Revert 240670 "Revert 239894 "Linux Sandbox: check no threads be..."

trunk/src/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>
@@ skipping 250 matching lines @@
   }
   int fds[2];
   if (pipe2(fds, O_NONBLOCK | O_CLOEXEC)) {
     SANDBOX_DIE("pipe() failed");
   }
 
   if (fds[0] <= 2 || fds[1] <= 2) {
     SANDBOX_DIE("Process started without standard file descriptors");
   }
 
+  // This code is using fork() and should only ever run single-threaded.
+  // Most of the code below is "async-signal-safe" and only minor changes
+  // would be needed to support threads.
+  DCHECK(IsSingleThreaded(proc_fd_));
   pid_t pid = fork();
   if (pid < 0) {
     // Die if we cannot fork(). We would probably fail a little later
     // anyway, as the machine is likely very close to running out of
     // memory.
     // But what we don't want to do is return "false", as a crafty
     // attacker might cause fork() to fail at will and could trick us
     // into running without a sandbox.
     sigprocmask(SIG_SETMASK, &old_mask, NULL);  // OK, if it fails
     SANDBOX_DIE("fork() failed unexpectedly");
@@ skipping 728 matching lines @@
                    &*conds_->insert(failed).first);
 }
 
 ErrorCode SandboxBPF::Kill(const char* msg) {
   return Trap(BPFFailure, const_cast<char*>(msg));
 }
 
 SandboxBPF::SandboxStatus SandboxBPF::status_ = STATUS_UNKNOWN;
 
 }  // namespace sandbox