/* switch_root.c - Switch from rootfs/initramfs to another filesystem
 *
 * Copyright 2005 Rob Landley <rob@landley.net>

USE_SWITCH_ROOT(NEWTOY(switch_root, "<2c:h", TOYFLAG_SBIN))

config SWITCH_ROOT
	bool "switch_root"
	default y
	help
	  usage: switch_root [-c /dev/console] NEW_ROOT NEW_INIT...

	  Use from PID 1 under initramfs to free initramfs, chroot to NEW_ROOT,
	  and exec NEW_INIT.

	  -c	Redirect console to device in NEW_ROOT
	  -h	Hang instead of exiting on failure (avoids kernel panic)
*/

#include "toys.h"
#include <sys/vfs.h>

DEFINE_GLOBALS(
   char *console;

   dev_t rootdev;
)

#define TT this.switch_root

#define FLAG_h  (1<<0)
#define FLAG_c	(1<<1)

static int del_node(struct dirtree *node)
{
   if (node->st.st_dev == TT.rootdev && dirtree_notdotdot(node)) {
      int flag = 0;
      if (S_ISDIR(node->st.st_mode)) {
         if (node->data != -1) return DIRTREE_COMEAGAIN;
         flag = AT_REMOVEDIR;
      }
      unlinkat(dirtree_parentfd(node), node->name, flag);
   }

   return 0;
}

void switch_root_main(void)
{
   char *newroot = *toys.optargs, **cmdline = toys.optargs+1;
   struct stat st1, st2;
   struct statfs stfs;
   int console = console; // gcc's "may be used" warnings are broken.

   if (getpid() != 1) error_exit("not pid 1");

   // Root filesystem we're leaving must be ramfs or tmpfs
   if (statfs("/", &stfs) ||
      (stfs.f_type != 0x858458f6 && stfs.f_type != 0x01021994))
   {
      error_msg("not ramfs");
      goto panic;
   }

   // New directory must be different filesystem instance
   if (chdir(newroot) || stat(".", &st1) || stat("/", &st2) ||
      st1.st_dev == st2.st_dev)
   {
      error_msg("bad newroot '%s'", newroot);
      goto panic;
   }
   TT.rootdev=st2.st_dev;

   // init program must exist and be an executable file
   if (stat("init", &st1) || !S_ISREG(st1.st_mode) || !(st1.st_mode&0100)) {
      error_msg("bad init");
      goto panic;
   }

   if (TT.console && -1 == (console = open(TT.console, O_RDWR))) {
      perror_msg("bad console '%s'", TT.console);
      goto panic;
   }
 
   // Ok, enough safety checks: wipe root partition.
   dirtree_read("/", del_node);

   if (TT.console) {
      int i;
      for (i=0; i<3; i++) if (console != i) dup2(console, i);
      if (console>2) close(console);
   }
   execv(*cmdline, cmdline);
   perror_msg("Failed to exec '%s'", *cmdline);
panic:
   if (toys.optflags & FLAG_h) for (;;) wait(NULL);
}