Merge pull request #336 from dqminh/execin-wait

nsenter waits for parent signal before forking

namespaces/execin.go

@@ -73,6 +73,11 @@ func ExecIn(container *libcontainer.Config, state *libcontainer.State, userArgs
 		return terminate(err)
 	}
 
+	// finish cgroups' setup, unblock the child process.
+	if _, err := parent.WriteString("1"); err != nil {
+		return terminate(err)
+	}
+
 	if err := json.NewEncoder(parent).Encode(container); err != nil {
 		return terminate(err)
 	}

namespaces/nsenter/nsenter.c

@@ -28,7 +28,6 @@ void get_args(int *argc, char ***argv)
 		pr_perror("Unable to open /proc/self/cmdline");
 		exit(1);
 	}
-
 	// Read the whole commandline.
 	ssize_t contents_size = 0;
 	ssize_t contents_offset = 0;
@@ -98,13 +97,12 @@ void nsenter()
 	if (strncmp(argv[0], kNsEnter, strlen(kNsEnter)) != 0) {
 		return;
 	}
-
-	#ifdef PR_SET_CHILD_SUBREAPER
+#ifdef PR_SET_CHILD_SUBREAPER
 	if (prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0) == -1) {
 		pr_perror("Failed to set child subreaper");
 		exit(1);
 	}
-	#endif
+#endif
 
 	static const struct option longopts[] = {
 		{"nspid", required_argument, NULL, 'n'},
@@ -155,6 +153,12 @@ void nsenter()
 			exit(1);
 		}
 	}
+	// blocking until the parent placed the process inside correct cgroups.
+	unsigned char s;
+	if (read(3, &s, 1) != 1 || s != '1') {
+		pr_perror("failed to receive synchronization data from parent");
+		exit(1);
+	}
 	// Setns on all supported namespaces.
 	char ns_dir[PATH_MAX];
 	memset(ns_dir, 0, PATH_MAX);
@@ -173,7 +177,8 @@ void nsenter()
 	for (i = 0; i < num; i++) {
 		// A zombie process has links on namespaces, but they can't be opened
 		struct stat st;
-		if (fstatat(ns_dir_fd, namespaces[i], &st, AT_SYMLINK_NOFOLLOW) == -1) {
+		if (fstatat(ns_dir_fd, namespaces[i], &st, AT_SYMLINK_NOFOLLOW)
+		    == -1) {
 			if (errno == ENOENT)
 				continue;
 			pr_perror("Failed to stat ns file %s for ns %s",

namespaces/nsenter/nsenter_test.go

@@ -12,15 +12,29 @@ import (
 
 func TestNsenterAlivePid(t *testing.T) {
 	args := []string{"nsenter-exec", "--nspid", fmt.Sprintf("%d", os.Getpid())}
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("failed to create pipe %v", err)
+	}
 
 	cmd := &exec.Cmd{
 		Path:       os.Args[0],
 		Args:       args,
+		ExtraFiles: []*os.File{r},
 	}
 
-	err := cmd.Run()
-	if err != nil {
-		t.Fatal("nsenter exits with a non-zero exit status")
+	if err := cmd.Start(); err != nil {
+		t.Fatalf("nsenter failed to start %v", err)
+	}
+	r.Close()
+
+	// unblock the child process
+	if _, err := w.WriteString("1"); err != nil {
+		t.Fatalf("parent failed to write synchronization data %v", err)
+	}
+
+	if err := cmd.Wait(); err != nil {
+		t.Fatalf("nsenter exits with a non-zero exit status")
 	}
 }