sandbox/linux/services/thread_helpers.cc

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "sandbox/linux/services/thread_helpers.h"
   6
   7 #include <errno.h>
   8 #include <fcntl.h>
   9 #include <signal.h>
  10 #include <sys/types.h>
  11 #include <sys/stat.h>
  12 #include <unistd.h>
  13
  14 #include <string>
  15
  16 #include "base/basictypes.h"
  17 #include "base/logging.h"
  18 #include "base/posix/eintr_wrapper.h"
  19 #include "base/strings/string_number_conversions.h"
  20 #include "base/threading/platform_thread.h"
  21 #include "base/threading/thread.h"
  22
  23 namespace sandbox {
  24
  25 namespace {
  26
  27 bool IsSingleThreadedImpl(int proc_self_task) {
  28   CHECK_LE(0, proc_self_task);
  29   struct stat task_stat;
  30   int fstat_ret = fstat(proc_self_task, &task_stat);
  31   PCHECK(0 == fstat_ret);
  32
  33   // At least "..", "." and the current thread should be present.
  34   CHECK_LE(3UL, task_stat.st_nlink);
  35   // Counting threads via /proc/self/task could be racy. For the purpose of
  36   // determining if the current proces is monothreaded it works: if at any
  37   // time it becomes monothreaded, it'll stay so.
  38   return task_stat.st_nlink == 3;
  39 }
  40
  41 }  // namespace
  42
  43 bool ThreadHelpers::IsSingleThreaded(int proc_self_task) {
  44   DCHECK_LE(-1, proc_self_task);
  45   if (-1 == proc_self_task) {
  46     const int task_fd = open("/proc/self/task/", O_RDONLY | O_DIRECTORY);
  47     PCHECK(0 <= task_fd);
  48     const bool result = IsSingleThreadedImpl(task_fd);
  49     PCHECK(0 == IGNORE_EINTR(close(task_fd)));
  50     return result;
  51   } else {
  52     return IsSingleThreadedImpl(proc_self_task);
  53   }
  54 }
  55
  56 bool ThreadHelpers::StopThreadAndWatchProcFS(int proc_self_task,
  57                                              base::Thread* thread) {
  58   DCHECK_LE(0, proc_self_task);
  59   DCHECK(thread);
  60   const base::PlatformThreadId thread_id = thread->thread_id();
  61   const std::string thread_id_dir_str = base::IntToString(thread_id) + "/";
  62
  63   // The kernel is at liberty to wake the thread id futex before updating
  64   // /proc. Following Stop(), the thread is joined, but entries in /proc may
  65   // not have been updated.
  66   thread->Stop();
  67
  68   unsigned int iterations = 0;
  69   bool thread_present_in_procfs = true;
  70   // Poll /proc with an exponential back-off, sleeping 2^iterations nanoseconds
  71   // in nanosleep(2).
  72   // Note: the clock may not allow for nanosecond granularity, in this case the
  73   // first iterations would sleep a tiny bit more instead, which would not
  74   // change the calculations significantly.
  75   while (thread_present_in_procfs) {
  76     struct stat task_stat;
  77     const int fstat_ret =
  78         fstatat(proc_self_task, thread_id_dir_str.c_str(), &task_stat, 0);
  79     if (fstat_ret < 0) {
  80       PCHECK(ENOENT == errno);
  81       // The thread disappeared from /proc, we're done.
  82       thread_present_in_procfs = false;
  83       break;
  84     }
  85     // Increase the waiting time exponentially.
  86     struct timespec ts = {0, 1L << iterations /* nanoseconds */};
  87     PCHECK(0 == HANDLE_EINTR(nanosleep(&ts, &ts)));
  88     ++iterations;
  89
  90     // Crash after 30 iterations, which means having spent roughly 2s in
  91     // nanosleep(2) cumulatively.
  92     CHECK_GT(30U, iterations);
  93     // In practice, this never goes through more than a couple iterations. In
  94     // debug mode, crash after 64ms (+ eventually 25 times the granularity of
  95     // the clock) in nanosleep(2).
  96     DCHECK_GT(25U, iterations);
  97   }
  98
  99   return true;
 100 }
 101
 102 }  // namespace sandbox