Re: [PATCH bpf-next V9 7/7] bpf/selftests: tests using bpf_check_mtu BPF-helper

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Dec 17, 2020 at 9:30 AM Jesper Dangaard Brouer
<brouer@redhat.com> wrote:
>
> Adding selftest for BPF-helper bpf_check_mtu(). Making sure
> it can be used from both XDP and TC.
>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>  tools/testing/selftests/bpf/prog_tests/check_mtu.c |  204 ++++++++++++++++++++
>  tools/testing/selftests/bpf/progs/test_check_mtu.c |  196 +++++++++++++++++++
>  2 files changed, 400 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/check_mtu.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_check_mtu.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/check_mtu.c b/tools/testing/selftests/bpf/prog_tests/check_mtu.c
> new file mode 100644
> index 000000000000..b5d0c3a9abe8
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/check_mtu.c
> @@ -0,0 +1,204 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2020 Jesper Dangaard Brouer */
> +
> +#include <linux/if_link.h> /* before test_progs.h, avoid bpf_util.h redefines */
> +
> +#include <test_progs.h>
> +#include "test_check_mtu.skel.h"
> +#include <network_helpers.h>
> +
> +#include <stdlib.h>
> +#include <inttypes.h>
> +
> +#define IFINDEX_LO 1
> +
> +static __u32 duration; /* Hint: needed for CHECK macro */
> +
> +static int read_mtu_device_lo(void)
> +{
> +       const char *filename = "/sys/devices/virtual/net/lo/mtu";
> +       char buf[11] = {};
> +       int value;
> +       int fd;
> +
> +       fd = open(filename, 0, O_RDONLY);
> +       if (fd == -1)
> +               return -1;
> +
> +       if (read(fd, buf, sizeof(buf)) == -1)

close fd missing here?

> +               return -2;
> +       close(fd);
> +
> +       value = strtoimax(buf, NULL, 10);
> +       if (errno == ERANGE)
> +               return -3;
> +
> +       return value;
> +}
> +
> +static void test_check_mtu_xdp_attach(struct bpf_program *prog)
> +{
> +       int err = 0;
> +       int fd;
> +
> +       fd = bpf_program__fd(prog);
> +       err = bpf_set_link_xdp_fd(IFINDEX_LO, fd, XDP_FLAGS_SKB_MODE);
> +       if (CHECK(err, "XDP-attach", "failed"))
> +               return;
> +
> +       bpf_set_link_xdp_fd(IFINDEX_LO, -1, 0);

can you please use bpf_link-based bpf_program__attach_xdp() which will
provide auto-cleanup in case of crash?

also check that it succeeded?

> +}
> +
> +static void test_check_mtu_run_xdp(struct test_check_mtu *skel,
> +                                  struct bpf_program *prog,
> +                                  __u32 mtu_expect)
> +{
> +       const char *prog_name = bpf_program__name(prog);
> +       int retval_expect = XDP_PASS;
> +       __u32 mtu_result = 0;
> +       char buf[256];
> +       int err;
> +
> +       struct bpf_prog_test_run_attr tattr = {
> +               .repeat = 1,
> +               .data_in = &pkt_v4,
> +               .data_size_in = sizeof(pkt_v4),
> +               .data_out = buf,
> +               .data_size_out = sizeof(buf),
> +               .prog_fd = bpf_program__fd(prog),
> +       };

nit: it's a variable declaration, so keep it all in one block. There
is also opts-based variant, which might be good to use here instead.

> +
> +       memset(buf, 0, sizeof(buf));

char buf[256] = {}; would make this unnecessary


> +
> +       err = bpf_prog_test_run_xattr(&tattr);
> +       CHECK_ATTR(err != 0 || errno != 0, "bpf_prog_test_run",
> +                  "prog_name:%s (err %d errno %d retval %d)\n",
> +                  prog_name, err, errno, tattr.retval);
> +
> +        CHECK(tattr.retval != retval_expect, "retval",

whitespaces are off?

> +             "progname:%s unexpected retval=%d expected=%d\n",
> +             prog_name, tattr.retval, retval_expect);
> +
> +       /* Extract MTU that BPF-prog got */
> +       mtu_result = skel->bss->global_bpf_mtu_xdp;
> +       CHECK(mtu_result != mtu_expect, "MTU-compare-user",
> +             "failed (MTU user:%d bpf:%d)", mtu_expect, mtu_result);

There is nicer ASSERT_EQ() macro for such cases:

ASSERT_EQ(mtu_result, mtu_expect, "MTU-compare-user"); it will format
sensible error message automatically

> +}
> +

[...]

> +       char buf[256];
> +       int err;
> +
> +       struct bpf_prog_test_run_attr tattr = {
> +               .repeat = 1,
> +               .data_in = &pkt_v4,
> +               .data_size_in = sizeof(pkt_v4),
> +               .data_out = buf,
> +               .data_size_out = sizeof(buf),
> +               .prog_fd = bpf_program__fd(prog),
> +       };
> +
> +       memset(buf, 0, sizeof(buf));
> +

same as above

> +       err = bpf_prog_test_run_xattr(&tattr);
> +       CHECK_ATTR(err != 0 || errno != 0, "bpf_prog_test_run",
> +                  "prog_name:%s (err %d errno %d retval %d)\n",
> +                  prog_name, err, errno, tattr.retval);
> +
> +        CHECK(tattr.retval != retval_expect, "retval",

same :)

> +             "progname:%s unexpected retval=%d expected=%d\n",
> +             prog_name, tattr.retval, retval_expect);
> +
> +       /* Extract MTU that BPF-prog got */
> +       mtu_result = skel->bss->global_bpf_mtu_tc;
> +       CHECK(mtu_result != mtu_expect, "MTU-compare-user",
> +             "failed (MTU user:%d bpf:%d)", mtu_expect, mtu_result);
> +}
> +
> +

[...]

> +
> +void test_check_mtu(void)
> +{
> +       struct test_check_mtu *skel;
> +       __u32 mtu_lo;
> +
> +       skel = test_check_mtu__open_and_load();
> +       if (CHECK(!skel, "open and load skel", "failed"))
> +               return; /* Exit if e.g. helper unknown to kernel */
> +
> +       if (test__start_subtest("bpf_check_mtu XDP-attach"))
> +               test_check_mtu_xdp_attach(skel->progs.xdp_use_helper_basic);
> +
> +       test_check_mtu__destroy(skel);

here it's not clear why you instantiate skeleton outside of
test_check_mtu_xdp_attach() subtest. Can you please move it in? It
will keep this failure local to that specific subtest, not the entire
test. And is just cleaner, of course.

> +
> +       mtu_lo = read_mtu_device_lo();
> +       if (CHECK(mtu_lo < 0, "reading MTU value", "failed (err:%d)", mtu_lo))

ASSERT_OK() could be used here

> +               return;
> +
> +       if (test__start_subtest("bpf_check_mtu XDP-run"))
> +               test_check_mtu_xdp(mtu_lo, 0);
> +
> +       if (test__start_subtest("bpf_check_mtu XDP-run ifindex-lookup"))
> +               test_check_mtu_xdp(mtu_lo, IFINDEX_LO);
> +
> +       if (test__start_subtest("bpf_check_mtu TC-run"))
> +               test_check_mtu_tc(mtu_lo, 0);
> +
> +       if (test__start_subtest("bpf_check_mtu TC-run ifindex-lookup"))
> +               test_check_mtu_tc(mtu_lo, IFINDEX_LO);
> +}

[...]

> +
> +       global_bpf_mtu_tc = mtu_len;
> +       return retval;
> +}
> +
> +SEC("classifier")

nice use of the same SEC()'tion BPF programs!


> +int tc_minus_delta(struct __sk_buff *ctx)
> +{
> +       int retval = BPF_OK; /* Expected retval on successful test */
> +       __u32 ifindex = GLOBAL_USER_IFINDEX;
> +       __u32 skb_len = ctx->len;
> +       __u32 mtu_len = 0;
> +       int delta;
> +
> +       /* Boarderline test case: Minus delta exceeding packet length allowed */
> +       delta = -((skb_len - ETH_HLEN) + 1);
> +
> +       /* Minus length (adjusted via delta) still pass MTU check, other helpers
> +        * are responsible for catching this, when doing actual size adjust
> +        */
> +       if (bpf_check_mtu(ctx, ifindex, &mtu_len, delta, 0))
> +               retval = BPF_DROP;
> +
> +       global_bpf_mtu_xdp = mtu_len;
> +       return retval;
> +}
>
>