On Wed, Apr 28, 2021 at 8:25 AM Florent Revest <revest@xxxxxxxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
> ---
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
>
