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package fusefrontend
// Helper functions for sparse files (files with holes)
import (
"context"
"runtime"
"syscall"
"github.com/hanwen/go-fuse/v2/fs"
"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) syscall.Errno {
// Get the current file size.
fi, err := f.fd.Stat()
if err != nil {
tlog.Warn.Printf("checkAndPadHole: Fstat failed: %v", err)
return fs.ToErrno(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 0
}
// 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.
errno := f.zeroPad(plainSize)
if errno != 0 {
return errno
}
return 0
}
// 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) syscall.Errno {
lastBlockLen := plainSize % f.contentEnc.PlainBS()
if lastBlockLen == 0 {
// Already block-aligned
return 0
}
missing := f.contentEnc.PlainBS() - lastBlockLen
pad := make([]byte, missing)
tlog.Debug.Printf("zeroPad: Writing %d bytes\n", missing)
_, errno := f.doWrite(pad, int64(plainSize))
return errno
}
// Lseek - FUSE call.
//
// Looking at
// fuse_file_llseek @ https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/fs/fuse/file.c?h=v5.12.7#n2634
// this function is only called for SEEK_HOLE & SEEK_DATA.
func (f *File) Lseek(ctx context.Context, off uint64, whence uint32) (uint64, syscall.Errno) {
const (
SEEK_DATA = 3 // find next data segment at or above `off`
SEEK_HOLE = 4 // find next hole at or above `off`
// On error, we return -1 as the offset as per man lseek.
MinusOne = ^uint64(0)
)
if whence != SEEK_DATA && whence != SEEK_HOLE {
tlog.Warn.Printf("BUG: Lseek was called with whence=%d. This is not supported!", whence)
return 0, syscall.EINVAL
}
if runtime.GOOS != "linux" {
// MacOS has broken (different?) SEEK_DATA / SEEK_HOLE semantics, see
// https://lists.gnu.org/archive/html/bug-gnulib/2018-09/msg00051.html
tlog.Warn.Printf("buggy on non-linux platforms, disabling SEEK_DATA & SEEK_HOLE")
return MinusOne, syscall.ENOSYS
}
// We will need the file size
var st syscall.Stat_t
err := syscall.Fstat(f.intFd(), &st)
if err != nil {
return 0, fs.ToErrno(err)
}
fileSize := st.Size
// Better safe than sorry. The logic is only tested for 4k blocks.
if st.Blksize != 4096 {
tlog.Warn.Printf("unsupported block size of %d bytes, disabling SEEK_DATA & SEEK_HOLE", st.Blksize)
return MinusOne, syscall.ENOSYS
}
// man lseek: offset beyond end of file -> ENXIO
if f.rootNode.contentEnc.PlainOffToCipherOff(off) >= uint64(fileSize) {
return MinusOne, syscall.ENXIO
}
// Round down to start of block:
cipherOff := f.rootNode.contentEnc.BlockNoToCipherOff(f.rootNode.contentEnc.PlainOffToBlockNo(off))
newCipherOff, err := syscall.Seek(f.intFd(), int64(cipherOff), int(whence))
if err != nil {
return MinusOne, fs.ToErrno(err)
}
// already in data/hole => return original offset
if newCipherOff == int64(cipherOff) {
return off, 0
}
// If there is no further hole, SEEK_HOLE returns the file size
// (SEEK_DATA returns ENXIO in this case).
if whence == SEEK_HOLE {
fi, err := f.fd.Stat()
if err != nil {
return MinusOne, fs.ToErrno(err)
}
if newCipherOff == fi.Size() {
return f.rootNode.contentEnc.CipherSizeToPlainSize(uint64(newCipherOff)), 0
}
}
// syscall.Seek gave us the beginning of the next ext4 data/hole section.
// The next gocryptfs data/hole block starts at the next block boundary,
// so we have to round up:
newBlockNo := f.rootNode.contentEnc.CipherOffToBlockNo(uint64(newCipherOff) + f.rootNode.contentEnc.CipherBS() - 1)
return f.rootNode.contentEnc.BlockNoToPlainOff(newBlockNo), 0
}
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