internal/exec/*: preserve special mode bits for a given file/dir

This allows Ignition to preserve the special mode bits for specs >=
3.4.0

Fixes: coreos#1301

internal/exec/stages/files/filesystemEntries.go

@@ -296,7 +296,7 @@ func (tmp fileEntry) create(l *log.Logger, u util.Util) error {
 			return fmt.Errorf("failed to create file %q: %v", op.Node.Path, err)
 		}
 	}
-	if err := u.SetPermissions(f.Mode, f.Node); err != nil {
+	if err := u.SetPermissions(cutil.IntToPtr(int(util.ToFileMode(f.Mode))), f.Node); err != nil {
 		return fmt.Errorf("error setting file permissions for %s: %v", f.Path, err)
 	}
 	return nil
@@ -323,7 +323,7 @@ func (tmp dirEntry) create(l *log.Logger, u util.Util) error {
 		return fmt.Errorf("error creating directory %s: A non-directory already exists and overwrite is false", d.Path)
 	}
 
-	if err := u.SetPermissions(d.Mode, d.Node); err != nil {
+	if err := u.SetPermissions(cutil.IntToPtr(int(util.ToFileMode(d.Mode))), d.Node); err != nil {
 		return fmt.Errorf("error setting directory permissions for %s: %v", d.Path, err)
 	}
 	return nil

internal/exec/util/file.go

@@ -25,6 +25,7 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
+	"syscall"
 
 	cutil "github.com/coreos/ignition/v2/config/util"
 	"github.com/coreos/ignition/v2/config/v3_4_experimental/types"
@@ -169,6 +170,25 @@ func (u Util) SetPermissions(mode *int, node types.Node) error {
 	return nil
 }
 
+// ToFileMode converts Go permission bits to POSIX permission bits.
+func ToFileMode(m *int) os.FileMode {
+	mode := uint32(*m)
+	res := os.FileMode(mode & 0777)
+
+	if mode&syscall.S_ISGID != 0 {
+		res |= os.ModeSetgid
+
+	}
+	if mode&syscall.S_ISUID != 0 {
+		res |= os.ModeSetuid
+	}
+	if mode&syscall.S_ISVTX != 0 {
+		res |= os.ModeSticky
+	}
+
+	return res
+}
+
 // PerformFetch performs a fetch operation generated by PrepareFetch, retrieving
 // the file and writing it to disk. Any encountered errors are returned.
 func (u Util) PerformFetch(f FetchOp) error {