1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
|
package mounttree // import "github.com/docker/docker/internal/mounttree"
import (
"fmt"
"os"
"path/filepath"
"github.com/moby/sys/mount"
"github.com/moby/sys/mountinfo"
"golang.org/x/sys/unix"
)
// SwitchRoot changes path to be the root of the mount tree and changes the
// current working directory to the new root.
//
// This function bind-mounts onto path; it is the caller's responsibility to set
// the desired propagation mode of path's parent mount beforehand to prevent
// unwanted propagation into different mount namespaces.
func SwitchRoot(path string) error {
if mounted, _ := mountinfo.Mounted(path); !mounted {
if err := mount.Mount(path, path, "bind", "rbind,rw"); err != nil {
return realChroot(path)
}
}
// setup oldRoot for pivot_root
pivotDir, err := os.MkdirTemp(path, ".pivot_root")
if err != nil {
return fmt.Errorf("Error setting up pivot dir: %v", err)
}
var mounted bool
defer func() {
if mounted {
// make sure pivotDir is not mounted before we try to remove it
if errCleanup := unix.Unmount(pivotDir, unix.MNT_DETACH); errCleanup != nil {
if err == nil {
err = errCleanup
}
return
}
}
errCleanup := os.Remove(pivotDir)
// pivotDir doesn't exist if pivot_root failed and chroot+chdir was successful
// because we already cleaned it up on failed pivot_root
if errCleanup != nil && !os.IsNotExist(errCleanup) {
errCleanup = fmt.Errorf("Error cleaning up after pivot: %v", errCleanup)
if err == nil {
err = errCleanup
}
}
}()
if err := unix.PivotRoot(path, pivotDir); err != nil {
// If pivot fails, fall back to the normal chroot after cleaning up temp dir
if err := os.Remove(pivotDir); err != nil {
return fmt.Errorf("Error cleaning up after failed pivot: %v", err)
}
return realChroot(path)
}
mounted = true
// This is the new path for where the old root (prior to the pivot) has been moved to
// This dir contains the rootfs of the caller, which we need to remove so it is not visible during extraction
pivotDir = filepath.Join("/", filepath.Base(pivotDir))
if err := unix.Chdir("/"); err != nil {
return fmt.Errorf("Error changing to new root: %v", err)
}
// Make the pivotDir (where the old root lives) private so it can be unmounted without propagating to the host
if err := unix.Mount("", pivotDir, "", unix.MS_PRIVATE|unix.MS_REC, ""); err != nil {
return fmt.Errorf("Error making old root private after pivot: %v", err)
}
// Now unmount the old root so it's no longer visible from the new root
if err := unix.Unmount(pivotDir, unix.MNT_DETACH); err != nil {
return fmt.Errorf("Error while unmounting old root after pivot: %v", err)
}
mounted = false
return nil
}
func realChroot(path string) error {
if err := unix.Chroot(path); err != nil {
return fmt.Errorf("Error after fallback to chroot: %v", err)
}
if err := unix.Chdir("/"); err != nil {
return fmt.Errorf("Error changing to new root after chroot: %v", err)
}
return nil
}
|
