perf tools: Use scandir() to iterate threads when synthesizing PERF_RECORD_ events

Like in __event__synthesize_thread(), I think it's better to use
scandir() instead of the readdir() loop. In case some malicious task
continues to create new threads, the readdir() loop will run over and
over to collect tids. The scandir() also has the problem but the window
is much smaller since it doesn't do much work during the iteration.

Also add filter_task() function as we only care the tasks.

Signed-off-by: Namhyung Kim <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Andi Kleen <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Stephane Eranian <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/synthetic-events.c

···
 	return rc;
 }
 
+static int filter_task(const struct dirent *dirent)
+{
+	return isdigit(dirent->d_name[0]);
+}
+
 static int __event__synthesize_thread(union perf_event *comm_event,
 				      union perf_event *mmap_event,
 				      union perf_event *fork_event,
···
 				      struct perf_tool *tool, struct machine *machine, bool mmap_data)
 {
 	char filename[PATH_MAX];
-	DIR *tasks;
-	struct dirent *dirent;
+	struct dirent **dirent;
 	pid_t tgid, ppid;
 	int rc = 0;
+	int i, n;
 
 	/* special case: only send one comm event using passed in pid */
 	if (!full) {
···
 	snprintf(filename, sizeof(filename), "%s/proc/%d/task",
 		 machine->root_dir, pid);
 
-	tasks = opendir(filename);
-	if (tasks == NULL) {
-		pr_debug("couldn't open %s\n", filename);
-		return 0;
-	}
+	n = scandir(filename, &dirent, filter_task, alphasort);
+	if (n < 0)
+		return n;
 
-	while ((dirent = readdir(tasks)) != NULL) {
+	for (i = 0; i < n; i++) {
 		char *end;
 		pid_t _pid;
 		bool kernel_thread;
 
-		_pid = strtol(dirent->d_name, &end, 10);
+		_pid = strtol(dirent[i]->d_name, &end, 10);
 		if (*end)
 			continue;
 
···
 		}
 	}
 
-	closedir(tasks);
+	for (i = 0; i < n; i++)
+		zfree(&dirent[i]);
+	free(dirent);
+
 	return rc;
 }
 
···
 		return 0;
 
 	snprintf(proc_path, sizeof(proc_path), "%s/proc", machine->root_dir);
-	n = scandir(proc_path, &dirent, 0, alphasort);
+	n = scandir(proc_path, &dirent, filter_task, alphasort);
 	if (n < 0)
 		return err;