[bug report] selftests/bpf: Tests using bpf_check_mtu BPF-helper

[Dan Carpenter <dan.carpenter@xxxxxxxxxx>]

Hello Jesper Dangaard Brouer,

The patch b62eba563229: "selftests/bpf: Tests using bpf_check_mtu
BPF-helper" from Feb 9, 2021, leads to the following Smatch static
checker warning:

	./tools/testing/selftests/bpf/prog_tests/check_mtu.c:192 serial_test_check_mtu()
	warn: unsigned 'mtu_lo' is never less than zero.

./tools/testing/selftests/bpf/prog_tests/check_mtu.c
    184 void serial_test_check_mtu(void)
    185 {
    186         __u32 mtu_lo;
                ^^^^^^^^^^^^

    187 
    188         if (test__start_subtest("bpf_check_mtu XDP-attach"))
    189                 test_check_mtu_xdp_attach();
    190 
    191         mtu_lo = read_mtu_device_lo();
--> 192         if (CHECK(mtu_lo < 0, "reading MTU value", "failed (err:%d)", mtu_lo))
                          ^^^^^^^^^^
Unsigned can't be less than zero.

    193                 return;
    194 
    195         if (test__start_subtest("bpf_check_mtu XDP-run"))
    196                 test_check_mtu_xdp(mtu_lo, 0);
    197 
    198         if (test__start_subtest("bpf_check_mtu XDP-run ifindex-lookup"))
    199                 test_check_mtu_xdp(mtu_lo, IFINDEX_LO);
    200 
    201         if (test__start_subtest("bpf_check_mtu TC-run"))
    202                 test_check_mtu_tc(mtu_lo, 0);
    203 
    204         if (test__start_subtest("bpf_check_mtu TC-run ifindex-lookup"))
    205                 test_check_mtu_tc(mtu_lo, IFINDEX_LO);
    206 }

regards,
dan carpenter