linux-kernel - [PATCH 2/4] perf probe: Fix failure to probe online module

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Date:	Wed, 1 Jul 2015 09:05:37 +0000
From:	He Kuang <hekuang@...wei.com>
To:	<acme@...nel.org>, <a.p.zijlstra@...llo.nl>, <mingo@...hat.com>,
	<masami.hiramatsu.pt@...achi.com>, <namhyung@...nel.org>,
	<jolsa@...nel.org>
CC:	<wangnan0@...wei.com>, <linux-kernel@...r.kernel.org>
Subject: [PATCH 2/4] perf probe: Fix failure to probe online module

Kernel module has the type DSO_TYPE_USER, the check on dso type in
kernel_get_module_dso() disable openning debuginfo of online kernel
modules.

Problem can be reproduced as:

  $ insmod test_bpf.ko

  $ perf probe -v -m test_bpf --add='skb_is_nonlinear'

  probe-definition(0): skb_is_nonlinear
  symbol:skb_is_nonlinear file:(null) line:0 offset:0 return:0 lazy:(null)
  0 arguments
  Failed to find module test_bpf.
  Could not open debuginfo. Try to use symbols.
  Failed to find symbol skb_is_nonlinear in test_bpf
    Error: Failed to add events. Reason: No such file or directory (Code: -2)

Infact kernel_get_module_dso() function is called in perf-probe, in this
condition, machine->dsos list contains only dsos which is either kmod or
kernel loaded by machine__create_kernel_maps(), so there's no need to
pick away the non-kmod user type dsos.

This patch removes the check.

After this patch:

  $ perf probe -v -m test_bpf --add='skb_is_nonlinear'

  probe-definition(0): skb_is_nonlinear
  symbol:skb_is_nonlinear file:(null) line:0 offset:0 return:0 lazy:(null)
  0 arguments
  Open Debuginfo file: /lib/modules/4.1.0+/test_bpf.ko
  Try to find probe point from debuginfo.
  Matched function: skb_is_nonlinear
  found inline addr: 0x10ab3
  Probe point found: test_bpf_init+585
  Found 1 probe_trace_events.
  Opening /sys/kernel/debug/tracing/kprobe_events write=1
  Added new event:
  Writing event: p:probe/skb_is_nonlinear test_bpf:test_bpf_init+585
    probe:skb_is_nonlinear (on skb_is_nonlinear in test_bpf)

  You can now use it in all perf tools, such as:

      perf record -e probe:skb_is_nonlinear -aR sleep 1

Signed-off-by: He Kuang <hekuang@...wei.com>
---
 tools/perf/util/probe-event.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index 381f23a..8ad2743 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -275,8 +275,6 @@ static int kernel_get_module_dso(const char *module, struct dso **pdso)
 
 	if (module) {
 		list_for_each_entry(dso, &host_machine->dsos.head, node) {
-			if (!dso->kernel)
-				continue;
 			if (strncmp(dso->short_name + 1, module,
 				    dso->short_name_len - 2) == 0)
 				goto found;
-- 
1.8.5.2

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