Commit 41ca1d1e authored by Ravi Bangoria's avatar Ravi Bangoria Committed by Arnaldo Carvalho de Melo
Browse files

perf probe: Provide more detail with relocation warning 



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.

Reported-by: default avatarAneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: default avatarRavi Bangoria <ravi.bangoria@linux.ibm.com>
Acked-by: default avatarMasami Hiramatsu <mhiramat@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/20210525043744.193297-1-ravi.bangoria@linux.ibm.com


Signed-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
parent 6793672a

tools/perf/util/probe-event.c

+8 −3
Original line number Diff line number Diff line
@@ -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;

		}