Date:   Mon, 6 Dec 2021 19:09:57 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Hou Tao <houtao1@...wei.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 5/5] selftests/bpf: add test cases for bpf_strncmp()

On Tue, Nov 30, 2021 at 6:07 AM Hou Tao <houtao1@...wei.com> wrote:
>
> Four test cases are added:
> (1) ensure the return value is expected
> (2) ensure no const size is rejected
> (3) ensure writable str is rejected
> (4) ensure no null-terminated str is rejected
>
> Signed-off-by: Hou Tao <houtao1@...wei.com>
> ---
>  .../selftests/bpf/prog_tests/test_strncmp.c   | 170 ++++++++++++++++++
>  .../selftests/bpf/progs/strncmp_test.c        |  59 ++++++
>  2 files changed, 229 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/test_strncmp.c
>  create mode 100644 tools/testing/selftests/bpf/progs/strncmp_test.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/test_strncmp.c b/tools/testing/selftests/bpf/prog_tests/test_strncmp.c
> new file mode 100644
> index 000000000000..3ed54b55f96a
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/test_strncmp.c
> @@ -0,0 +1,170 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (C) 2021. Huawei Technologies Co., Ltd */
> +#include <test_progs.h>
> +#include "strncmp_test.skel.h"
> +
> +static struct strncmp_test *strncmp_test_open_and_disable_autoload(void)
> +{
> +       struct strncmp_test *skel;
> +       struct bpf_program *prog;
> +
> +       skel = strncmp_test__open();
> +       if (libbpf_get_error(skel))
> +               return skel;
> +
> +       bpf_object__for_each_program(prog, skel->obj)
> +               bpf_program__set_autoload(prog, false);

I think this is a wrong "code economy". You save few lines of code,
but make tests harder to follow. Just do 4 lines of code for each
subtest:

skel = strncmp_test__open();
if (!ASSERT_OK_PTR(skel, "skel_open"))
    return;

bpf_object__for_each_program(prog, skel->obj)
    bpf_program__set_autoload(prog, false);


It makes tests more self-contained and easier to follow. Also if some
tests need to do something slightly different it's easier to modify
them, as they are not coupled to some common helper. DRY is good where
it makes sense, but it also increases code coupling and more "jumping
around" in code, so it shouldn't be applied blindly.

> +
> +       return skel;
> +}
> +
> +static inline int to_tristate_ret(int ret)
> +{
> +       if (ret > 0)
> +               return 1;
> +       if (ret < 0)
> +               return -1;
> +       return 0;
> +}
> +
> +static int trigger_strncmp(const struct strncmp_test *skel)
> +{
> +       struct timespec wait = {.tv_sec = 0, .tv_nsec = 1};
> +
> +       nanosleep(&wait, NULL);

all the other tests are just doing usleep(1), why using this more verbose way?

> +       return to_tristate_ret(skel->bss->cmp_ret);
> +}
> +
> +/*
> + * Compare str and target after making str[i] != target[i].
> + * When exp is -1, make str[i] < target[i] and delta is -1.
> + */
> +static void strncmp_full_str_cmp(struct strncmp_test *skel, const char *name,
> +                                int exp)
> +{
> +       size_t nr = sizeof(skel->bss->str);
> +       char *str = skel->bss->str;
> +       int delta = exp;
> +       int got;
> +       size_t i;
> +
> +       memcpy(str, skel->rodata->target, nr);
> +       for (i = 0; i < nr - 1; i++) {
> +               str[i] += delta;
> +
> +               got = trigger_strncmp(skel);
> +               ASSERT_EQ(got, exp, name);
> +
> +               str[i] -= delta;
> +       }
> +}
> +

[...]

> diff --git a/tools/testing/selftests/bpf/progs/strncmp_test.c b/tools/testing/selftests/bpf/progs/strncmp_test.c
> new file mode 100644
> index 000000000000..8cdf950a0ce1
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/strncmp_test.c
> @@ -0,0 +1,59 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/* Copyright (C) 2021. Huawei Technologies Co., Ltd */
> +#include <stdbool.h>
> +#include <linux/types.h>
> +#include <linux/bpf.h>
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +
> +#define STRNCMP_STR_SZ 8
> +
> +const char target[STRNCMP_STR_SZ] = "EEEEEEE";
> +
> +char str[STRNCMP_STR_SZ];
> +int cmp_ret = 0;
> +int target_pid = 0;
> +
> +char bad_target[STRNCMP_STR_SZ];
> +unsigned int bad_cmp_str_size = STRNCMP_STR_SZ;
> +
> +char _license[] SEC("license") = "GPL";
> +
> +static __always_inline bool called_by_target_pid(void)
> +{
> +       __u32 pid = bpf_get_current_pid_tgid() >> 32;
> +
> +       return pid == target_pid;
> +}

again, what's the point of this helper? it's used once and you'd
actually save the code by doing the following inline:

if ((bpf_get_current_pid_tgid() >> 32) != target_pid)
    return 0;

> +
> +SEC("tp/syscalls/sys_enter_nanosleep")
> +int do_strncmp(void *ctx)
> +{
> +       if (!called_by_target_pid())
> +               return 0;
> +
> +       cmp_ret = bpf_strncmp(str, STRNCMP_STR_SZ, target);
> +
> +       return 0;
> +}
> +
> +SEC("tp/syscalls/sys_enter_nanosleep")
> +int strncmp_bad_not_const_str_size(void *ctx)
> +{

probably worth leaving a short comment explaining that this program
should fail because ...

> +       cmp_ret = bpf_strncmp(str, bad_cmp_str_size, target);
> +       return 0;
> +}
> +
> +SEC("tp/syscalls/sys_enter_nanosleep")
> +int strncmp_bad_writable_target(void *ctx)
> +{
> +       cmp_ret = bpf_strncmp(str, STRNCMP_STR_SZ, bad_target);
> +       return 0;
> +}
> +
> +SEC("tp/syscalls/sys_enter_nanosleep")
> +int strncmp_bad_not_null_term_target(void *ctx)
> +{
> +       cmp_ret = bpf_strncmp(str, STRNCMP_STR_SZ, target);
> +       return 0;
> +}
> --
> 2.29.2
>

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