {
+		bpf_cgroup_release(invalid);
+		err = 4;
+		return 0;
+	}
+
+	invalid = bpf_cgroup_ancestor(cgrp, -1);
+	if (invalid) {
+		bpf_cgroup_release(invalid);
+		err = 5;
+		return 0;
+	}
+
+	return 0;
+}