Re: [PATCH 1/2] perf/probe: Report possible permission error for map__load failure

[Namhyung Kim]

Hi Arnaldo and Masami,

On Fri, Jun 4, 2021 at 12:18 PM Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx> wrote:
>
> Em Sat, Jun 05, 2021 at 12:30:58AM +0900, Masami Hiramatsu escreveu:
> > Report possible permission error including kptr_restrict setting
> > for map__load() failure. This can happen when non-superuser runs
> > perf probe.
> >
> > With this patch, perf probe shows the following message.
> >
> >  $ perf probe vfs_read
> >  Failed to load symbols from /proc/kallsyms
> >  Please ensure you can read the /proc/kallsyms symbol addresses.
> >  If the /proc/sys/kernel/kptr_restrict is '2', you can not read
> >  kernel symbol address even if you are a superuser. Please change
> >  it to '1'. If kptr_restrict is '1', the superuser can read the
> >  symbol addresses.
> >  In that case, please run this command again with sudo.
> >    Error: Failed to add events.

Maybe we can read the kptr_restrict file and (e)uid to suggest
the specific message for the situation only.

Thanks,
Namhyung

>
>
> > Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > ---
> >  tools/perf/util/probe-event.c |   25 ++++++++++++++++++++++---
> >  1 file changed, 22 insertions(+), 3 deletions(-)
> >
> > diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> > index 3a7649835ec9..8fe179d671c3 100644
> > --- a/tools/perf/util/probe-event.c
> > +++ b/tools/perf/util/probe-event.c
> > @@ -2936,7 +2936,7 @@ static int find_probe_functions(struct map *map, char *name,
> >       bool cut_version = true;
> >
> >       if (map__load(map) < 0)
> > -             return 0;
> > +             return -EACCES; /* Possible permission error to load symbols */
> >
> >       /* If user gives a version, don't cut off the version from symbols */
> >       if (strchr(name, '@'))
> > @@ -2975,6 +2975,17 @@ void __weak arch__fix_tev_from_maps(struct perf_probe_event *pev __maybe_unused,
> >                               struct map *map __maybe_unused,
> >                               struct symbol *sym __maybe_unused) { }
> >
> > +
> > +static void pr_kallsyms_access_error(void)
> > +{
> > +     pr_err("Please ensure you can read the /proc/kallsyms symbol addresses.\n"
> > +            "If the /proc/sys/kernel/kptr_restrict is '2', you can not read\n"
> > +            "kernel symbol address even if you are a superuser. Please change\n"
> > +            "it to '1'. If kptr_restrict is '1', the superuser can read the\n"
> > +            "symbol addresses.\n"
> > +            "In that case, please run this command again with sudo.\n");
> > +}
> > +
> >  /*
> >   * Find probe function addresses from map.
> >   * Return an error or the number of found probe_trace_event
> > @@ -3011,8 +3022,16 @@ static int find_probe_trace_events_from_map(struct perf_probe_event *pev,
> >        */
> >       num_matched_functions = find_probe_functions(map, pp->function, syms);
> >       if (num_matched_functions <= 0) {
> > -             pr_err("Failed to find symbol %s in %s\n", pp->function,
> > -                     pev->target ? : "kernel");
> > +             if (num_matched_functions == -EACCES) {
> > +                     pr_err("Failed to load symbols from %s\n",
> > +                            pev->target ?: "/proc/kallsyms");
> > +                     if (pev->target)
> > +                             pr_err("Please ensure the file is not stripped.\n");
> > +                     else
> > +                             pr_kallsyms_access_error();
> > +             } else
> > +                     pr_err("Failed to find symbol %s in %s\n", pp->function,
> > +                             pev->target ? : "kernel");
> >               ret = -ENOENT;
> >               goto out;
> >       } else if (num_matched_functions > probe_conf.max_probes) {
> >
>
> --
>
> - Arnaldo