libc/bionic/bionic_systrace.cpp

/*
 * Copyright (C) 2014 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 <cutils/trace.h>
#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include "private/bionic_lock.h"
#include "private/bionic_systrace.h"
#include "private/libc_logging.h"

#define _REALLY_INCLUDE_SYS__SYSTEM_PROPERTIES_H_
#include <sys/_system_properties.h>

#define WRITE_OFFSET   32

constexpr char SYSTRACE_PROPERTY_NAME[] = "debug.atrace.tags.enableflags";

static Lock g_lock;
static const prop_info* g_pinfo;
static uint32_t g_property_serial = -1;
static uint32_t g_property_area_serial = -1;
static uint64_t g_tags;
static int g_trace_marker_fd = -1;

static bool should_trace() {
  bool result = false;
  g_lock.lock();
  // debug.atrace.tags.enableflags is set to a safe non-tracing value during property
  // space initialization, so it should only be null in two cases, if there are
  // insufficient permissions for this process to access the property, in which
  // case an audit will be logged, and during boot before the property server has
  // been started, in which case we store the global property_area serial to prevent
  // the costly find operation until we see a changed property_area.
  if (!g_pinfo && g_property_area_serial != __system_property_area_serial()) {
    g_property_area_serial = __system_property_area_serial();
    g_pinfo = __system_property_find(SYSTRACE_PROPERTY_NAME);
  }
  if (g_pinfo) {
    // Find out which tags have been enabled on the command line and set
    // the value of tags accordingly.  If the value of the property changes,
    // the serial will also change, so the costly system_property_read function
    // can be avoided by calling the much cheaper system_property_serial
    // first.  The values within pinfo may change, but its location is guaranteed
    // not to move.
    uint32_t cur_serial = __system_property_serial(g_pinfo);
    if (cur_serial != g_property_serial) {
      g_property_serial = cur_serial;
      char value[PROP_VALUE_MAX];
      __system_property_read(g_pinfo, 0, value);
      g_tags = strtoull(value, nullptr, 0);
    }
    result = ((g_tags & ATRACE_TAG_BIONIC) != 0);
  }
  g_lock.unlock();
  return result;
}

static int get_trace_marker_fd() {
  g_lock.lock();
  if (g_trace_marker_fd == -1) {
    g_trace_marker_fd = open("/sys/kernel/debug/tracing/trace_marker", O_CLOEXEC | O_WRONLY);
  }
  g_lock.unlock();
  return g_trace_marker_fd;
}

ScopedTrace::ScopedTrace(const char* message) {
  if (!should_trace()) {
    return;
  }

  int trace_marker_fd = get_trace_marker_fd();
  if (trace_marker_fd == -1) {
    return;
  }

  // If bionic tracing has been enabled, then write the message to the
  // kernel trace_marker.
  int length = strlen(message);
  char buf[length + WRITE_OFFSET];
  size_t len = snprintf(buf, length + WRITE_OFFSET, "B|%d|%s", getpid(), message);

  // Tracing may stop just after checking property and before writing the message.
  // So the write is acceptable to fail. See b/20666100.
  TEMP_FAILURE_RETRY(write(trace_marker_fd, buf, len));
}

ScopedTrace::~ScopedTrace() {
  if (!should_trace()) {
    return;
  }

  int trace_marker_fd = get_trace_marker_fd();
  if (trace_marker_fd == -1) {
    return;
  }

  TEMP_FAILURE_RETRY(write(trace_marker_fd, "E", 1));
}