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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
}
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