finit/src/initramfs.c

/* Initramfs switch_root support
 *
 * Copyright (c) 2008-2025  Joachim Wiberg <troglobit@gmail.com>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#include "config.h"		/* Generated by configure script */

#include <errno.h>
#include <fcntl.h>
#include <ftw.h>
#include <limits.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/vfs.h>
#include <sys/wait.h>
#include <unistd.h>
#include <linux/magic.h>
#ifdef _LIBITE_LITE
# include <libite/lite.h>
#else
# include <lite/lite.h>
#endif

#include "finit.h"
#include "cond.h"
#include "helpers.h"
#include "log.h"
#include "plugin.h"
#include "private.h"
#include "sig.h"
#include "util.h"

/* State for delete_cb(), nftw() doesn't support passing user data */
static const char *delete_skip;
static dev_t delete_rootdev;

/*
 * nftw() callback to delete initramfs contents before switch_root.
 * Skips the new root mount point and stays on the same filesystem.
 */
static int delete_cb(const char *fpath, const struct stat *sb, int tflag, struct FTW *ftw)
{
	(void)tflag;

	/* Skip root directory itself */
	if (ftw->level == 0)
		return 0;

	/* Skip the new root mount point */
	if (delete_skip && !strncmp(fpath, delete_skip, strlen(delete_skip)))
		return 0;

	/* Stay on the same filesystem (initramfs) */
	if (sb->st_dev != delete_rootdev)
		return 0;

	remove(fpath);
	return 0;
}

/*
 * Recursively delete initramfs contents to free memory before switch_root.
 */
static void delete_initramfs_contents(dev_t rootdev, const char *skip)
{
	delete_rootdev = rootdev;
	delete_skip = skip;

	nftw("/", delete_cb, 20, FTW_DEPTH | FTW_PHYS | FTW_MOUNT);
}

/*
 * Check if we're running on a tmpfs/ramfs (initramfs)
 */
static int is_initramfs(void)
{
	struct statfs sfs;

	if (statfs("/", &sfs))
		return 0;

	return sfs.f_type == RAMFS_MAGIC || sfs.f_type == TMPFS_MAGIC;
}

/*
 * Move a mount point from oldpath to newpath under newroot
 */
static int do_move_mount(const char *oldpath, const char *newroot)
{
	char newpath[PATH_MAX];
	struct stat st;

	if (stat(oldpath, &st))
		return 0;		/* Not mounted, skip */

	snprintf(newpath, sizeof(newpath), "%s%s", newroot, oldpath);

	/* Create target directory if needed */
	makedir(newpath, 0755);

	if (mount(oldpath, newpath, NULL, MS_MOVE, NULL)) {
		dbg("Failed to move %s to %s: %s", oldpath, newpath, strerror(errno));
		return -1;
	}

	dbg("Moved %s to %s", oldpath, newpath);
	return 0;
}

static int kill_cb(int pid, void *data)
{
	int signo = *(int *)data;

	kill(pid, signo);

	return 0;
}

/*
 * Perform switch_root to a new root filesystem
 *
 * This function does not return on success - it exec's the new init.
 * On failure, it returns -1 and sets errno.
 */
int switch_root(const char *newroot, const char *newinit)
{
	struct stat newroot_st, oldroot_st;
	char init_path[PATH_MAX];
	int console_fd;
	dev_t rootdev;
	int signo;

	if (!newroot || !newroot[0]) {
		errno = EINVAL;
		return -1;
	}

	/* Default to /sbin/init if not specified */
	if (!newinit || !newinit[0])
		newinit = "/sbin/init";

	/* Verify we're PID 1 */
	if (getpid() != 1) {
		logit(LOG_ERR, "switch_root must be run as PID 1");
		errno = EPERM;
		return -1;
	}

	/* Verify newroot exists and is a directory */
	if (stat(newroot, &newroot_st) || !S_ISDIR(newroot_st.st_mode)) {
		logit(LOG_ERR, "switch_root: %s is not a directory", newroot);
		errno = ENOTDIR;
		return -1;
	}

	/* Verify newroot is a mount point (different device than parent) */
	if (stat("/", &oldroot_st)) {
		logit(LOG_ERR, "switch_root: cannot stat /");
		return -1;
	}

	if (newroot_st.st_dev == oldroot_st.st_dev) {
		logit(LOG_ERR, "switch_root: %s is not a mount point", newroot);
		errno = EINVAL;
		return -1;
	}

	/* Verify init exists in new root */
	snprintf(init_path, sizeof(init_path), "%s%s", newroot, newinit);
	if (access(init_path, X_OK)) {
		logit(LOG_ERR, "switch_root: %s not found or not executable", init_path);
		errno = ENOENT;
		return -1;
	}

	logit(LOG_NOTICE, "Performing switch_root to %s, init %s", newroot, newinit);

	/* Run switch_root hook before we start tearing things down */
	plugin_run_hooks(HOOK_SWITCH_ROOT);

	/* Stop all services gracefully */
	dbg("Stopping all services...");
	halt = SHUT_OFF;		/* Prevent actual shutdown */

	/* Use existing shutdown logic to stop services */
	api_exit();
	log_exit();
	plugin_run_hooks(HOOK_SHUTDOWN);

	/* Kill remaining processes (except kernel threads and ourselves) */
	signo = SIGTERM;
	iterate_proc(kill_cb, &signo);
	do_usleep(500000);	/* Give them 500ms */

	signo = SIGKILL;
	iterate_proc(kill_cb, &signo);

	/* Reap zombies */
	while (waitpid(-1, NULL, WNOHANG) > 0)
		;

	/* Exit plugins */
	plugin_exit();
	cond_exit();

	/* Move virtual filesystems to new root */
	dbg("Moving virtual filesystems...");
	do_move_mount("/dev", newroot);
	do_move_mount("/proc", newroot);
	do_move_mount("/sys", newroot);
	do_move_mount("/run", newroot);

	/* Change to new root directory */
	if (chdir(newroot)) {
		err(1, "Failed to chdir to %s", newroot);
		return -1;
	}

	/* Delete contents of old root if we're on initramfs */
	rootdev = oldroot_st.st_dev;
	if (is_initramfs()) {
		dbg("Deleting initramfs contents...");
		delete_initramfs_contents(rootdev, newroot);
	}

	/* Mount --move newroot to / */
	if (mount(newroot, "/", NULL, MS_MOVE, NULL)) {
		err(1, "Failed to move %s to /", newroot);
		return -1;
	}

	/* chroot to new root */
	if (chroot(".")) {
		err(1, "Failed to chroot to new root");
		return -1;
	}

	if (chdir("/")) {
		err(1, "Failed to chdir to /");
		return -1;
	}

	/* Reopen console */
	close(STDIN_FILENO);
	close(STDOUT_FILENO);
	close(STDERR_FILENO);

	console_fd = open("/dev/console", O_RDWR);
	if (console_fd >= 0) {
		dup2(console_fd, STDIN_FILENO);
		dup2(console_fd, STDOUT_FILENO);
		dup2(console_fd, STDERR_FILENO);
		if (console_fd > STDERR_FILENO)
			close(console_fd);
	}

	/* Reset signals to default */
	sig_unblock();

	/* Exec the new init - this does not return on success */
	dbg("Executing %s...", newinit);
	execl(newinit, newinit, NULL);

	/* If we get here, exec failed */
	err(1, "Failed to exec %s", newinit);
	return -1;
}

/**
 * Local Variables:
 *  indent-tabs-mode: t
 *  c-file-style: "linux"
 * End:
 */