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

@@ -393,6 +393,9 @@ func (s stage) getOrderedCreationList(config types.Config) ([]filesystemEntry, e
 	// Map from paths in the config to where they resolve for duplicate checking
 	paths := map[string]string{}
 	for _, d := range config.Storage.Directories {
+		if d.Mode != nil {
+			d.Mode = cutil.IntToPtr(int(util.ToFileMode(d.Mode)))
+		}
 		path, err := s.JoinPath(d.Path)
 		if err != nil {
 			return nil, err
@@ -407,6 +410,9 @@ func (s stage) getOrderedCreationList(config types.Config) ([]filesystemEntry, e
 	}
 
 	for _, f := range config.Storage.Files {
+		if f.Mode != nil {
+			f.Mode = cutil.IntToPtr(int(util.ToFileMode(f.Mode)))
+		}
 		path, err := s.JoinPath(f.Path)
 		if err != nil {
 			return nil, err

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(int(*m))
+	res := os.FileMode(mode & 0000777)
+
+	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 {