nametransform: WriteDirIV: replace ioutil.WriteFile

As reported at https://github.com/rfjakob/gocryptfs/issues/105 ,
the "ioutil.WriteFile(file, iv, 0400)" call causes "permissions denied"
errors on an NFSv4 setup.

"strace"ing diriv creation and gocryptfs.conf creation shows this:

conf (works on the user's NFSv4 mount):
openat(AT_FDCWD, "/tmp/a/gocryptfs.conf.tmp", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0400) = 3

diriv (fails):
openat(AT_FDCWD, "/tmp/a/gocryptfs.diriv", O_WRONLY|O_CREAT|O_TRUNC|O_CLOEXEC, 0400) = 3

This patch creates the diriv file with the same flags that are used for
creating the conf:
openat(AT_FDCWD, "/tmp/a/gocryptfs.diriv", O_WRONLY|O_CREAT|O_EXCL|O_CLOEXEC, 0400) = 3

Closes https://github.com/rfjakob/gocryptfs/issues/105

1 files changed, 16 insertions, 4 deletions

diff --git a/internal/nametransform/diriv.go b/internal/nametransform/diriv.go
index e4670c8..00d059f 100644
--- a/internal/nametransform/diriv.go
+++ b/internal/nametransform/diriv.go
@@ -2,7 +2,6 @@ package nametransform
 
 import (
 	"io"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strings"
@@ -71,11 +70,24 @@ func fdReadDirIV(fd *os.File) (iv []byte, err error) {
 func WriteDirIV(dir string) error {
 	iv := cryptocore.RandBytes(DirIVLen)
 	file := filepath.Join(dir, DirIVFilename)
-	err := ioutil.WriteFile(file, iv, 0400)
+	// 0400 permissions: gocryptfs.diriv should never be modified after creation.
+	// Don't use "ioutil.WriteFile", it causes trouble on NFS: https://github.com/rfjakob/gocryptfs/issues/105
+	fd, err := os.OpenFile(file, os.O_WRONLY|os.O_CREATE|os.O_EXCL, 0400)
 	if err != nil {
-		tlog.Warn.Printf("WriteDirIV: %v", err)
+		tlog.Warn.Printf("WriteDirIV: OpenFile: %v", err)
+		return err
 	}
-	return err
+	_, err = fd.Write(iv)
+	if err != nil {
+		tlog.Warn.Printf("WriteDirIV: Write: %v", err)
+		return err
+	}
+	err = fd.Close()
+	if err != nil {
+		tlog.Warn.Printf("WriteDirIV: Close: %v", err)
+		return err
+	}
+	return nil
 }
 
 // EncryptPathDirIV - encrypt relative plaintext path using EME with DirIV.