93 lines
3.1 KiB
Go
93 lines
3.1 KiB
Go
package fusefrontend
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// Helper functions for sparse files (files with holes)
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import (
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"runtime"
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"syscall"
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"github.com/hanwen/go-fuse/v2/fuse"
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"github.com/rfjakob/gocryptfs/internal/tlog"
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)
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// Will a write to plaintext offset "targetOff" create a file hole in the
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// ciphertext? If yes, zero-pad the last ciphertext block.
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func (f *File) writePadHole(targetOff int64) fuse.Status {
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// Get the current file size.
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fi, err := f.fd.Stat()
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if err != nil {
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tlog.Warn.Printf("checkAndPadHole: Fstat failed: %v", err)
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return fuse.ToStatus(err)
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}
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plainSize := f.contentEnc.CipherSizeToPlainSize(uint64(fi.Size()))
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// Appending a single byte to the file (equivalent to writing to
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// offset=plainSize) would write to "nextBlock".
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nextBlock := f.contentEnc.PlainOffToBlockNo(plainSize)
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// targetBlock is the block the user wants to write to.
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targetBlock := f.contentEnc.PlainOffToBlockNo(uint64(targetOff))
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// The write goes into an existing block or (if the last block was full)
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// starts a new one directly after the last block. Nothing to do.
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if targetBlock <= nextBlock {
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return fuse.OK
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}
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// The write goes past the next block. nextBlock has
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// to be zero-padded to the block boundary and (at least) nextBlock+1
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// will contain a file hole in the ciphertext.
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status := f.zeroPad(plainSize)
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if status != fuse.OK {
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return status
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}
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return fuse.OK
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}
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// Zero-pad the file of size plainSize to the next block boundary. This is a no-op
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// if the file is already block-aligned.
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func (f *File) zeroPad(plainSize uint64) fuse.Status {
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lastBlockLen := plainSize % f.contentEnc.PlainBS()
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if lastBlockLen == 0 {
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// Already block-aligned
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return fuse.OK
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}
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missing := f.contentEnc.PlainBS() - lastBlockLen
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pad := make([]byte, missing)
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tlog.Debug.Printf("zeroPad: Writing %d bytes\n", missing)
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_, status := f.doWrite(pad, int64(plainSize))
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return status
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}
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// SeekData calls the lseek syscall with SEEK_DATA. It returns the offset of the
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// next data bytes, skipping over file holes.
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func (f *File) SeekData(oldOffset int64) (int64, error) {
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if runtime.GOOS != "linux" {
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// Does MacOS support something like this?
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return 0, syscall.EOPNOTSUPP
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}
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const SEEK_DATA = 3
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// Convert plaintext offset to ciphertext offset and round down to the
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// start of the current block. File holes smaller than a full block will
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// be ignored.
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blockNo := f.contentEnc.PlainOffToBlockNo(uint64(oldOffset))
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oldCipherOff := int64(f.contentEnc.BlockNoToCipherOff(blockNo))
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// Determine the next data offset. If the old offset points to (or beyond)
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// the end of the file, the Seek syscall fails with syscall.ENXIO.
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newCipherOff, err := syscall.Seek(f.intFd(), oldCipherOff, SEEK_DATA)
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if err != nil {
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return 0, err
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}
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// Convert ciphertext offset back to plaintext offset. At this point,
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// newCipherOff should always be >= contentenc.HeaderLen. Round down,
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// but ensure that the result is never smaller than the initial offset
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// (to avoid endless loops).
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blockNo = f.contentEnc.CipherOffToBlockNo(uint64(newCipherOff))
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newOffset := int64(f.contentEnc.BlockNoToPlainOff(blockNo))
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if newOffset < oldOffset {
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newOffset = oldOffset
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}
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return newOffset, nil
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}
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