selftests/bpf: add loop test 4

Add a test that returns a 'random' number between [0, 2^20)
If state pruning is not working correctly for loop body the number of
processed insns will be 2^20 * num_of_insns_in_loop_body and the program
will be rejected.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Yonghong Song <yhs@fb.com>

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

@@ -71,6 +71,7 @@ void test_bpf_verif_scale(void)
 
 		{ "loop1.o", BPF_PROG_TYPE_RAW_TRACEPOINT },
 		{ "loop2.o", BPF_PROG_TYPE_RAW_TRACEPOINT },
+		{ "loop4.o", BPF_PROG_TYPE_SCHED_CLS },
 
 		/* partial unroll. 19k insn in a loop.
 		 * Total program size 20.8k insn.

tools/testing/selftests/bpf/progs/loop4.c

@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2019 Facebook
+#include <linux/bpf.h>
+#include "bpf_helpers.h"
+
+char _license[] SEC("license") = "GPL";
+
+SEC("socket")
+int combinations(volatile struct __sk_buff* skb)
+{
+	int ret = 0, i;
+
+#pragma nounroll
+	for (i = 0; i < 20; i++)
+		if (skb->len)
+			ret |= 1 << i;
+	return ret;
+}