// SPDX-License-Identifier: GPL-2.0

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/syscall.h>
#include <sys/mount.h>
#include <sys/reboot.h>
#include <linux/kexec.h>

/* from arch/x86/include/asm/setup.h */
#define COMMAND_LINE_SIZE       2048

#define KHO_FINALIZE "/debugfs/kho/out/finalize"
#define KERNEL_IMAGE "/kernel"

static int mount_filesystems(void)
{
        if (mount("debugfs", "/debugfs", "debugfs", 0, NULL) < 0)
                return -1;

        return mount("proc", "/proc", "proc", 0, NULL);
}

static int kho_enable(void)
{
        const char enable[] = "1";
        int fd;

        fd = open(KHO_FINALIZE, O_RDWR);
        if (fd < 0)
                return -1;

        if (write(fd, enable, sizeof(enable)) != sizeof(enable))
                return 1;

        close(fd);
        return 0;
}

static long kexec_file_load(int kernel_fd, int initrd_fd,
                            unsigned long cmdline_len, const char *cmdline,
                            unsigned long flags)
{
        return syscall(__NR_kexec_file_load, kernel_fd, initrd_fd, cmdline_len,
                       cmdline, flags);
}

static int kexec_load(void)
{
        char cmdline[COMMAND_LINE_SIZE];
        ssize_t len;
        int fd, err;

        fd = open("/proc/cmdline", O_RDONLY);
        if (fd < 0)
                return -1;

        len = read(fd, cmdline, sizeof(cmdline));
        close(fd);
        if (len < 0)
                return -1;

        /* replace \n with \0 */
        cmdline[len - 1] = 0;
        fd = open(KERNEL_IMAGE, O_RDONLY);
        if (fd < 0)
                return -1;

        err = kexec_file_load(fd, -1, len, cmdline, KEXEC_FILE_NO_INITRAMFS);
        close(fd);

        return err ? : 0;
}

int main(int argc, char *argv[])
{
        if (mount_filesystems())
                goto err_reboot;

        if (kho_enable())
                goto err_reboot;

        if (kexec_load())
                goto err_reboot;

        if (reboot(RB_KEXEC))
                goto err_reboot;

        return 0;

err_reboot:
        reboot(RB_AUTOBOOT);
        return -1;
}