sandbox/src/sharedmem_ipc_client.cc

// Copyright (c) 2006-2008 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 <string.h>
#include "sandbox/src/sharedmem_ipc_client.h"
#include "sandbox/src/sandbox.h"
#include "sandbox/src/crosscall_client.h"
#include "sandbox/src/crosscall_params.h"
#include "base/logging.h"

namespace sandbox {

// Get the base of the data buffer of the channel; this is where the input
// parameters get serialized. Since they get serialized directly into the
// channel we avoid one copy.
void* SharedMemIPCClient::GetBuffer() {
  bool failure = false;
  size_t ix = LockFreeChannel(&failure);
  if (failure) {
    return NULL;
  }
  return reinterpret_cast<char*>(control_) +
         control_->channels[ix].channel_base;
}

// If we need to cancel an IPC before issuing DoCall
// our client should call FreeBuffer with the same pointer
// returned by GetBuffer.
void SharedMemIPCClient::FreeBuffer(void* buffer) {
  size_t num = ChannelIndexFromBuffer(buffer);
  ChannelControl* channel = control_->channels;
  LONG result = ::InterlockedExchange(&channel[num].state, kFreeChannel);
  DCHECK(kFreeChannel != result);
  result;
}

// The constructor simply casts the shared memory to the internal
// structures. This is a cheap step that is why this IPC object can
// and should be constructed per call.
SharedMemIPCClient::SharedMemIPCClient(void* shared_mem)
    : control_(reinterpret_cast<IPCControl*>(shared_mem)) {
  first_base_ = reinterpret_cast<char*>(shared_mem) +
               control_->channels[0].channel_base;
  // There must be at least one channel.
  DCHECK(0 != control_->channels_count);
}

// Do the IPC. At this point the channel should have already been
// filled with the serialized input parameters.
// We follow the pattern explained in the header file.
ResultCode SharedMemIPCClient::DoCall(CrossCallParams* params,
                                      CrossCallReturn* answer) {
  if (!control_->server_alive)
    return SBOX_ERROR_CHANNEL_ERROR;

  size_t num = ChannelIndexFromBuffer(params->GetBuffer());
  ChannelControl* channel = control_->channels;
  // Note that the IPC tag goes outside the buffer as well inside
  // the buffer. This should enable the server to prioritize based on
  // IPC tags without having to de-serialize the entire message.
  channel[num].ipc_tag = params->GetTag();

  // Wait for the server to service this IPC call. After kIPCWaitTimeOut1
  // we check if the server_alive mutex was abandoned which will indicate
  // that the server has died.

  // While the atomic signaling and waiting is not a requirement, it
  // is nice because we save a trip to kernel.
  DWORD wait = ::SignalObjectAndWait(channel[num].ping_event,
                                     channel[num].pong_event,
                                     kIPCWaitTimeOut1, FALSE);
  if (WAIT_TIMEOUT == wait) {
    // The server is taking too long. Enter a loop were we check if the
    // server_alive mutex has been abandoned which would signal a server crash
    // or else we keep waiting for a response.
    while (true) {
      wait = ::WaitForSingleObject(control_->server_alive, 0);
      if (WAIT_TIMEOUT == wait) {
        // Server seems still alive. We already signaled so here we just wait.
        wait = ::WaitForSingleObject(channel[num].pong_event, kIPCWaitTimeOut1);
        if (WAIT_OBJECT_0 == wait) {
          // The server took a long time but responded.
          break;
        } else if (WAIT_TIMEOUT == wait) {
          continue;
        } else {
          return SBOX_ERROR_CHANNEL_ERROR;
        }
      } else {
        // The server has crashed and windows has signaled the mutex as
        // abandoned.
        ::InterlockedExchange(&channel[num].state, kAbandonnedChannel);
        control_->server_alive = 0;
        return SBOX_ERROR_CHANNEL_ERROR;
      }
    }
  } else if (WAIT_OBJECT_0 != wait) {
    // Probably the server crashed before the kIPCWaitTimeOut1 occurred.
    return SBOX_ERROR_CHANNEL_ERROR;
  }

  // The server has returned an answer, copy it and free the channel.
  memcpy(answer, params->GetCallReturn(), sizeof(CrossCallReturn));

  // Return the IPC state It can indicate that while the IPC has
  // completed some error in the Broker has caused to not return valid
  // results.
  return answer->call_outcome;
}

// Locking a channel is a simple as looping over all the channels
// looking for one that is has state = kFreeChannel and atomically
// swapping it to kBusyChannel.
// If there is no free channel, then we must back off so some other
// thread makes progress and frees a channel. To back off we sleep.
size_t SharedMemIPCClient::LockFreeChannel(bool* severe_failure) {
  if (0 == control_->channels_count) {
    *severe_failure = true;
    return 0;
  }
  ChannelControl* channel = control_->channels;
  do {
    for (size_t ix = 0; ix != control_->channels_count; ++ix) {
      if (kFreeChannel == ::InterlockedCompareExchange(&channel[ix].state,
                                                       kBusyChannel,
                                                       kFreeChannel)) {
          *severe_failure = false;
          return ix;
      }
    }
    // We did not find any available channel, maybe the server is dead.
    DWORD wait = ::WaitForSingleObject(control_->server_alive,
                                       kIPCWaitTimeOut2);
    if (WAIT_TIMEOUT != wait) {
      // The server is dead and we outlive it enough to get in trouble.
      *severe_failure = true;
      return 0;
    }
  }
  while (true);
}

// Find out which channel we are from the pointer returned by GetBuffer.
size_t SharedMemIPCClient::ChannelIndexFromBuffer(const void* buffer) {
  ptrdiff_t d = reinterpret_cast<const char*>(buffer) - first_base_;
  size_t num = d/kIPCChannelSize;
  DCHECK(num < control_->channels_count);
  return (num);
}

}  // namespace sandbox