[PATCH] perf probe: Try to use symbol table if searching debug info failed

[Wang Nan]

A problem can occure in statically linked perf when vmlinux can be found:

 # perf probe --add sys_epoll_pwait
 probe-definition(0): sys_epoll_pwait
 symbol:sys_epoll_pwait file:(null) line:0 offset:0 return:0 lazy:(null)
 0 arguments
 Looking at the vmlinux_path (7 entries long)
 Using /lib/modules/4.2.0-rc1+/build/vmlinux for symbols
 Open Debuginfo file: /lib/modules/4.2.0-rc1+/build/vmlinux
 Try to find probe point from debuginfo.
 Symbol sys_epoll_pwait address found : ffffffff8122bd40
 Matched function: SyS_epoll_pwait
 Failed to get call frame on 0xffffffff8122bd40
 An error occurred in debuginfo analysis (-2).
   Error: Failed to add events. Reason: No such file or directory (Code: -2)

The reason is caused by libdw that, if libdw is statically linked, it can't
load libebl_{arch}.so reliable.

In this case it is still possible to get the address from /proc/kalksyms.
However, perf tries that only when libdw returns -EBADF.

This patch gives it another chance to utilize symbol table, even if libdw
return error code other than -EBADF.

After applying this patch:

 # perf probe -nv --add sys_epoll_pwait
 probe-definition(0): sys_epoll_pwait
 symbol:sys_epoll_pwait file:(null) line:0 offset:0 return:0 lazy:(null)
 0 arguments
 Looking at the vmlinux_path (7 entries long)
 Using /lib/modules/4.2.0-rc1+/build/vmlinux for symbols
 Open Debuginfo file: /lib/modules/4.2.0-rc1+/build/vmlinux
 Try to find probe point from debuginfo.
 Symbol sys_epoll_pwait address found : ffffffff8122bd40
 Matched function: SyS_epoll_pwait
 Failed to get call frame on 0xffffffff8122bd40
 An error occurred in debuginfo analysis (-2).
 Trying to use symbols.
 Opening /sys/kernel/debug/tracing/kprobe_events write=1
 Added new event:
 Writing event: p:probe/sys_epoll_pwait _text+2276672
   probe:sys_epoll_pwait (on sys_epoll_pwait)

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

 	perf record -e probe:sys_epoll_pwait -aR sleep 1

Although libdw returns an error (Failed to get call frame),
perf tries symbol table and finally gets correct address.

Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
---
 tools/perf/util/probe-event.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
index fe4941a..f07374b 100644
--- a/tools/perf/util/probe-event.c
+++ b/tools/perf/util/probe-event.c
@@ -705,9 +705,10 @@ static int try_to_find_probe_trace_events(struct perf_probe_event *pev,
 	}
 	/* Error path : ntevs < 0 */
 	pr_debug("An error occurred in debuginfo analysis (%d).\n", ntevs);
-	if (ntevs == -EBADF) {
-		pr_warning("Warning: No dwarf info found in the vmlinux - "
-			"please rebuild kernel with CONFIG_DEBUG_INFO=y.\n");
+	if (ntevs < 0) {
+		if (ntevs == -EBADF)
+			pr_warning("Warning: No dwarf info found in the vmlinux - "
+				"please rebuild kernel with CONFIG_DEBUG_INFO=y.\n");
 		if (!need_dwarf) {
 			pr_debug("Trying to use symbols.\n");
 			return 0;
-- 
1.8.3.4

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