Re: [PATCH v3] perf test: Skip sigtrap test on old kernels

[Ian Rogers]

On Wed, Sep 7, 2022 at 12:12 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
>
> If it runs on an old kernel, perf_event_open would fail because of the
> new fields sigtrap and sig_data.  Just skipping the test could miss an
> actual bug in the kernel.
>
> Let's check BTF if it has the perf_event_attr.sigtrap field.
>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Acked-by: Song Liu <song@xxxxxxxxxx>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
> * move #include under #ifdef
> * return true when BPF is not supported
> * update comment
>
>  tools/perf/tests/sigtrap.c | 50 +++++++++++++++++++++++++++++++++++++-
>  1 file changed, 49 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/tests/sigtrap.c b/tools/perf/tests/sigtrap.c
> index e32ece90e164..cdf75eab6a8a 100644
> --- a/tools/perf/tests/sigtrap.c
> +++ b/tools/perf/tests/sigtrap.c
> @@ -54,6 +54,48 @@ static struct perf_event_attr make_event_attr(void)
>         return attr;
>  }
>
> +#ifdef HAVE_BPF_SKEL
> +#include <bpf/btf.h>
> +
> +static bool attr_has_sigtrap(void)
> +{
> +       bool ret = false;
> +       struct btf *btf;
> +       const struct btf_type *t;
> +       const struct btf_member *m;
> +       const char *name;
> +       int i, id;
> +
> +       btf = btf__load_vmlinux_btf();
> +       if (btf == NULL) {
> +               /* should be an old kernel */
> +               return false;
> +       }
> +
> +       id = btf__find_by_name_kind(btf, "perf_event_attr", BTF_KIND_STRUCT);
> +       if (id < 0)
> +               goto out;
> +
> +       t = btf__type_by_id(btf, id);
> +       for (i = 0, m = btf_members(t); i < btf_vlen(t); i++, m++) {
> +               name = btf__name_by_offset(btf, m->name_off);
> +               if (!strcmp(name, "sigtrap")) {
> +                       ret = true;
> +                       break;
> +               }
> +       }
> +out:
> +       btf__free(btf);
> +       return ret;
> +}
> +#else  /* !HAVE_BPF_SKEL */
> +static bool attr_has_sigtrap(void)
> +{
> +       /* to maintain current behavior */

nit: I don't think this comment will age well and the behavior of the
function is a bit counterintuitive. Perhaps:

/* If we don't have libbpf then guess we're on a newer kernel with
sigtrap support. This will mean the test will fail on older kernels.
*/

> +       return true;
> +}
> +#endif  /* HAVE_BPF_SKEL */
> +
>  static void
>  sigtrap_handler(int signum __maybe_unused, siginfo_t *info, void *ucontext __maybe_unused)
>  {
> @@ -139,7 +181,13 @@ static int test__sigtrap(struct test_suite *test __maybe_unused, int subtest __m
>
>         fd = sys_perf_event_open(&attr, 0, -1, -1, perf_event_open_cloexec_flag());
>         if (fd < 0) {
> -               pr_debug("FAILED sys_perf_event_open(): %s\n", str_error_r(errno, sbuf, sizeof(sbuf)));
> +               if (attr_has_sigtrap()) {
> +                       pr_debug("FAILED sys_perf_event_open(): %s\n",
> +                                str_error_r(errno, sbuf, sizeof(sbuf)));
> +               } else {
> +                       pr_debug("perf_event_attr doesn't have sigtrap\n");
> +                       ret = TEST_SKIP;
> +               }
>                 goto out_restore_sigaction;
>         }
>
> --
> 2.37.2.789.g6183377224-goog
>