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Diffstat (limited to 'internal/fusefrontend/file2_holes.go')
-rw-r--r--internal/fusefrontend/file2_holes.go92
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
+}