1 files changed, 92 insertions, 0 deletions
diff --git a/internal/fusefrontend/file2_holes.go b/internal/fusefrontend/file2_holes.go
new file mode 100644
index 0000000..5e06981
--- /dev/null
+++ b/internal/fusefrontend/file2_holes.go
@@ -0,0 +1,92 @@
+package fusefrontend
+
+// Helper functions for sparse files (files with holes)
+
+import (
+	"runtime"
+	"syscall"
+
+	"github.com/hanwen/go-fuse/v2/fuse"
+
+	"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 *File2) writePadHole(targetOff int64) fuse.Status {
+	// Get the current file size.
+	fi, err := f.fd.Stat()
+	if err != nil {
+		tlog.Warn.Printf("checkAndPadHole: Fstat failed: %v", err)
+		return fuse.ToStatus(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 fuse.OK
+	}
+	// 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.
+	status := f.zeroPad(plainSize)
+	if status != fuse.OK {
+		return status
+	}
+	return fuse.OK
+}
+
+// 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 *File2) zeroPad(plainSize uint64) fuse.Status {
+	lastBlockLen := plainSize % f.contentEnc.PlainBS()
+	if lastBlockLen == 0 {
+		// Already block-aligned
+		return fuse.OK
+	}
+	missing := f.contentEnc.PlainBS() - lastBlockLen
+	pad := make([]byte, missing)
+	tlog.Debug.Printf("zeroPad: Writing %d bytes\n", missing)
+	_, status := f.doWrite(pad, int64(plainSize))
+	return status
+}
+
+// SeekData calls the lseek syscall with SEEK_DATA. It returns the offset of the
+// next data bytes, skipping over file holes.
+func (f *File2) SeekData(oldOffset int64) (int64, error) {
+	if runtime.GOOS != "linux" {
+		// Does MacOS support something like this?
+		return 0, syscall.EOPNOTSUPP
+	}
+	const SEEK_DATA = 3
+
+	// Convert plaintext offset to ciphertext offset and round down to the
+	// start of the current block. File holes smaller than a full block will
+	// be ignored.
+	blockNo := f.contentEnc.PlainOffToBlockNo(uint64(oldOffset))
+	oldCipherOff := int64(f.contentEnc.BlockNoToCipherOff(blockNo))
+
+	// Determine the next data offset. If the old offset points to (or beyond)
+	// the end of the file, the Seek syscall fails with syscall.ENXIO.
+	newCipherOff, err := syscall.Seek(f.intFd(), oldCipherOff, SEEK_DATA)
+	if err != nil {
+		return 0, err
+	}
+
+	// Convert ciphertext offset back to plaintext offset. At this point,
+	// newCipherOff should always be >= contentenc.HeaderLen. Round down,
+	// but ensure that the result is never smaller than the initial offset
+	// (to avoid endless loops).
+	blockNo = f.contentEnc.CipherOffToBlockNo(uint64(newCipherOff))
+	newOffset := int64(f.contentEnc.BlockNoToPlainOff(blockNo))
+	if newOffset < oldOffset {
+		newOffset = oldOffset
+	}
+
+	return newOffset, nil
+}