Re: [PATCH] perf probe: Provide more detail with relocation warning

From: Masami Hiramatsu
Date: Tue May 25 2021 - 08:49:05 EST


On Tue, 25 May 2021 10:07:44 +0530
Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx> wrote:

> When run as normal user with default sysctl kernel.kptr_restrict=0
> and kernel.perf_event_paranoid=2, perf probe fails with:
>
> $ ./perf probe move_page_tables
> Relocated base symbol is not found!
>
> The warning message is not much informative. The reason perf fails
> is because /proc/kallsyms is restricted by perf_event_paranoid=2
> for normal user and thus perf fails to read relocated address of
> the base symbol.
>
> Tweaking kptr_restrict and perf_event_paranoid can change the
> behavior of perf probe. Also, running as root or privileged user
> works too. Add these details in the warning message.
>
> Plus, kmap->ref_reloc_sym might not be always set even if
> host_machine is initialized. Above is the example of the same.
> Remove that comment.

Yes, those are restricted in some cases. Anyway without priviledged
(super) user, perf probe can not set the probe in ftrace.

Hmm, I think it should check the effective user-id at first. If it
is not super user and the action will access tracefs and kallsyms,
it should warn at that point.

Thank you,

>
> Reported-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx>
> ---
> tools/perf/util/probe-event.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> index a78c8d59a555..3a7649835ec9 100644
> --- a/tools/perf/util/probe-event.c
> +++ b/tools/perf/util/probe-event.c
> @@ -108,7 +108,6 @@ void exit_probe_symbol_maps(void)
>
> static struct ref_reloc_sym *kernel_get_ref_reloc_sym(struct map **pmap)
> {
> - /* kmap->ref_reloc_sym should be set if host_machine is initialized */
> struct kmap *kmap;
> struct map *map = machine__kernel_map(host_machine);
>
> @@ -819,7 +818,10 @@ post_process_kernel_probe_trace_events(struct probe_trace_event *tevs,
>
> reloc_sym = kernel_get_ref_reloc_sym(&map);
> if (!reloc_sym) {
> - pr_warning("Relocated base symbol is not found!\n");
> + pr_warning("Relocated base symbol is not found! "
> + "Check /proc/sys/kernel/kptr_restrict\n"
> + "and /proc/sys/kernel/perf_event_paranoid. "
> + "Or run as privileged perf user.\n\n");
> return -EINVAL;
> }
>
> @@ -3025,7 +3027,10 @@ static int find_probe_trace_events_from_map(struct perf_probe_event *pev,
> (!pp->retprobe || kretprobe_offset_is_supported())) {
> reloc_sym = kernel_get_ref_reloc_sym(NULL);
> if (!reloc_sym) {
> - pr_warning("Relocated base symbol is not found!\n");
> + pr_warning("Relocated base symbol is not found! "
> + "Check /proc/sys/kernel/kptr_restrict\n"
> + "and /proc/sys/kernel/perf_event_paranoid. "
> + "Or run as privileged perf user.\n\n");
> ret = -EINVAL;
> goto out;
> }
> --
> 2.31.1
>


--
Masami Hiramatsu <mhiramat@xxxxxxxxxx>