selftests/bpf: add proper section name to bpf prog and rename it

test_skb_cgroup_id_kern.c is currently involved in a manual test. In its
current form, it can not be used with the auto-generated skeleton APIs,
because the section name is not valid to allow libbpf to deduce the program
type.

Update section name to allow skeleton APIs usage. Also rename the program
name to make it shorter and more straighforward regarding the API it is
testing. While doing so, make sure that test_skb_cgroup_id.sh passes to get
a working reference before converting it to test_progs
- update the obj name
- fix loading issue (verifier rejecting the program when loaded through tc,
  because of map not found), by preloading the whole obj with bpftool
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20240813-convert_cgroup_tests-v4-3-a33c03458cf6@bootlin.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/progs/test_skb_cgroup_id_kern.c → tools/testing/selftests/bpf/progs/cgroup_ancestor.c

@@ -28,7 +28,7 @@ static __always_inline void log_nth_level(struct __sk_buff *skb, __u32 level)
 	bpf_map_update_elem(&cgroup_ids, &level, &id, 0);
 }
 
-SEC("cgroup_id_logger")
+SEC("tc")
 int log_cgroup_id(struct __sk_buff *skb)
 {
 	/* Loop unroll can't be used here due to [1]. Unrolling manually.

tools/testing/selftests/bpf/test_skb_cgroup_id.sh

@@ -30,8 +30,10 @@ setup()
 	wait_for_ip
 
 	tc qdisc add dev ${TEST_IF} clsact
-	tc filter add dev ${TEST_IF} egress bpf obj ${BPF_PROG_OBJ} \
-		sec ${BPF_PROG_SECTION} da
+	mkdir -p /sys/fs/bpf/${BPF_PROG_PIN}
+	bpftool prog loadall ${BPF_PROG_OBJ} /sys/fs/bpf/${BPF_PROG_PIN} type tc
+	tc filter add dev ${TEST_IF} egress bpf da object-pinned \
+		/sys/fs/bpf/${BPF_PROG_PIN}/${BPF_PROG_NAME}
 
 	BPF_PROG_ID=$(tc filter show dev ${TEST_IF} egress | \
 			awk '/ id / {sub(/.* id /, "", $0); print($1)}')
@@ -41,6 +43,7 @@ cleanup()
 {
 	ip link del ${TEST_IF} 2>/dev/null || :
 	ip link del ${TEST_IF_PEER} 2>/dev/null || :
+	rm -rf /sys/fs/bpf/${BPF_PROG_PIN}
 }
 
 main()
@@ -54,8 +57,9 @@ DIR=$(dirname $0)
 TEST_IF="test_cgid_1"
 TEST_IF_PEER="test_cgid_2"
 MAX_PING_TRIES=5
-BPF_PROG_OBJ="${DIR}/test_skb_cgroup_id_kern.bpf.o"
-BPF_PROG_SECTION="cgroup_id_logger"
+BPF_PROG_PIN="cgroup_ancestor"
+BPF_PROG_OBJ="${DIR}/${BPF_PROG_PIN}.bpf.o"
+BPF_PROG_NAME="log_cgroup_id"
 BPF_PROG_ID=0
 PROG="${DIR}/test_skb_cgroup_id_user"
 type ping6 >/dev/null 2>&1 && PING6="ping6" || PING6="ping -6"