package fusefrontend // Helper functions for sparse files (files with holes) import ( "runtime" "syscall" "github.com/hanwen/go-fuse/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 *File) 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 *File) 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 *File) 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 }