test_bpf: Add backward jump test case

Currently the testsuite does not have a test case with a backward jump.
The s390x JIT (kernel 4.0) had a bug in that area.
So add one new test case for this now.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

lib/test_bpf.c

@@ -3940,6 +3940,22 @@ static struct bpf_test tests[] = {
 		{ },
 		{ { 0, 1 } },
 	},
+	/* BPF_JMP | BPF_JGT | BPF_K jump backwards */
+	{
+		"JMP_JGT_K: if (3 > 2) return 1 (jump backwards)",
+		.u.insns_int = {
+			BPF_JMP_IMM(BPF_JA, 0, 0, 2), /* goto start */
+			BPF_ALU32_IMM(BPF_MOV, R0, 1), /* out: */
+			BPF_EXIT_INSN(),
+			BPF_ALU32_IMM(BPF_MOV, R0, 0), /* start: */
+			BPF_LD_IMM64(R1, 3), /* note: this takes 2 insns */
+			BPF_JMP_IMM(BPF_JGT, R1, 2, -6), /* goto out */
+			BPF_EXIT_INSN(),
+		},
+		INTERNAL,
+		{ },
+		{ { 0, 1 } },
+	},
 	{
 		"JMP_JGE_K: if (3 >= 3) return 1",
 		.u.insns_int = {