fusefrontend: -sharedstorage: present stable inode numbers

Use the Gen field (inode generation) to distinguish hard links
while passing the real inode numbers to userspace.

Fixes https://github.com/rfjakob/gocryptfs/issues/584

internal/fusefrontend/node.go

@@ -40,6 +40,24 @@ func (n *Node) Lookup(ctx context.Context, name string, out *fuse.EntryOut) (ch
 	// Translate ciphertext size in `out.Attr.Size` to plaintext size
 	n.translateSize(dirfd, cName, &out.Attr)
 
+	rn := n.rootNode()
+	if rn.args.SharedStorage {
+		// If we already have a child node that matches what we found on disk*
+		// (as reflected in `ch`), return it here.
+		//
+		// This keeps the Node ID for each directory entry stable
+		// (until forgotten).
+		//
+		// *We compare `name`, `Ino`, `Mode` (but not `Gen`!)
+		old := n.Inode.GetChild(name)
+		if old != nil &&
+			old.StableAttr().Ino == ch.StableAttr().Ino &&
+			// `Mode` has already been masked with syscall.S_IFMT by n.newChild()
+			old.StableAttr().Mode == ch.StableAttr().Mode {
+			return old, 0
+		}
+	}
+
 	return ch, 0
 }
 

internal/fusefrontend/node_helpers.go

@@ -2,6 +2,7 @@ package fusefrontend
 
 import (
 	"context"
+	"sync/atomic"
 	"syscall"
 
 	"github.com/hanwen/go-fuse/v2/fs"
@@ -82,13 +83,20 @@ func (n *Node) rootNode() *RootNode {
 func (n *Node) newChild(ctx context.Context, st *syscall.Stat_t, out *fuse.EntryOut) *fs.Inode {
 	rn := n.rootNode()
 	// Get stable inode number based on underlying (device,ino) pair
-	// (or set to zero in case of `-sharestorage`)
 	rn.inoMap.TranslateStat(st)
 	out.Attr.FromStat(st)
+
+	var gen uint64 = 1
+	if rn.args.SharedStorage {
+		// Make each directory entry a unique node by using a unique generation
+		// value - see the comment at RootNode.gen for details.
+		gen = atomic.AddUint64(&rn.gen, 1)
+	}
+
 	// Create child node
 	id := fs.StableAttr{
 		Mode: uint32(st.Mode),
-		Gen:  1,
+		Gen:  gen,
 		Ino:  st.Ino,
 	}
 	node := &Node{}

internal/fusefrontend/root_node.go

@@ -50,7 +50,13 @@ type RootNode struct {
 	dirCache dirCache
 	// inoMap translates inode numbers from different devices to unique inode
 	// numbers.
-	inoMap inomap.TranslateStater
+	inoMap *inomap.InoMap
+	// gen is the node generation numbers. Normally, it is always set to 1,
+	// but -sharestorage uses an incrementing counter for new nodes.
+	// This makes each directory entry unique (even hard links),
+	// makes go-fuse hand out separate FUSE Node IDs for each, and prevents
+	// bizarre problems when inode numbers are reused behind our back.
+	gen uint64
 }
 
 func NewRootNode(args Args, c *contentenc.ContentEnc, n *nametransform.NameTransform) *RootNode {
@@ -71,11 +77,6 @@ func NewRootNode(args Args, c *contentenc.ContentEnc, n *nametransform.NameTrans
 		inoMap:        inomap.New(),
 		dirCache:      dirCache{ivLen: ivLen},
 	}
-	// In `-sharedstorage` mode we always set the inode number to zero.
-	// This makes go-fuse generate a new inode number for each lookup.
-	if args.SharedStorage {
-		rn.inoMap = &inomap.TranslateStatZero{}
-	}
 	return rn
 }
 

internal/inomap/inomap.go

@@ -104,14 +104,3 @@ func (m *InoMap) TranslateStat(st *syscall.Stat_t) {
 	in := QInoFromStat(st)
 	st.Ino = m.Translate(in)
 }
-
-type TranslateStater interface {
-	TranslateStat(st *syscall.Stat_t)
-}
-
-// TranslateStatZero always sets st.Ino to zero. Used for `-sharedstorage`.
-type TranslateStatZero struct{}
-
-func (z TranslateStatZero) TranslateStat(st *syscall.Stat_t) {
-	st.Ino = 0
-}

tests/cli/cli_test.go

@@ -929,8 +929,8 @@ func TestInitNotEmpty(t *testing.T) {
 	}
 }
 
-// TestSharedstorage checks that `-sharedstorage` hands out arbitrary inode
-// numbers (no hard link tracking)
+// TestSharedstorage checks that `-sharedstorage` shows stable inode numbers to
+// userpsace despite having hard link tracking disabled
 func TestSharedstorage(t *testing.T) {
 	dir := test_helpers.InitFS(t)
 	mnt := dir + ".mnt"
@@ -944,7 +944,7 @@ func TestSharedstorage(t *testing.T) {
 	if err := os.Link(foo1, foo2); err != nil {
 		t.Fatal(err)
 	}
-	var st1, st2, st3 syscall.Stat_t
+	var st1, st2 syscall.Stat_t
 	if err := syscall.Stat(foo1, &st1); err != nil {
 		t.Fatal(err)
 	}
@@ -952,12 +952,8 @@ func TestSharedstorage(t *testing.T) {
 	if err := syscall.Stat(foo2, &st2); err != nil {
 		t.Fatal(err)
 	}
-	// Stat()'ing again should give us again a new inode number
-	if err := syscall.Stat(foo2, &st3); err != nil {
-		t.Fatal(err)
-	}
-	if st1.Ino == st2.Ino || st2.Ino == st3.Ino || st1.Ino == st3.Ino {
-		t.Error(st1.Ino, st2.Ino, st3.Ino)
+	if st1.Ino != st2.Ino {
+		t.Errorf("unstable inode number: changed from %d to %d", st1.Ino, st2.Ino)
 	}
 	// Check that we we don't have stat caching. New length should show up
 	// on the hard link immediately.