linux-kernel - Re: [PATCH bpf-next] selftests/bpf: Fix the snprintf test

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Message-ID: <CAEf4BzbXqSDL84j3Cb5WOCcghqN=C7eUEPKmw-h8bmN6EyyPwQ@mail.gmail.com>
Date:   Fri, 30 Apr 2021 10:46:07 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Florent Revest <revest@...omium.org>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        KP Singh <kpsingh@...nel.org>,
        Brendan Jackman <jackmanb@...gle.com>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] selftests/bpf: Fix the snprintf test

On Wed, Apr 28, 2021 at 8:25 AM Florent Revest <revest@...omium.org> wrote:
>
> The BPF program for the snprintf selftest runs on all syscall entries.
> On busy multicore systems this can cause concurrency issues.
>
> For example it was observed that sometimes the userspace part of the
> test reads "    4 0000" instead of "    4 000" (extra '0' at the end)
> which seems to happen just before snprintf on another core sets
> end[-1] = '\0'.
>
> This patch adds a pid filter to the test to ensure that no
> bpf_snprintf() will write over the test's output buffers while the
> userspace reads the values.
>
> Fixes: c2e39c6bdc7e ("selftests/bpf: Add a series of tests for bpf_snprintf")
> Reported-by: Andrii Nakryiko <andrii@...nel.org>
> Signed-off-by: Florent Revest <revest@...omium.org>
> ---

Applied to bpf tree. Thanks.

>  tools/testing/selftests/bpf/prog_tests/snprintf.c | 2 ++
>  tools/testing/selftests/bpf/progs/test_snprintf.c | 5 +++++
>  2 files changed, 7 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/snprintf.c b/tools/testing/selftests/bpf/prog_tests/snprintf.c
> index a958c22aec75..dffbcaa1ec98 100644
> --- a/tools/testing/selftests/bpf/prog_tests/snprintf.c
> +++ b/tools/testing/selftests/bpf/prog_tests/snprintf.c
> @@ -43,6 +43,8 @@ void test_snprintf_positive(void)
>         if (!ASSERT_OK_PTR(skel, "skel_open"))
>                 return;
>
> +       skel->bss->pid = getpid();
> +
>         if (!ASSERT_OK(test_snprintf__attach(skel), "skel_attach"))
>                 goto cleanup;
>
> diff --git a/tools/testing/selftests/bpf/progs/test_snprintf.c b/tools/testing/selftests/bpf/progs/test_snprintf.c
> index 951a0301c553..e35129bea0a0 100644
> --- a/tools/testing/selftests/bpf/progs/test_snprintf.c
> +++ b/tools/testing/selftests/bpf/progs/test_snprintf.c
> @@ -5,6 +5,8 @@
>  #include <bpf/bpf_helpers.h>
>  #include <bpf/bpf_tracing.h>
>
> +__u32 pid = 0;
> +
>  char num_out[64] = {};
>  long num_ret = 0;
>
> @@ -42,6 +44,9 @@ int handler(const void *ctx)
>         static const char str1[] = "str1";
>         static const char longstr[] = "longstr";
>
> +       if ((int)bpf_get_current_pid_tgid() != pid)
> +               return 0;
> +
>         /* Integer types */
>         num_ret  = BPF_SNPRINTF(num_out, sizeof(num_out),
>                                 "%d %u %x %li %llu %lX",
> --
> 2.31.1.498.g6c1eba8ee3d-goog
>