1 files changed, 68 insertions, 0 deletions
diff --git a/internal/fusefrontend/file_holes.go b/internal/fusefrontend/file_holes.go
new file mode 100644
index 0000000..85792f9
--- /dev/null
+++ b/internal/fusefrontend/file_holes.go
@@ -0,0 +1,68 @@
+package fusefrontend
+
+// Helper functions for sparse files (files with holes)
+
+import (
+	"context"
+	"syscall"
+
+	"github.com/hanwen/go-fuse/v2/fs"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+// Will a write to plaintext offset "targetOff" create a file hole in the
+// ciphertext? If yes, zero-pad the last ciphertext block.
+func (f *File) writePadHole(targetOff int64) syscall.Errno {
+	// Get the current file size.
+	fi, err := f.fd.Stat()
+	if err != nil {
+		tlog.Warn.Printf("checkAndPadHole: Fstat failed: %v", err)
+		return fs.ToErrno(err)
+	}
+	plainSize := f.contentEnc.CipherSizeToPlainSize(uint64(fi.Size()))
+	// Appending a single byte to the file (equivalent to writing to
+	// offset=plainSize) would write to "nextBlock".
+	nextBlock := f.contentEnc.PlainOffToBlockNo(plainSize)
+	// targetBlock is the block the user wants to write to.
+	targetBlock := f.contentEnc.PlainOffToBlockNo(uint64(targetOff))
+	// The write goes into an existing block or (if the last block was full)
+	// starts a new one directly after the last block. Nothing to do.
+	if targetBlock <= nextBlock {
+		return 0
+	}
+	// The write goes past the next block. nextBlock has
+	// to be zero-padded to the block boundary and (at least) nextBlock+1
+	// will contain a file hole in the ciphertext.
+	errno := f.zeroPad(plainSize)
+	if errno != 0 {
+		return errno
+	}
+	return 0
+}
+
+// Zero-pad the file of size plainSize to the next block boundary. This is a no-op
+// if the file is already block-aligned.
+func (f *File) zeroPad(plainSize uint64) syscall.Errno {
+	lastBlockLen := plainSize % f.contentEnc.PlainBS()
+	if lastBlockLen == 0 {
+		// Already block-aligned
+		return 0
+	}
+	missing := f.contentEnc.PlainBS() - lastBlockLen
+	pad := make([]byte, missing)
+	tlog.Debug.Printf("zeroPad: Writing %d bytes\n", missing)
+	_, errno := f.doWrite(pad, int64(plainSize))
+	return errno
+}
+
+// Lseek - FUSE call.
+func (f *File) Lseek(ctx context.Context, off uint64, whence uint32) (uint64, syscall.Errno) {
+	cipherOff := f.rootNode.contentEnc.PlainSizeToCipherSize(off)
+	newCipherOff, err := syscall.Seek(f.intFd(), int64(cipherOff), int(whence))
+	if err != nil {
+		return uint64(newCipherOff), fs.ToErrno(err)
+	}
+	newOff := f.contentEnc.CipherSizeToPlainSize(uint64(newCipherOff))
+	return newOff, 0
+}