113 lines
3.8 KiB
Go
113 lines
3.8 KiB
Go
package chrootarchive // import "github.com/docker/docker/pkg/chrootarchive"
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import (
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"fmt"
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"io/ioutil"
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"os"
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"path/filepath"
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"github.com/docker/docker/pkg/mount"
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rsystem "github.com/opencontainers/runc/libcontainer/system"
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"golang.org/x/sys/unix"
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)
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// chroot on linux uses pivot_root instead of chroot
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// pivot_root takes a new root and an old root.
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// Old root must be a sub-dir of new root, it is where the current rootfs will reside after the call to pivot_root.
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// New root is where the new rootfs is set to.
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// Old root is removed after the call to pivot_root so it is no longer available under the new root.
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// This is similar to how libcontainer sets up a container's rootfs
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func chroot(path string) (err error) {
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// if the engine is running in a user namespace we need to use actual chroot
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if rsystem.RunningInUserNS() {
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return realChroot(path)
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}
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if err := unix.Unshare(unix.CLONE_NEWNS); err != nil {
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return fmt.Errorf("Error creating mount namespace before pivot: %v", err)
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}
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// Make everything in new ns slave.
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// Don't use `private` here as this could race where the mountns gets a
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// reference to a mount and an unmount from the host does not propagate,
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// which could potentially cause transient errors for other operations,
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// even though this should be relatively small window here `slave` should
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// not cause any problems.
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if err := mount.MakeRSlave("/"); err != nil {
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return err
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}
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if mounted, _ := mount.Mounted(path); !mounted {
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if err := mount.Mount(path, path, "bind", "rbind,rw"); err != nil {
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return realChroot(path)
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}
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}
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// setup oldRoot for pivot_root
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pivotDir, err := ioutil.TempDir(path, ".pivot_root")
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if err != nil {
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return fmt.Errorf("Error setting up pivot dir: %v", err)
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}
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var mounted bool
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defer func() {
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if mounted {
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// make sure pivotDir is not mounted before we try to remove it
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if errCleanup := unix.Unmount(pivotDir, unix.MNT_DETACH); errCleanup != nil {
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if err == nil {
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err = errCleanup
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}
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return
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}
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}
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errCleanup := os.Remove(pivotDir)
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// pivotDir doesn't exist if pivot_root failed and chroot+chdir was successful
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// because we already cleaned it up on failed pivot_root
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if errCleanup != nil && !os.IsNotExist(errCleanup) {
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errCleanup = fmt.Errorf("Error cleaning up after pivot: %v", errCleanup)
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if err == nil {
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err = errCleanup
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}
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}
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}()
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if err := unix.PivotRoot(path, pivotDir); err != nil {
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// If pivot fails, fall back to the normal chroot after cleaning up temp dir
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if err := os.Remove(pivotDir); err != nil {
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return fmt.Errorf("Error cleaning up after failed pivot: %v", err)
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}
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return realChroot(path)
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}
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mounted = true
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// This is the new path for where the old root (prior to the pivot) has been moved to
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// This dir contains the rootfs of the caller, which we need to remove so it is not visible during extraction
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pivotDir = filepath.Join("/", filepath.Base(pivotDir))
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if err := unix.Chdir("/"); err != nil {
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return fmt.Errorf("Error changing to new root: %v", err)
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}
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// Make the pivotDir (where the old root lives) private so it can be unmounted without propagating to the host
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if err := unix.Mount("", pivotDir, "", unix.MS_PRIVATE|unix.MS_REC, ""); err != nil {
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return fmt.Errorf("Error making old root private after pivot: %v", err)
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}
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// Now unmount the old root so it's no longer visible from the new root
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if err := unix.Unmount(pivotDir, unix.MNT_DETACH); err != nil {
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return fmt.Errorf("Error while unmounting old root after pivot: %v", err)
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}
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mounted = false
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return nil
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}
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func realChroot(path string) error {
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if err := unix.Chroot(path); err != nil {
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return fmt.Errorf("Error after fallback to chroot: %v", err)
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}
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if err := unix.Chdir("/"); err != nil {
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return fmt.Errorf("Error changing to new root after chroot: %v", err)
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}
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return nil
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}
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