LXR toybox/toys/other/switch

   1/* switch_root.c - Switch from rootfs/initramfs to another filesystem
   2 *
   3 * Copyright 2005 Rob Landley <rob@landley.net>
   4
   5USE_SWITCH_ROOT(NEWTOY(switch_root, "<2c:h", TOYFLAG_SBIN))
   6
   7config SWITCH_ROOT
   8  bool "switch_root"
   9  default y
  10  help
  11    usage: switch_root [-c /dev/console] NEW_ROOT NEW_INIT...
  12
  13    Use from PID 1 under initramfs to free initramfs, chroot to NEW_ROOT,
  14    and exec NEW_INIT.
  15
  16    -c  Redirect console to device in NEW_ROOT
  17    -h  Hang instead of exiting on failure (avoids kernel panic)
  18*/
  19
  20#define FOR_switch_root
  21#include "toys.h"
  22#include <sys/vfs.h>
  23
  24GLOBALS(
  25  char *c;
  26
  27  dev_t rootdev;
  28)
  29
  30static int del_node(struct dirtree *node)
  31{
  32  if (node->st.st_dev == TT.rootdev && dirtree_notdotdot(node)) {
  33    int flag = 0;
  34    if (S_ISDIR(node->st.st_mode)) {
  35      if (!node->again) return DIRTREE_COMEAGAIN;
  36      flag = AT_REMOVEDIR;
  37    }
  38    unlinkat(dirtree_parentfd(node), node->name, flag);
  39  }
  40
  41  return 0;
  42}
  43
  44void switch_root_main(void)
  45{
  46  char *newroot = *toys.optargs, **cmdline = toys.optargs+1;
  47  struct stat st1, st2;
  48  struct statfs stfs;
  49  int console = console; // gcc's "may be used" warnings are broken.
  50
  51  if (getpid() != 1) error_exit("not pid 1");
  52
  53  // Root filesystem we're leaving must be ramfs or tmpfs
  54  if (statfs("/", &stfs) ||
  55    (stfs.f_type != 0x858458f6 && stfs.f_type != 0x01021994))
  56  {
  57    error_msg("not ramfs");
  58    goto panic;
  59  }
  60
  61  // New directory must be different filesystem instance
  62  if (chdir(newroot) || stat(".", &st1) || stat("/", &st2) ||
  63    st1.st_dev == st2.st_dev)
  64  {
  65    error_msg("bad newroot '%s'", newroot);
  66    goto panic;
  67  }
  68  TT.rootdev=st2.st_dev;
  69
  70  // trim any / characters from the init cmdline, as we want to test it with
  71  // stat(), relative to newroot. *cmdline is also used below, but by that
  72  // point we are in the chroot, so a relative path is still OK.
  73  while (**cmdline == '/') (*cmdline)++;
  74
  75  // init program must exist and be an executable file
  76  if (stat(*cmdline, &st1) || !S_ISREG(st1.st_mode) || !(st1.st_mode&0100)) {
  77    error_msg("bad init");
  78    goto panic;
  79  }
  80
  81  if (TT.c && -1 == (console = open(TT.c, O_RDWR))) {
  82    perror_msg("bad console '%s'", TT.c);
  83    goto panic;
  84  }
  85 
  86  // Ok, enough safety checks: wipe root partition.
  87  dirtree_read("/", del_node);
  88
  89  // Fix the appearance of the mount table in the newroot chroot
  90  if (mount(".", "/", NULL, MS_MOVE, NULL)) {
  91    perror_msg("mount");
  92    goto panic;
  93  }
  94
  95  // Enter the new root before starting init
  96  if (chroot(".")) {
  97    perror_msg("chroot");
  98    goto panic;
  99  }
 100
 101  // Make sure cwd does not point outside of the chroot
 102  if (chdir("/")) {
 103    perror_msg("chdir");
 104    goto panic;
 105  }
 106
 107  if (TT.c) {
 108    int i;
 109    for (i=0; i<3; i++) if (console != i) dup2(console, i);
 110    if (console>2) close(console);
 111  }
 112  execv(*cmdline, cmdline);
 113  perror_msg("Failed to exec '%s'", *cmdline);
 114panic:
 115  if (toys.optflags & FLAG_h) for (;;) wait(NULL);
 116}
 117