fallocate the space needed for the file header beforehand

This makes sure writing to a file fails early if the underlying
filesystem does not support fallocate. It also prevents partial header
write due to ENOSPC.

cryptfs/names_diriv.go

@@ -22,6 +22,7 @@ type DirIVCache struct {
 	lock sync.RWMutex
 }
 
+// lookup - fetch entry for "dir" from the cache
 func (c *DirIVCache) lookup(dir string) (bool, []byte, string) {
 	c.lock.RLock()
 	defer c.lock.RUnlock()
@@ -31,6 +32,7 @@ func (c *DirIVCache) lookup(dir string) (bool, []byte, string) {
 	return false, nil, ""
 }
 
+// store - write entry for "dir" into the caches
 func (c *DirIVCache) store(dir string, iv []byte, translatedDir string) {
 	c.lock.Lock()
 	defer c.lock.Unlock()

pathfs_frontend/file.go

@@ -93,7 +93,18 @@ func (f *file) readHeader() error {
 func (f *file) createHeader() error {
 	h := cryptfs.RandomHeader()
 	buf := h.Pack()
-	_, err := f.fd.WriteAt(buf, 0)
+
+	// Prevent partially written (=corrupt) header by preallocating the space beforehand
+	f.fdLock.Lock()
+	defer f.fdLock.Unlock()
+	err := syscall.Fallocate(int(f.fd.Fd()), FALLOC_FL_KEEP_SIZE, 0, cryptfs.HEADER_LEN)
+	if err != nil {
+			cryptfs.Warn.Printf("createHeader: Fallocate failed: %s\n", err.Error())
+			return err
+	}
+
+	// Actually write header
+	_, err = f.fd.WriteAt(buf, 0)
 	if err != nil {
 		return err
 	}