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libct: use preopened rootfs more

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This uses preopened rootfs in Chdir and pivotRoot.

While at it, add O_PATH when opening oldroot in pivotRoot.

Co-authored-by: Kir Kolyshkin <kolyshkin@gmail.com>
Signed-off-by: lfbzhm <lifubang@acmcoder.com>
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
This commit is contained in:
lfbzhm
2026-03-27 13:27:43 -07:00
committed by Kir Kolyshkin
parent 28cb321887
commit 5b094ed1ac
1 changed files with 6 additions and 12 deletions
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libcontainer/rootfs_linux.go
+6 -12
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@@ -212,7 +212,7 @@ func prepareRootfs(pipe *syncSocket, iConfig *initConfig) (err error) {
// container. It's just cleaner to do this here (at the expense of the // container. It's just cleaner to do this here (at the expense of the
// operation not being perfectly split). // operation not being perfectly split).
if err := unix.Chdir(config.Rootfs); err != nil { if err := unix.Fchdir(int(rootFd.Fd())); err != nil {
return &os.PathError{Op: "chdir", Path: config.Rootfs, Err: err} return &os.PathError{Op: "chdir", Path: config.Rootfs, Err: err}
} }
@@ -227,7 +227,7 @@ func prepareRootfs(pipe *syncSocket, iConfig *initConfig) (err error) {
if config.NoPivotRoot { if config.NoPivotRoot {
err = msMoveRoot(config.Rootfs) err = msMoveRoot(config.Rootfs)
} else if config.Namespaces.Contains(configs.NEWNS) { } else if config.Namespaces.Contains(configs.NEWNS) {
err = pivotRoot(config.Rootfs) err = pivotRoot(rootFd)
} else { } else {
err = chroot() err = chroot()
} }
@@ -1134,28 +1134,22 @@ func setupPtmx(config *configs.Config) error {
// pivotRoot will call pivot_root such that rootfs becomes the new root // pivotRoot will call pivot_root such that rootfs becomes the new root
// filesystem, and everything else is cleaned up. // filesystem, and everything else is cleaned up.
func pivotRoot(rootfs string) error { func pivotRoot(root *os.File) error {
// While the documentation may claim otherwise, pivot_root(".", ".") is // While the documentation may claim otherwise, pivot_root(".", ".") is
// actually valid. What this results in is / being the new root but // actually valid. What this results in is / being the new root but
// /proc/self/cwd being the old root. Since we can play around with the cwd // /proc/self/cwd being the old root. Since we can play around with the cwd
// with pivot_root this allows us to pivot without creating directories in // with pivot_root this allows us to pivot without creating directories in
// the rootfs. Shout-outs to the LXC developers for giving us this idea. // the rootfs. Shout-outs to the LXC developers for giving us this idea.
oldroot, err := linux.Open("/", unix.O_DIRECTORY|unix.O_RDONLY, 0) oldroot, err := linux.Open("/", unix.O_DIRECTORY|unix.O_RDONLY|unix.O_PATH, 0)
if err != nil { if err != nil {
return err return err
} }
defer unix.Close(oldroot) defer unix.Close(oldroot)
newroot, err := linux.Open(rootfs, unix.O_DIRECTORY|unix.O_RDONLY, 0)
if err != nil {
return err
}
defer unix.Close(newroot)
// Change to the new root so that the pivot_root actually acts on it. // Change to the new root so that the pivot_root actually acts on it.
if err := unix.Fchdir(newroot); err != nil { if err := unix.Fchdir(int(root.Fd())); err != nil {
return &os.PathError{Op: "fchdir", Path: "fd " + strconv.Itoa(newroot), Err: err} return &os.PathError{Op: "chdir", Path: root.Name(), Err: err}
} }
if err := unix.PivotRoot(".", "."); err != nil { if err := unix.PivotRoot(".", "."); err != nil {