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/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"
@@ -152,8 +153,7 @@ func (u Util) WriteLink(s types.Link) error {
 
 func (u Util) SetPermissions(mode *int, node types.Node) error {
 	if mode != nil {
-		mode := os.FileMode(*mode)
-		if err := os.Chmod(node.Path, mode); err != nil {
+		if err := os.Chmod(node.Path, toFileMode(*mode)); err != nil {
 			return fmt.Errorf("failed to change mode of %s: %v", node.Path, err)
 		}
 	}
@@ -169,6 +169,24 @@ 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 {