correct container "/" ownership with userns

Previously "`/`" in a container was always owned by `root:root` (0/0), even if
`withRemappedSnapshot` had been used. Meaning that if `withUserNamespace` is
used then `/` can be remapped to `nobody:nogroup` (65534/65534).

The fix is is twofold:

- incrementFS should operate on the root of the tree.
- when creating a new snapshot we must propagate the ownership of the topmost
  "lower" directory into the new "upper".

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

snapshot/overlay/overlay.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"syscall"
 
 	"github.com/containerd/containerd/fs"
 	"github.com/containerd/containerd/log"
@@ -274,7 +275,8 @@ func (o *snapshotter) createSnapshot(ctx context.Context, kind snapshot.Kind, ke
 		}
 	}()
 
-	if err = os.MkdirAll(filepath.Join(td, "fs"), 0755); err != nil {
+	fs := filepath.Join(td, "fs")
+	if err = os.MkdirAll(fs, 0755); err != nil {
 		return nil, err
 	}
 
@@ -297,6 +299,25 @@ func (o *snapshotter) createSnapshot(ctx context.Context, kind snapshot.Kind, ke
 		return nil, errors.Wrap(err, "failed to create active")
 	}
 
+	if len(s.ParentIDs) > 0 {
+		st, err := os.Stat(filepath.Join(o.upperPath(s.ParentIDs[0])))
+		if err != nil {
+			if rerr := t.Rollback(); rerr != nil {
+				log.G(ctx).WithError(rerr).Warn("Failure rolling back transaction")
+			}
+			return nil, errors.Wrap(err, "failed to stat parent")
+		}
+
+		stat := st.Sys().(*syscall.Stat_t)
+
+		if err := os.Lchown(fs, int(stat.Uid), int(stat.Gid)); err != nil {
+			if rerr := t.Rollback(); rerr != nil {
+				log.G(ctx).WithError(rerr).Warn("Failure rolling back transaction")
+			}
+			return nil, errors.Wrap(err, "failed to chown")
+		}
+	}
+
 	path = filepath.Join(snapshotDir, s.ID)
 	if err = os.Rename(td, path); err != nil {
 		if rerr := t.Rollback(); rerr != nil {

spec_unix.go

@@ -171,9 +171,6 @@ func incrementFS(root string, uidInc, gidInc uint32) filepath.WalkFunc {
 		if err != nil {
 			return err
 		}
-		if root == path {
-			return nil
-		}
 		var (
 			stat = info.Sys().(*syscall.Stat_t)
 			u, g = int(stat.Uid + uidInc), int(stat.Gid + gidInc)