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"
@@ -129,7 +130,7 @@ func (u Util) WriteLink(s types.Link) error {
 	path := s.Path
 
 	if err := MkdirForFile(path); err != nil {
-		return fmt.Errorf("Could not create leading directories: %v", err)
+		return fmt.Errorf("could not create leading directories: %v", err)
 	}
 
 	if cutil.IsTrue(s.Hard) {
@@ -141,7 +142,7 @@ func (u Util) WriteLink(s types.Link) error {
 	}
 
 	if err := os.Symlink(*s.Target, path); err != nil {
-		return fmt.Errorf("Could not create symlink: %v", err)
+		return fmt.Errorf("could not create symlink: %v", err)
 	}
 
 	if err := u.SetPermissions(nil, s.Node); err != nil {
@@ -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 +169,23 @@ 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 {
@@ -337,23 +354,23 @@ func (u Util) ResolveNodeUidAndGid(n types.Node, defaultUid, defaultGid int) (in
 func (u Util) getUserID(name string) (int, error) {
 	usr, err := u.userLookup(name)
 	if err != nil {
-		return 0, fmt.Errorf("No such user %q: %v", name, err)
+		return 0, fmt.Errorf("no such user %q: %v", name, err)
 	}
 	uid, err := strconv.ParseInt(usr.Uid, 0, 0)
 	if err != nil {
-		return 0, fmt.Errorf("Couldn't parse uid %q: %v", usr.Uid, err)
+		return 0, fmt.Errorf("couldn't parse uid %q: %v", usr.Uid, err)
 	}
 	return int(uid), nil
 }
 
 func (u Util) getGroupID(name string) (int, error) {
 	g, err := u.groupLookup(name)
 	if err != nil {
-		return 0, fmt.Errorf("No such group %q: %v", name, err)
+		return 0, fmt.Errorf("no such group %q: %v", name, err)
 	}
 	gid, err := strconv.ParseInt(g.Gid, 0, 0)
 	if err != nil {
-		return 0, fmt.Errorf("Couldn't parse gid %q: %v", g.Gid, err)
+		return 0, fmt.Errorf("couldn't parse gid %q: %v", g.Gid, err)
 	}
 	return int(gid), nil
 }