libcutils/android_reboot.c

/*
 * Copyright 2011, 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 <unistd.h>
#include <sys/reboot.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <mntent.h>
#include <stdio.h>
#include <string.h>

#include <cutils/android_reboot.h>

#define UNUSED __attribute__((unused))

/* Check to see if /proc/mounts contains any writeable filesystems
 * backed by a block device.
 * Return true if none found, else return false.
 */
static int remount_ro_done(void)
{
    FILE* fp;
    struct mntent* mentry;
    int found_rw_fs = 0;

    if ((fp = setmntent("/proc/mounts", "r")) == NULL) {
        /* If we can't read /proc/mounts, just give up. */
        return 1;
    }
    while ((mentry = getmntent(fp)) != NULL) {
        if (!strncmp(mentry->mnt_fsname, "/dev/block", 10) && strstr(mentry->mnt_opts, "rw,")) {
            found_rw_fs = 1;
            break;
        }
    }
    endmntent(fp);

    return !found_rw_fs;
}

/* Remounting filesystems read-only is difficult when there are files
 * opened for writing or pending deletes on the filesystem.  There is
 * no way to force the remount with the mount(2) syscall.  The magic sysrq
 * 'u' command does an emergency remount read-only on all writable filesystems
 * that have a block device (i.e. not tmpfs filesystems) by calling
 * emergency_remount(), which knows how to force the remount to read-only.
 * Unfortunately, that is asynchronous, and just schedules the work and
 * returns.  The best way to determine if it is done is to read /proc/mounts
 * repeatedly until there are no more writable filesystems mounted on
 * block devices.
 */
static void remount_ro(void)
{
    int fd, cnt = 0;

    /* Trigger the remount of the filesystems as read-only,
     * which also marks them clean.
     */
    fd = open("/proc/sysrq-trigger", O_WRONLY);
    if (fd < 0) {
        return;
    }
    write(fd, "u", 1);
    close(fd);


    /* Now poll /proc/mounts till it's done */
    while (!remount_ro_done() && (cnt < 50)) {
        usleep(100000);
        cnt++;
    }

    return;
}


int android_reboot(int cmd, int flags UNUSED, const char *arg)
{
    int ret;

    sync();
    remount_ro();

    switch (cmd) {
        case ANDROID_RB_RESTART:
            ret = reboot(RB_AUTOBOOT);
            break;

        case ANDROID_RB_POWEROFF:
            ret = reboot(RB_POWER_OFF);
            break;

        case ANDROID_RB_RESTART2:
            ret = syscall(__NR_reboot, LINUX_REBOOT_MAGIC1, LINUX_REBOOT_MAGIC2,
                           LINUX_REBOOT_CMD_RESTART2, arg);
            break;

        default:
            ret = -1;
    }

    return ret;
}