reverse: use unique generation number for all nodes

We used to present gocryptfs.longname.*.name files for hardlinked
files as hardlinked to the kernel (same Node ID) which is wrong.

Fix this by using a unique generation number for all nodes, which
also fixes possible issues with inode reuse.

Basically what 1bc1db620b061aabf59469a5eb4fb60e3e1701a3 did
for forward mode with -sharedstorage.

Fixes https://github.com/rfjakob/gocryptfs/issues/802

1 files changed, 24 insertions, 0 deletions

diff --git a/internal/fusefrontend_reverse/root_node.go b/internal/fusefrontend_reverse/root_node.go
index 8a2afd9..1a68ffd 100644
--- a/internal/fusefrontend_reverse/root_node.go
+++ b/internal/fusefrontend_reverse/root_node.go
@@ -5,6 +5,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"sync/atomic"
 	"syscall"
 
 	"github.com/rfjakob/gocryptfs/v2/internal/exitcodes"
@@ -45,6 +46,13 @@ type RootNode struct {
 	// If a file name length is shorter than shortNameMax, there is no need to
 	// hash it.
 	shortNameMax int
+	// gen is the node generation number. Normally, it is always set to 1,
+	// but reverse mode, like -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,
+	// like this one: https://github.com/rfjakob/gocryptfs/issues/802
+	gen uint64
 }
 
 // NewRootNode returns an encrypted FUSE overlay filesystem.
@@ -149,3 +157,19 @@ func (rn *RootNode) excludeDirEntries(d *dirfdPlus, entries []fuse.DirEntry) (fi
 	}
 	return filtered
 }
+
+// uniqueStableAttr returns a fs.StableAttr struct with a unique generation number,
+// preventing files to appear hard-linked, even when they have the same inode number.
+//
+// This is good because inode numbers can be reused behind our back, which could make
+// unrelated files appear hard-linked.
+// Example: https://github.com/rfjakob/gocryptfs/issues/802
+func (rn *RootNode) uniqueStableAttr(mode uint32, ino uint64) fs.StableAttr {
+	return fs.StableAttr{
+		Mode: mode,
+		Ino:  ino,
+		// Make each directory entry a unique node by using a unique generation
+		// value. Also see the comment at RootNode.gen for details.
+		Gen: atomic.AddUint64(&rn.gen, 1),
+	}
+}