[seccomp-sandbox] Fix test_prctl so that it does not leave behind a zombie process

tests/test_syscalls.cc

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <assert.h>
 #include <dirent.h>
 #include <errno.h>
 #include <pthread.h>
 #include <pty.h>
 #include <sys/prctl.h>
@@ skipping 689 matching lines @@
     write(fds[0], &ch, 1);
     read(fds[0], &ch, 1);
     _exit(1);
   }
   rc = ptrace(PTRACE_ATTACH, pid, 0, 0);
   assert(rc == -1 && errno == EPERM);
   write(fds[1], &ch, 1);
   read(fds[1], &ch, 1);
   rc = ptrace(PTRACE_ATTACH, pid, 0, 0);
   assert(rc == 0);
+
+  // Now clean up.  We have to collect the subprocess's stopped state
+  // with waitpid() otherwise PTRACE_KILL will not successfully kill
+  // the subprocess.
+  int status;
+  rc = waitpid(pid, &status, 0);
+  assert(rc == pid);
+  assert(WIFSTOPPED(status));
+  assert(WSTOPSIG(status) == SIGSTOP);
+
   rc = ptrace(PTRACE_KILL, pid, 0, 0);
   assert(rc == 0);
+
+  rc = waitpid(pid, &status, 0);
+  assert(rc == pid);
+  assert(WIFSIGNALED(status));
+  assert(WTERMSIG(status) == SIGKILL);
 }
 
 struct testcase {
   const char *test_name;
   void (*test_func)();
 };
 
 struct testcase all_tests[] = {
 #include "test-list.h"
   { NULL, NULL },
@@ skipping 84 matching lines @@
     if (failures == 0) {
       printf("OK\n");
       return 0;
     }
     else {
       printf("%i FAILURE(S)\n", failures);
       return 1;
     }
   }
 }