netdev - Re: [PATCH bpf-next V7 8/8] bpf/selftests: activating bpf_check

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Date:   Fri, 20 Nov 2020 13:50:51 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Daniel Borkmann <borkmann@...earbox.net>, maze@...gle.com,
        lmb@...udflare.com, shaun@...era.io,
        Lorenzo Bianconi <lorenzo@...nel.org>, marek@...udflare.com,
        John Fastabend <john.fastabend@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, eyal.birger@...il.com,
        colrack@...il.com
Subject: Re: [PATCH bpf-next V7 8/8] bpf/selftests: activating bpf_check_mtu
 BPF-helper

On Fri, Nov 20, 2020 at 05:18:47PM +0100, Jesper Dangaard Brouer 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@...hat.com>
> ---
>  tools/testing/selftests/bpf/prog_tests/check_mtu.c |   37 ++++++++++++++++++++
>  tools/testing/selftests/bpf/progs/test_check_mtu.c |   33 ++++++++++++++++++
>  2 files changed, 70 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..09b8f986a17b
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/check_mtu.c
> @@ -0,0 +1,37 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2020 Red Hat */
> +#include <uapi/linux/bpf.h>
> +#include <linux/if_link.h>
> +#include <test_progs.h>
> +
> +#include "test_check_mtu.skel.h"
> +#define IFINDEX_LO 1
> +
> +void test_check_mtu_xdp(struct test_check_mtu *skel)
> +{
> +	int err = 0;
> +	int fd;
> +
> +	fd = bpf_program__fd(skel->progs.xdp_use_helper);
> +	err = bpf_set_link_xdp_fd(IFINDEX_LO, fd, XDP_FLAGS_SKB_MODE);
> +	if (CHECK_FAIL(err))
> +		return;
> +
> +	bpf_set_link_xdp_fd(IFINDEX_LO, -1, 0);
> +}
> +
> +void test_check_mtu(void)
> +{
> +	struct test_check_mtu *skel;
> +
> +	skel = test_check_mtu__open_and_load();
> +	if (CHECK_FAIL(!skel)) {
> +		perror("test_check_mtu__open_and_load");
> +		return;
> +	}
> +
> +	if (test__start_subtest("bpf_check_mtu XDP-attach"))
> +		test_check_mtu_xdp(skel);
> +
> +	test_check_mtu__destroy(skel);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/test_check_mtu.c b/tools/testing/selftests/bpf/progs/test_check_mtu.c
> new file mode 100644
> index 000000000000..ab97ec925a32
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_check_mtu.c
> @@ -0,0 +1,33 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (c) 2020 Red Hat */
> +#include <linux/bpf.h>
> +#include <bpf/bpf_helpers.h>
> +
> +#include <stddef.h>
> +#include <stdint.h>
> +
> +char _license[] SEC("license") = "GPL";
> +
> +SEC("xdp")
> +int xdp_use_helper(struct xdp_md *ctx)
> +{
> +	uint32_t mtu_len = 0;
> +	int delta = 20;
> +
> +	if (bpf_check_mtu(ctx, 0, &mtu_len, delta, 0)) {
> +		return XDP_ABORTED;
> +	}
> +	return XDP_PASS;
> +}
> +
> +SEC("classifier")
> +int tc_use_helper(struct __sk_buff *ctx)
> +{
> +	uint32_t mtu_len = 0;
> +	int delta = -20;
> +
> +	if (bpf_check_mtu(ctx, 0, &mtu_len, delta, 0)) {
> +		return BPF_DROP;
> +	}
> +	return BPF_OK;
> +}

Patches 7 and 8 are not adequate as tests.
They do not testing the functionality of earlier patches.
bpf_check_mtu() could be returning random value and "tests" 7 and 8 would still pass.
Please fix.