Re: [PATCH v5 2/8] perf trace: BTF-based enum pretty printing for syscall args

[Ian Rogers]

On Fri, Jul 5, 2024 at 2:43 AM Howard Chu <howardchu95@xxxxxxxxx> wrote:
>
> In this patch, BTF is used to turn enum value to the corresponding
> name. There is only one system call that uses enum value as its
> argument, that is `landlock_add_rule()`.
>
> The vmlinux btf is loaded lazily, when user decided to trace the
> `landlock_add_rule` syscall. But if one decide to run `perf trace`
> without any arguments, the behaviour is to trace `landlock_add_rule`,
> so vmlinux btf will be loaded by default.
>
> The laziest behaviour is to load vmlinux btf when a
> `landlock_add_rule` syscall hits. But I think you could lose some
> samples when loading vmlinux btf at run time, for it can delay the
> handling of other samples. I might need your precious opinions on
> this...
>
> before:
>
> ```
> perf $ ./perf trace -e landlock_add_rule
>      0.000 ( 0.008 ms): ldlck-test/438194 landlock_add_rule(rule_type: 2) = -1 EBADFD (File descriptor in bad state)
>      0.010 ( 0.001 ms): ldlck-test/438194 landlock_add_rule(rule_type: 1) = -1 EBADFD (File descriptor in bad state)
> ```
>
> after:
>
> ```
> perf $ ./perf trace -e landlock_add_rule
>      0.000 ( 0.029 ms): ldlck-test/438194 landlock_add_rule(rule_type: LANDLOCK_RULE_NET_PORT)     = -1 EBADFD (File descriptor in bad state)
>      0.036 ( 0.004 ms): ldlck-test/438194 landlock_add_rule(rule_type: LANDLOCK_RULE_PATH_BENEATH) = -1 EBADFD (File descriptor in bad state)
> ```
>
> Committer notes:
>
> Made it build with NO_LIBBPF=1, simplified btf_enum_fprintf(), see [1]
> for the discussion.
>
> Signed-off-by: Howard Chu <howardchu95@xxxxxxxxx>
> Tested-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
> Cc: Günther Noack <gnoack@xxxxxxxxxx>
> Cc: Ian Rogers <irogers@xxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Mickaël Salaün <mic@xxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/20240613022757.3589783-1-howardchu95@xxxxxxxxx
> Link: https://lore.kernel.org/lkml/ZnXAhFflUl_LV1QY@x1 # [1]
> Link: https://lore.kernel.org/r/20240624181345.124764-3-howardchu95@xxxxxxxxx
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> ---
>  tools/perf/builtin-trace.c | 110 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 106 insertions(+), 4 deletions(-)
>
> diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
> index 8449f2beb54d..1391564911d9 100644
> --- a/tools/perf/builtin-trace.c
> +++ b/tools/perf/builtin-trace.c
> @@ -19,6 +19,7 @@
>  #ifdef HAVE_LIBBPF_SUPPORT
>  #include <bpf/bpf.h>
>  #include <bpf/libbpf.h>
> +#include <bpf/btf.h>
>  #ifdef HAVE_BPF_SKEL
>  #include "bpf_skel/augmented_raw_syscalls.skel.h"
>  #endif
> @@ -110,6 +111,10 @@ struct syscall_arg_fmt {
>         const char *name;
>         u16        nr_entries; // for arrays
>         bool       show_zero;
> +       bool       is_enum;
> +#ifdef HAVE_LIBBPF_SUPPORT
> +       const struct btf_type *type;
> +#endif
>  };
>
>  struct syscall_fmt {
> @@ -139,6 +144,9 @@ struct trace {
>         } syscalls;
>  #ifdef HAVE_BPF_SKEL
>         struct augmented_raw_syscalls_bpf *skel;
> +#endif
> +#ifdef HAVE_LIBBPF_SUPPORT
> +       struct btf              *btf;
>  #endif
>         struct record_opts      opts;
>         struct evlist   *evlist;
> @@ -204,6 +212,20 @@ struct trace {
>         } oe;
>  };
>
> +static void trace__load_vmlinux_btf(struct trace *trace __maybe_unused)
> +{
> +#ifdef HAVE_LIBBPF_SUPPORT
> +       if (trace->btf != NULL)
> +               return;
> +
> +       trace->btf = btf__load_vmlinux_btf();
> +       if (verbose > 0) {
> +               fprintf(trace->output, trace->btf ? "vmlinux BTF loaded\n" :
> +                                                   "Failed to load vmlinux BTF\n");
> +       }
> +#endif
> +}
> +
>  struct tp_field {
>         int offset;
>         union {
> @@ -887,6 +909,64 @@ static size_t syscall_arg__scnprintf_getrandom_flags(char *bf, size_t size,
>
>  #define SCA_GETRANDOM_FLAGS syscall_arg__scnprintf_getrandom_flags
>
> +#ifdef HAVE_LIBBPF_SUPPORT
> +static int syscall_arg_fmt__cache_btf_enum(struct syscall_arg_fmt *arg_fmt, struct btf *btf, char *type)
> +{
> +       int id;
> +
> +       // Already cached?

Hi Howard,

I'm on nit patrol again, just to note that most comments are /* */
style although we do have // comments - the ratio of /* */ to // is
about 10:1. The kernel is similar in this regard. There's no clear
guidance so we could ignore this, but fwiw I generally try to stick
with /* */ for consistency with the rest of the surrounding code.

Thanks,
Ian
> +       if (arg_fmt->type != NULL)
> +               return 0;
> +
> +       type = strstr(type, "enum ");
> +       if (type == NULL)
> +               return -1;
> +
> +       type += 5; // skip "enum " to get the enumeration name
> +
> +       id = btf__find_by_name(btf, type);
> +       if (id < 0)
> +               return -1;
> +
> +       arg_fmt->type = btf__type_by_id(btf, id);
> +       return arg_fmt->type == NULL ? -1 : 0;
> +}
> +
> +static size_t btf_enum_scnprintf(const struct btf_type *type, struct btf *btf, char *bf, size_t size, int val)
> +{
> +       struct btf_enum *be = btf_enum(type);
> +       const int nr_entries = btf_vlen(type);
> +
> +       for (int i = 0; i < nr_entries; ++i, ++be) {
> +               if (be->val == val) {
> +                       return scnprintf(bf, size, "%s",
> +                                        btf__name_by_offset(btf, be->name_off));
> +               }
> +       }
> +
> +       return 0;
> +}
> +
> +static size_t trace__btf_enum_scnprintf(struct trace *trace, struct syscall_arg_fmt *arg_fmt, char *bf,
> +                                       size_t size, int val, char *type)
> +{
> +       if (trace->btf == NULL)
> +               return 0;
> +
> +       if (syscall_arg_fmt__cache_btf_enum(arg_fmt, trace->btf, type) < 0)
> +               return 0;
> +
> +       return btf_enum_scnprintf(arg_fmt->type, trace->btf, bf, size, val);
> +}
> +#else // HAVE_LIBBPF_SUPPORT
> +static size_t trace__btf_enum_scnprintf(struct trace *trace __maybe_unused, struct syscall_arg_fmt *arg_fmt __maybe_unused,
> +                                       char *bf __maybe_unused, size_t size __maybe_unused, int val __maybe_unused,
> +                                       char *type __maybe_unused)
> +{
> +       return 0;
> +}
> +#endif // HAVE_LIBBPF_SUPPORT
> +
>  #define STRARRAY(name, array) \
>           { .scnprintf  = SCA_STRARRAY, \
>             .strtoul    = STUL_STRARRAY, \
> @@ -1238,6 +1318,7 @@ struct syscall {
>         bool                is_exit;
>         bool                is_open;
>         bool                nonexistent;
> +       bool                use_btf;
>         struct tep_format_field *args;
>         const char          *name;
>         const struct syscall_fmt  *fmt;
> @@ -1744,7 +1825,8 @@ static const struct syscall_arg_fmt *syscall_arg_fmt__find_by_name(const char *n
>  }
>
>  static struct tep_format_field *
> -syscall_arg_fmt__init_array(struct syscall_arg_fmt *arg, struct tep_format_field *field)
> +syscall_arg_fmt__init_array(struct syscall_arg_fmt *arg, struct tep_format_field *field,
> +                           bool *use_btf)
>  {
>         struct tep_format_field *last_field = NULL;
>         int len;
> @@ -1756,6 +1838,7 @@ syscall_arg_fmt__init_array(struct syscall_arg_fmt *arg, struct tep_format_field
>                         continue;
>
>                 len = strlen(field->name);
> +               arg->is_enum = false;
>
>                 if (strcmp(field->type, "const char *") == 0 &&
>                     ((len >= 4 && strcmp(field->name + len - 4, "name") == 0) ||
> @@ -1782,6 +1865,8 @@ syscall_arg_fmt__init_array(struct syscall_arg_fmt *arg, struct tep_format_field
>                          * 7 unsigned long
>                          */
>                         arg->scnprintf = SCA_FD;
> +               } else if (strstr(field->type, "enum") && use_btf != NULL) {
> +                       *use_btf = arg->is_enum = true;
>                 } else {
>                         const struct syscall_arg_fmt *fmt =
>                                 syscall_arg_fmt__find_by_name(field->name);
> @@ -1798,7 +1883,8 @@ syscall_arg_fmt__init_array(struct syscall_arg_fmt *arg, struct tep_format_field
>
>  static int syscall__set_arg_fmts(struct syscall *sc)
>  {
> -       struct tep_format_field *last_field = syscall_arg_fmt__init_array(sc->arg_fmt, sc->args);
> +       struct tep_format_field *last_field = syscall_arg_fmt__init_array(sc->arg_fmt, sc->args,
> +                                                                         &sc->use_btf);
>
>         if (last_field)
>                 sc->args_size = last_field->offset + last_field->size;
> @@ -1811,6 +1897,7 @@ static int trace__read_syscall_info(struct trace *trace, int id)
>         char tp_name[128];
>         struct syscall *sc;
>         const char *name = syscalltbl__name(trace->sctbl, id);
> +       int err;
>
>  #ifdef HAVE_SYSCALL_TABLE_SUPPORT
>         if (trace->syscalls.table == NULL) {
> @@ -1883,7 +1970,13 @@ static int trace__read_syscall_info(struct trace *trace, int id)
>         sc->is_exit = !strcmp(name, "exit_group") || !strcmp(name, "exit");
>         sc->is_open = !strcmp(name, "open") || !strcmp(name, "openat");
>
> -       return syscall__set_arg_fmts(sc);
> +       err = syscall__set_arg_fmts(sc);
> +
> +       /* after calling syscall__set_arg_fmts() we'll know whether use_btf is true */
> +       if (sc->use_btf)
> +               trace__load_vmlinux_btf(trace);
> +
> +       return err;
>  }
>
>  static int evsel__init_tp_arg_scnprintf(struct evsel *evsel)
> @@ -1891,7 +1984,7 @@ static int evsel__init_tp_arg_scnprintf(struct evsel *evsel)
>         struct syscall_arg_fmt *fmt = evsel__syscall_arg_fmt(evsel);
>
>         if (fmt != NULL) {
> -               syscall_arg_fmt__init_array(fmt, evsel->tp_format->format.fields);
> +               syscall_arg_fmt__init_array(fmt, evsel->tp_format->format.fields, NULL);
>                 return 0;
>         }
>
> @@ -2103,6 +2196,15 @@ static size_t syscall__scnprintf_args(struct syscall *sc, char *bf, size_t size,
>                         if (trace->show_arg_names)
>                                 printed += scnprintf(bf + printed, size - printed, "%s: ", field->name);
>
> +                       if (sc->arg_fmt[arg.idx].is_enum) {
> +                               size_t p = trace__btf_enum_scnprintf(trace, &sc->arg_fmt[arg.idx], bf + printed,
> +                                                                    size - printed, val, field->type);
> +                               if (p) {
> +                                       printed += p;
> +                                       continue;
> +                               }
> +                       }
> +
>                         printed += syscall_arg_fmt__scnprintf_val(&sc->arg_fmt[arg.idx],
>                                                                   bf + printed, size - printed, &arg, val);
>                 }
> --
> 2.45.2
>