cgroups: fix cgroup_event_listener error handling

The error handling in cgroup_event_listener.c did not correctly deal
with either an error opening either <control_file> or
cgroup.event_control.  Due to an uninitialized variable the program
exit code was undefined if either of these opens failed.

This patch simplifies and corrects cgroup_event_listener.c error
handling by:
1. using err*() rather than printf(),exit()
2. depending on process exit to close open files

With this patch failures always return non-zero error.
Signed-off-by: Greg Thelen <gthelen@google.com>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

tools/cgroup/cgroup_event_listener.c

@@ -5,6 +5,7 @@
  */
 
 #include <assert.h>
+#include <err.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <libgen.h>
@@ -15,7 +16,7 @@
 
 #include <sys/eventfd.h>
 
-#define USAGE_STR "Usage: cgroup_event_listener <path-to-control-file> <args>\n"
+#define USAGE_STR "Usage: cgroup_event_listener <path-to-control-file> <args>"
 
 int main(int argc, char **argv)
 {
@@ -26,49 +27,33 @@ int main(int argc, char **argv)
 	char line[LINE_MAX];
 	int ret;
 
-	if (argc != 3) {
-		fputs(USAGE_STR, stderr);
-		return 1;
-	}
+	if (argc != 3)
+		errx(1, "%s", USAGE_STR);
 
 	cfd = open(argv[1], O_RDONLY);
-	if (cfd == -1) {
-		fprintf(stderr, "Cannot open %s: %s\n", argv[1],
-				strerror(errno));
-		goto out;
-	}
+	if (cfd == -1)
+		err(1, "Cannot open %s", argv[1]);
 
 	ret = snprintf(event_control_path, PATH_MAX, "%s/cgroup.event_control",
 			dirname(argv[1]));
-	if (ret >= PATH_MAX) {
-		fputs("Path to cgroup.event_control is too long\n", stderr);
-		goto out;
-	}
+	if (ret >= PATH_MAX)
+		errx(1, "Path to cgroup.event_control is too long");
 
 	event_control = open(event_control_path, O_WRONLY);
-	if (event_control == -1) {
-		fprintf(stderr, "Cannot open %s: %s\n", event_control_path,
-				strerror(errno));
-		goto out;
-	}
+	if (event_control == -1)
+		err(1, "Cannot open %s", event_control_path);
 
 	efd = eventfd(0, 0);
-	if (efd == -1) {
-		perror("eventfd() failed");
-		goto out;
-	}
+	if (efd == -1)
+		err(1, "eventfd() failed");
 
 	ret = snprintf(line, LINE_MAX, "%d %d %s", efd, cfd, argv[2]);
-	if (ret >= LINE_MAX) {
-		fputs("Arguments string is too long\n", stderr);
-		goto out;
-	}
+	if (ret >= LINE_MAX)
+		errx(1, "Arguments string is too long");
 
 	ret = write(event_control, line, strlen(line) + 1);
-	if (ret == -1) {
-		perror("Cannot write to cgroup.event_control");
-		goto out;
-	}
+	if (ret == -1)
+		err(1, "Cannot write to cgroup.event_control");
 
 	while (1) {
 		uint64_t result;
@@ -77,34 +62,21 @@ int main(int argc, char **argv)
 		if (ret == -1) {
 			if (errno == EINTR)
 				continue;
-			perror("Cannot read from eventfd");
-			break;
+			err(1, "Cannot read from eventfd");
 		}
 		assert(ret == sizeof(result));
 
 		ret = access(event_control_path, W_OK);
 		if ((ret == -1) && (errno == ENOENT)) {
-				puts("The cgroup seems to have removed.");
-				ret = 0;
-				break;
-		}
-
-		if (ret == -1) {
-			perror("cgroup.event_control "
-					"is not accessible any more");
+			puts("The cgroup seems to have removed.");
 			break;
 		}
 
+		if (ret == -1)
+			err(1, "cgroup.event_control is not accessible any more");
+
 		printf("%s %s: crossed\n", argv[1], argv[2]);
 	}
 
-out:
-	if (efd >= 0)
-		close(efd);
-	if (event_control >= 0)
-		close(event_control);
-	if (cfd >= 0)
-		close(cfd);
-
-	return (ret != 0);
+	return 0;
 }