system/logging/logd/LogListener.cpp

/*
 * Copyright (C) 2012-2014 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 <limits.h>
#include <sys/cdefs.h>
#include <sys/prctl.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <sys/un.h>
#include <unistd.h>

#include <thread>

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

#include "LogBuffer.h"
#include "LogListener.h"
#include "LogPermissions.h"

LogListener::LogListener(LogBuffer* buf) : socket_(GetLogSocket()), logbuf_(buf) {}

bool LogListener::StartListener() {
    if (socket_ <= 0) {
        return false;
    }
    auto thread = std::thread(&LogListener::ThreadFunction, this);
    thread.detach();
    return true;
}

void LogListener::ThreadFunction() {
    prctl(PR_SET_NAME, "logd.writer");

    while (true) {
        HandleData();
    }
}

void LogListener::HandleData() {
    // + 1 to ensure null terminator if MAX_PAYLOAD buffer is received
    __attribute__((uninitialized)) char
            buffer[sizeof(android_log_header_t) + LOGGER_ENTRY_MAX_PAYLOAD + 1];
    struct iovec iov = {buffer, sizeof(buffer) - 1};

    alignas(4) char control[CMSG_SPACE(sizeof(struct ucred))];
    struct msghdr hdr = {
        nullptr, 0, &iov, 1, control, sizeof(control), 0,
    };

    ssize_t n = recvmsg(socket_, &hdr, 0);
    if (n <= (ssize_t)(sizeof(android_log_header_t))) {
        return;
    }

    // To clear the entire buffer would be safe, but this contributes to 1.68%
    // overhead under logging load. We are safe because we check counts, but
    // still need to clear null terminator
    buffer[n] = 0;

    struct ucred* cred = nullptr;

    struct cmsghdr* cmsg = CMSG_FIRSTHDR(&hdr);
    while (cmsg != nullptr) {
        if (cmsg->cmsg_level == SOL_SOCKET &&
            cmsg->cmsg_type == SCM_CREDENTIALS) {
            cred = (struct ucred*)CMSG_DATA(cmsg);
            break;
        }
        cmsg = CMSG_NXTHDR(&hdr, cmsg);
    }

    if (cred == nullptr) {
        return;
    }

    if (cred->uid == AID_LOGD) {
        // ignore log messages we send to ourself.
        // Such log messages are often generated by libraries we depend on
        // which use standard Android logging.
        return;
    }

    android_log_header_t* header =
        reinterpret_cast<android_log_header_t*>(buffer);
    log_id_t logId = static_cast<log_id_t>(header->id);
    if (/* logId < LOG_ID_MIN || */ logId >= LOG_ID_MAX ||
        logId == LOG_ID_KERNEL) {
        return;
    }

    if (logId == LOG_ID_SECURITY) {
        if (!__android_log_security()) {
            return;
        }
        if (!clientCanWriteSecurityLog(cred->uid, cred->gid, cred->pid)) {
            return;
        }
    }

    char* msg = ((char*)buffer) + sizeof(android_log_header_t);
    n -= sizeof(android_log_header_t);

    // NB: hdr.msg_flags & MSG_TRUNC is not tested, silently passing a
    // truncated message to the logs.

    logbuf_->Log(logId, header->realtime, cred->uid, cred->pid, header->tid, msg,
                 ((size_t)n <= UINT16_MAX) ? (uint16_t)n : UINT16_MAX);
}

int LogListener::GetLogSocket() {
    static const char socketName[] = "logdw";
    int sock = android_get_control_socket(socketName);

    if (sock < 0) {  // logd started up in init.sh
        sock = socket_local_server(
            socketName, ANDROID_SOCKET_NAMESPACE_RESERVED, SOCK_DGRAM);

        int on = 1;
        if (setsockopt(sock, SOL_SOCKET, SO_PASSCRED, &on, sizeof(on))) {
            return -1;
        }
    }
    return sock;
}