perf probe: Show the error reason comes from invalid DSO

Show the reason of error when dso__load* fails. This shows when user
gives wrong kernel image or wrong path.

Without this, perf probe shows an obscure message:

  ----
  $ perf probe -k ~/kbin/linux-3.x86_64/vmlinux -L vfs_read
  Failed to find path of kernel module.
    Error: Failed to show lines.
  ----

With this, perf shows appropriate error message:

  ----
  $ perf probe -k ~/kbin/linux-3.x86_64/vmlinux -L vfs_read
  Failed to find the path for kernel: Mismatching build id
    Error: Failed to show lines.
  ----

And:

  ----
  $ perf probe -k /non-exist/kernel/vmlinux -L vfs_read
  Failed to find the path for kernel: No such file or directory
    Error: Failed to show lines.
  ----

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Richard Weinberger <richard@nod.at>
Link: http://lkml.kernel.org/r/20150527083718.23880.84100.stgit@localhost.localdomain
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.c

@@ -200,11 +200,12 @@ static void put_target_map(struct map *map, bool user)
 }
 
 
-static struct dso *kernel_get_module_dso(const char *module)
+static int kernel_get_module_dso(const char *module, struct dso **pdso)
 {
 	struct dso *dso;
 	struct map *map;
 	const char *vmlinux_name;
+	int ret = 0;
 
 	if (module) {
 		list_for_each_entry(dso, &host_machine->kernel_dsos.head,
@@ -214,30 +215,21 @@ static struct dso *kernel_get_module_dso(const char *module)
 				goto found;
 		}
 		pr_debug("Failed to find module %s.\n", module);
-		return NULL;
+		return -ENOENT;
 	}
 
 	map = host_machine->vmlinux_maps[MAP__FUNCTION];
 	dso = map->dso;
 
 	vmlinux_name = symbol_conf.vmlinux_name;
-	if (vmlinux_name) {
-		if (dso__load_vmlinux(dso, map, vmlinux_name, false, NULL) <= 0)
-			return NULL;
-	} else {
-		if (dso__load_vmlinux_path(dso, map, NULL) <= 0) {
-			pr_debug("Failed to load kernel map.\n");
-			return NULL;
-		}
-	}
+	dso->load_errno = 0;
+	if (vmlinux_name)
+		ret = dso__load_vmlinux(dso, map, vmlinux_name, false, NULL);
+	else
+		ret = dso__load_vmlinux_path(dso, map, NULL);
 found:
-	return dso;
-}
-
-const char *kernel_get_module_path(const char *module)
-{
-	struct dso *dso = kernel_get_module_dso(module);
-	return (dso) ? dso->long_name : NULL;
+	*pdso = dso;
+	return ret;
 }
 
 static int convert_exec_to_group(const char *exec, char **result)
@@ -389,16 +381,25 @@ static int get_alternative_line_range(struct debuginfo *dinfo,
 static struct debuginfo *open_debuginfo(const char *module, bool silent)
 {
 	const char *path = module;
-	struct debuginfo *ret;
+	char reason[STRERR_BUFSIZE];
+	struct debuginfo *ret = NULL;
+	struct dso *dso = NULL;
+	int err;
 
 	if (!module || !strchr(module, '/')) {
-		path = kernel_get_module_path(module);
-		if (!path) {
+		err = kernel_get_module_dso(module, &dso);
+		if (err < 0) {
+			if (!dso || dso->load_errno == 0) {
+				if (!strerror_r(-err, reason, STRERR_BUFSIZE))
+					strcpy(reason, "(unknown)");
+			} else
+				dso__strerror_load(dso, reason, STRERR_BUFSIZE);
 			if (!silent)
-				pr_err("Failed to find path of %s module.\n",
-				       module ?: "kernel");
+				pr_err("Failed to find the path for %s: %s\n",
+					module ?: "kernel", reason);
 			return NULL;
 		}
+		path = dso->long_name;
 	}
 	ret = debuginfo__new(path);
 	if (!ret && !silent) {

tools/perf/util/probe-event.h

@@ -131,9 +131,6 @@ extern void line_range__clear(struct line_range *lr);
 /* Initialize line range */
 extern int line_range__init(struct line_range *lr);
 
-/* Internal use: Return kernel/module path */
-extern const char *kernel_get_module_path(const char *module);
-
 extern int add_perf_probe_events(struct perf_probe_event *pevs, int npevs);
 extern int del_perf_probe_events(struct strfilter *filter);
 extern int show_perf_probe_events(struct strfilter *filter);