Fall back to chroot() if setns() fails for mnt namespace

fixes #347

mnexec.c

@@ -152,15 +152,16 @@ int main(int argc, char *argv[])
                 perror("setns");
                 return 1;
             }
+            /* Plan A: call setns() to attach to mount namespace */
             sprintf(path, "/proc/%d/ns/mnt", pid);
             nsid = open(path, O_RDONLY);
-            if (nsid < 0) {
+            if (nsid < 0 || setns(nsid, 0) != 0) {
+                /* Plan B: chroot into pid's root file system */
+                sprintf(path, "/proc/%d/root", pid);
+                if (chroot(path) < 0) {
                     perror(path);
                     return 1;
                 }
-            if (setns(nsid, 0) != 0) {
-                perror("setns");
-                return 1;
             }
             break;
         case 'g':