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      1 /* switch_root.c - Switch from rootfs/initramfs to another filesystem
      2  *
      3  * Copyright 2005 Rob Landley <rob (at) landley.net>
      4 
      5 USE_SWITCH_ROOT(NEWTOY(switch_root, "<2c:h", TOYFLAG_SBIN))
      6 
      7 config 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 
     24 GLOBALS(
     25   char *console;
     26 
     27   dev_t rootdev;
     28 )
     29 
     30 static 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 
     44 void 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.console && -1 == (console = open(TT.console, O_RDWR))) {
     82     perror_msg("bad console '%s'", TT.console);
     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.console) {
    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);
    114 panic:
    115   if (toys.optflags & FLAG_h) for (;;) wait(NULL);
    116 }
    117