rootless: exec handle processes that create an user namespace

Manage the case where the main process of the container creates and
joins a new user namespace.

In this case we want to join only the first child in the new
hierarchy, which is the user namespace that was used to create the
container.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

Closes: #1331
Approved by: rhatdan

pkg/rootless/rootless_unsupported.go

@@ -3,6 +3,8 @@
 package rootless
 
 import (
+	"os"
+
 	"github.com/pkg/errors"
 )
 
@@ -30,3 +32,8 @@ func SetSkipStorageSetup(bool) {
 func SkipStorageSetup() bool {
 	return false
 }
+
+// GetUserNSForPid returns an open FD for the first direct child user namespace that created the process
+func GetUserNSForPid(pid uint) (*os.File, error) {
+	return nil, errors.New("this function is not supported on this os")
+}