mirror of https://github.com/docker/docs.git
96 lines
3.2 KiB
Go
96 lines
3.2 KiB
Go
package 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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"syscall"
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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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// Create new mount namespace so mounts don't leak
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if err := syscall.Unshare(syscall.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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// path must be a different fs for pivot_root, so bind-mount to itself to ensure this
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if err := syscall.Mount(path, path, "", syscall.MS_BIND, ""); err != nil {
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return fmt.Errorf("Error mounting pivot dir before pivot: %v", err)
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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 := syscall.Unmount(pivotDir, syscall.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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// but 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 := syscall.PivotRoot(path, pivotDir); err != nil {
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// If pivot fails, fall back to the normal chroot after cleaning up temp dir for pivot_root
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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 := syscall.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 := syscall.Mount("", pivotDir, "", syscall.MS_PRIVATE|syscall.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 := syscall.Unmount(pivotDir, syscall.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 := syscall.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 := syscall.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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