Update from https://crrev.com/316786

sandbox/linux/seccomp-bpf/sandbox_bpf.cc

 // Copyright (c) 2012 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 "sandbox/linux/seccomp-bpf/sandbox_bpf.h"
 
 // Some headers on Android are missing cdefs: crbug.com/172337.
 // (We can't use OS_ANDROID here since build_config.h is not included).
 #if defined(ANDROID)
 #include <sys/cdefs.h>
 #endif
 
 #include <errno.h>
 #include <linux/filter.h>
 #include <sys/prctl.h>
 #include <sys/types.h>
 #include <unistd.h>
 
 #include "base/compiler_specific.h"
 #include "base/files/scoped_file.h"
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/third_party/valgrind/valgrind.h"
 #include "sandbox/linux/bpf_dsl/codegen.h"
 #include "sandbox/linux/bpf_dsl/dump_bpf.h"
 #include "sandbox/linux/bpf_dsl/policy.h"
 #include "sandbox/linux/bpf_dsl/policy_compiler.h"
+#include "sandbox/linux/bpf_dsl/seccomp_macros.h"
 #include "sandbox/linux/bpf_dsl/syscall_set.h"
 #include "sandbox/linux/seccomp-bpf/die.h"
 #include "sandbox/linux/seccomp-bpf/errorcode.h"
-#include "sandbox/linux/seccomp-bpf/linux_seccomp.h"
 #include "sandbox/linux/seccomp-bpf/syscall.h"
 #include "sandbox/linux/seccomp-bpf/trap.h"
 #include "sandbox/linux/seccomp-bpf/verifier.h"
-#include "sandbox/linux/services/linux_syscalls.h"
+#include "sandbox/linux/services/proc_util.h"
 #include "sandbox/linux/services/syscall_wrappers.h"
 #include "sandbox/linux/services/thread_helpers.h"
+#include "sandbox/linux/system_headers/linux_seccomp.h"
+#include "sandbox/linux/system_headers/linux_syscalls.h"
 
 namespace sandbox {
 
 namespace {
 
 bool IsRunningOnValgrind() { return RUNNING_ON_VALGRIND; }
 
 bool IsSingleThreaded(int proc_task_fd) {
   return ThreadHelpers::IsSingleThreaded(proc_task_fd);
 }
@@ skipping 59 matching lines @@
   CHECK(seccomp_level == SeccompLevel::SINGLE_THREADED ||
         seccomp_level == SeccompLevel::MULTI_THREADED);
 
   if (sandbox_has_started_) {
     SANDBOX_DIE(
         "Cannot repeatedly start sandbox. Create a separate Sandbox "
         "object instead.");
     return false;
   }
 
+  if (!proc_task_fd_.is_valid()) {
+    SetProcTaskFd(ProcUtil::OpenProcSelfTask());
+  }
+
   const bool supports_tsync = KernelSupportsSeccompTsync();
 
   if (seccomp_level == SeccompLevel::SINGLE_THREADED) {
-    if (!IsSingleThreaded(proc_task_fd_.get())) {
-      SANDBOX_DIE("Cannot start sandbox; process is already multi-threaded");
-      return false;
-    }
+    // Wait for /proc/self/task/ to update if needed and assert the
+    // process is single threaded.
+    ThreadHelpers::AssertSingleThreaded(proc_task_fd_.get());
   } else if (seccomp_level == SeccompLevel::MULTI_THREADED) {
     if (IsSingleThreaded(proc_task_fd_.get())) {
       SANDBOX_DIE("Cannot start sandbox; "
                   "process may be single-threaded when reported as not");
       return false;
     }
     if (!supports_tsync) {
       SANDBOX_DIE("Cannot start sandbox; kernel does not support synchronizing "
                   "filters for a threadgroup");
       return false;
@@ skipping 106 matching lines @@
   } else {
     if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
       SANDBOX_DIE("Kernel refuses to turn on BPF filters");
     }
   }
 
   sandbox_has_started_ = true;
 }
 
 }  // namespace sandbox