1 /* switch_root.c - Switch from rootfs/initramfs to another filesystem
2 *
3 * Copyright 2005 Rob Landley <rob@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
GLOBALS(char * c;dev_t rootdev;)24 GLOBALS(
25 char *c;
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
switch_root_main(void)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.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);
114 panic:
115 if (toys.optflags & FLAG_h) for (;;) wait(NULL);
116 }
117