fusefrontend: add fallocate support

Mode=0 (default) and mode=1 (keep size) are supported.
The patch includes test cases and the whole thing passed xfstests.

Fixes https://github.com/rfjakob/gocryptfs/issues/1 .

2 files changed, 122 insertions, 39 deletions

diff --git a/internal/fusefrontend/file.go b/internal/fusefrontend/file.go
index 1835b53..ead253f 100644
--- a/internal/fusefrontend/file.go
+++ b/internal/fusefrontend/file.go
@@ -208,8 +208,6 @@ func (f *file) Read(buf []byte, off int64) (resultData fuse.ReadResult, code fus
 	return fuse.ReadResultData(out), status
 }
 
-const FALLOC_FL_KEEP_SIZE = 0x01
-
 // doWrite - encrypt "data" and write it to plaintext offset "off"
 //
 // Arguments do not have to be block-aligned, read-modify-write is
diff --git a/internal/fusefrontend/file_allocate_truncate.go b/internal/fusefrontend/file_allocate_truncate.go
index 5be7df4..65d6df6 100644
--- a/internal/fusefrontend/file_allocate_truncate.go
+++ b/internal/fusefrontend/file_allocate_truncate.go
@@ -4,6 +4,7 @@ package fusefrontend
 // i.e. ftruncate and fallocate
 
 import (
+	"log"
 	"sync"
 	"syscall"
 
@@ -12,17 +13,78 @@ import (
 	"github.com/rfjakob/gocryptfs/internal/tlog"
 )
 
+const FALLOC_DEFAULT = 0x00
+const FALLOC_FL_KEEP_SIZE = 0x01
+
 // Only warn once
 var allocateWarnOnce sync.Once
 
-// Allocate - FUSE call, fallocate(2)
-// This is not implemented yet in gocryptfs, but it is neither in EncFS. This
-// suggests that the user demand is low.
+// Allocate - FUSE call for fallocate(2)
+//
+// mode=FALLOC_FL_KEEP_SIZE is implemented directly.
+//
+// mode=FALLOC_DEFAULT is implemented as a two-step process:
+//
+//   (1) Allocate the space using FALLOC_FL_KEEP_SIZE
+//   (2) Set the file size using ftruncate (via truncateGrowFile)
+//
+// This allows us to reuse the file grow mechanics from Truncate as they are
+// complicated and hard to get right.
+//
+// Other modes (hole punching, zeroing) are not supported.
 func (f *file) Allocate(off uint64, sz uint64, mode uint32) fuse.Status {
-	allocateWarnOnce.Do(func() {
-		tlog.Warn.Printf("fallocate(2) is not supported, returning ENOSYS - see https://github.com/rfjakob/gocryptfs/issues/1")
-	})
-	return fuse.ENOSYS
+	if mode != FALLOC_DEFAULT && mode != FALLOC_FL_KEEP_SIZE {
+		f := func() {
+			tlog.Warn.Print("fallocate: only mode 0 (default) and 1 (keep size) are supported")
+		}
+		allocateWarnOnce.Do(f)
+		return fuse.Status(syscall.EOPNOTSUPP)
+	}
+
+	f.fdLock.RLock()
+	defer f.fdLock.RUnlock()
+	if f.released {
+		return fuse.EBADF
+	}
+	wlock.lock(f.ino)
+	defer wlock.unlock(f.ino)
+
+	blocks := f.contentEnc.ExplodePlainRange(off, sz)
+	firstBlock := blocks[0]
+	lastBlock := blocks[len(blocks)-1]
+
+	// Step (1): Allocate the space the user wants using FALLOC_FL_KEEP_SIZE.
+	// This will fill file holes and/or allocate additional space past the end of
+	// the file.
+	cipherOff := firstBlock.BlockCipherOff()
+	cipherSz := lastBlock.BlockCipherOff() - cipherOff +
+		f.contentEnc.PlainSizeToCipherSize(lastBlock.Skip+lastBlock.Length)
+	err := syscall.Fallocate(f.intFd(), FALLOC_FL_KEEP_SIZE, int64(cipherOff), int64(cipherSz))
+	tlog.Debug.Printf("Allocate off=%d sz=%d mode=%x cipherOff=%d cipherSz=%d\n",
+		off, sz, mode, cipherOff, cipherSz)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	if mode == FALLOC_FL_KEEP_SIZE {
+		// The user did not want to change the apparent size. We are done.
+		return fuse.OK
+	}
+	// Step (2): Grow the apparent file size
+	// We need the old file size to determine if we are growing the file at all.
+	newPlainSz := off + sz
+	oldPlainSz, err := f.statPlainSize()
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	if newPlainSz <= oldPlainSz {
+		// The new size is smaller (or equal). Fallocate with mode = 0 never
+		// truncates a file, so we are done.
+		return fuse.OK
+	}
+	// The file grows. The space has already been allocated in (1), so what is
+	// left to do is to pad the first and last block and call truncate.
+	// truncateGrowFile does just that.
+	return f.truncateGrowFile(oldPlainSz, newPlainSz)
 }
 
 // Truncate - FUSE call
@@ -50,13 +112,10 @@ func (f *file) Truncate(newSize uint64) fuse.Status {
 	}
 	// We need the old file size to determine if we are growing or shrinking
 	// the file
-	fi, err := f.fd.Stat()
+	oldSize, err := f.statPlainSize()
 	if err != nil {
-		tlog.Warn.Printf("ino%d fh%d: Truncate: Fstat failed: %v", f.ino, f.intFd(), err)
 		return fuse.ToStatus(err)
-	}
-	oldSize := f.contentEnc.CipherSizeToPlainSize(uint64(fi.Size()))
-	{
+	} else {
 		oldB := float32(oldSize) / float32(f.contentEnc.PlainBS())
 		newB := float32(newSize) / float32(f.contentEnc.PlainBS())
 		tlog.Debug.Printf("ino%d: FUSE Truncate from %.2f to %.2f blocks (%d to %d bytes)", f.ino, oldB, newB, oldSize, newSize)
@@ -67,31 +126,7 @@ func (f *file) Truncate(newSize uint64) fuse.Status {
 	}
 	// File grows
 	if newSize > oldSize {
-		// File was empty, create new header
-		if oldSize == 0 {
-			err = f.createHeader()
-			if err != nil {
-				return fuse.ToStatus(err)
-			}
-		}
-		// New blocks to add
-		addBlocks := f.contentEnc.ExplodePlainRange(oldSize, newSize-oldSize)
-		if len(addBlocks) >= 2 {
-			f.zeroPad(oldSize)
-		}
-		lastBlock := addBlocks[len(addBlocks)-1]
-		if lastBlock.IsPartial() {
-			off := lastBlock.BlockPlainOff()
-			_, status := f.doWrite(make([]byte, lastBlock.Length), int64(off+lastBlock.Skip))
-			return status
-		} else {
-			off := lastBlock.BlockCipherOff()
-			err = syscall.Ftruncate(f.intFd(), int64(off+f.contentEnc.CipherBS()))
-			if err != nil {
-				tlog.Warn.Printf("Truncate: grow Ftruncate returned error: %v", err)
-			}
-			return fuse.ToStatus(err)
-		}
+		return f.truncateGrowFile(oldSize, newSize)
 	} else {
 		// File shrinks
 		blockNo := f.contentEnc.PlainOffToBlockNo(newSize)
@@ -121,3 +156,53 @@ func (f *file) Truncate(newSize uint64) fuse.Status {
 		return fuse.OK
 	}
 }
+
+// statPlainSize stats the file and returns the plaintext size
+func (f *file) statPlainSize() (uint64, error) {
+	fi, err := f.fd.Stat()
+	if err != nil {
+		tlog.Warn.Printf("ino%d fh%d: statPlainSize: %v", f.ino, f.intFd(), err)
+		return 0, err
+	}
+	cipherSz := uint64(fi.Size())
+	plainSz := uint64(f.contentEnc.CipherSizeToPlainSize(cipherSz))
+	return plainSz, nil
+}
+
+// truncateGrowFile extends a file using seeking or ftruncate performing RMW on
+// the first and last block as neccessary. New blocks in the middle become
+// file holes unless they have been fallocate()'d beforehand.
+func (f *file) truncateGrowFile(oldPlainSz uint64, newPlainSz uint64) fuse.Status {
+	if newPlainSz <= oldPlainSz {
+		log.Panicf("BUG: newSize=%d <= oldSize=%d", newPlainSz, oldPlainSz)
+	}
+	var err error
+	// File was empty, create new header
+	if oldPlainSz == 0 {
+		err = f.createHeader()
+		if err != nil {
+			return fuse.ToStatus(err)
+		}
+	}
+	// New blocks to add
+	addBlocks := f.contentEnc.ExplodePlainRange(oldPlainSz, newPlainSz-oldPlainSz)
+	if oldPlainSz > 0 && len(addBlocks) >= 2 {
+		// Zero-pad the first block (unless the first block is also the last block)
+		f.zeroPad(oldPlainSz)
+	}
+	lastBlock := addBlocks[len(addBlocks)-1]
+	if lastBlock.IsPartial() {
+		// Write at the new end of the file. The seek implicitly grows the file
+		// (creates a file hole) and doWrite() takes care of RMW.
+		off := lastBlock.BlockPlainOff()
+		_, status := f.doWrite(make([]byte, lastBlock.Length), int64(off+lastBlock.Skip))
+		return status
+	} else {
+		off := lastBlock.BlockCipherOff()
+		err = syscall.Ftruncate(f.intFd(), int64(off+f.contentEnc.CipherBS()))
+		if err != nil {
+			tlog.Warn.Printf("Truncate: grow Ftruncate returned error: %v", err)
+		}
+		return fuse.ToStatus(err)
+	}
+}