liblog/logd_writer.cpp

/*
 * Copyright (C) 2007-2016 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "logd_writer.h"

#include <errno.h>
#include <fcntl.h>
#include <inttypes.h>
#include <poll.h>
#include <stdarg.h>
#include <stdatomic.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/un.h>
#include <time.h>
#include <unistd.h>

#include <private/android_filesystem_config.h>
#include <private/android_logger.h>

#include "logger.h"
#include "uio.h"

static atomic_int logd_socket;

// Note that it is safe to call connect() multiple times on DGRAM Unix domain sockets, so this
// function is used to reconnect to logd without requiring a new socket.
static void LogdConnect() {
  sockaddr_un un = {};
  un.sun_family = AF_UNIX;
  strcpy(un.sun_path, "/dev/socket/logdw");
  TEMP_FAILURE_RETRY(connect(logd_socket, reinterpret_cast<sockaddr*>(&un), sizeof(sockaddr_un)));
}

// logd_socket should only be opened once.  If we see that logd_socket is uninitialized, we create a
// new socket and attempt to exchange it into the atomic logd_socket.  If the compare/exchange was
// successful, then that will be the socket used for the duration of the program, otherwise a
// different thread has already opened and written the socket to the atomic, so close the new socket
// and return.
static void GetSocket() {
  if (logd_socket != 0) {
    return;
  }

  int new_socket = TEMP_FAILURE_RETRY(socket(PF_UNIX, SOCK_DGRAM | SOCK_CLOEXEC, 0));
  if (new_socket <= 0) {
    return;
  }

  int uninitialized_value = 0;
  if (!logd_socket.compare_exchange_strong(uninitialized_value, new_socket)) {
    close(new_socket);
    return;
  }

  LogdConnect();
}

// This is the one exception to the above.  Zygote uses this to clean up open FD's after fork() and
// before specialization.  It is single threaded at this point and therefore this function is
// explicitly not thread safe.  It sets logd_socket to 0, so future logs will be safely initialized
// whenever they happen.
void LogdClose() {
  if (logd_socket > 0) {
    close(logd_socket);
  }
  logd_socket = 0;
}

int LogdWrite(log_id_t logId, struct timespec* ts, struct iovec* vec, size_t nr) {
  ssize_t ret;
  static const unsigned headerLength = 1;
  struct iovec newVec[nr + headerLength];
  android_log_header_t header;
  size_t i, payloadSize;

  GetSocket();

  if (logd_socket <= 0) {
    return -EBADF;
  }

  /* logd, after initialization and priv drop */
  if (getuid() == AID_LOGD) {
    /*
     * ignore log messages we send to ourself (logd).
     * Such log messages are often generated by libraries we depend on
     * which use standard Android logging.
     */
    return 0;
  }

  header.id = logId;
  header.tid = gettid();
  header.realtime.tv_sec = ts->tv_sec;
  header.realtime.tv_nsec = ts->tv_nsec;

  newVec[0].iov_base = (unsigned char*)&header;
  newVec[0].iov_len = sizeof(header);

  for (payloadSize = 0, i = headerLength; i < nr + headerLength; i++) {
    newVec[i].iov_base = vec[i - headerLength].iov_base;
    payloadSize += newVec[i].iov_len = vec[i - headerLength].iov_len;

    if (payloadSize > LOGGER_ENTRY_MAX_PAYLOAD) {
      newVec[i].iov_len -= payloadSize - LOGGER_ENTRY_MAX_PAYLOAD;
      if (newVec[i].iov_len) {
        ++i;
      }
      break;
    }
  }

  ret = TEMP_FAILURE_RETRY(writev(logd_socket, newVec, i));
  if (ret < 0) {
    LogdConnect();

    ret = TEMP_FAILURE_RETRY(writev(logd_socket, newVec, i));
  }

  if (ret < 0) {
    ret = -errno;
  }

  return ret;
}