Merge pull request #4255 from greatroar/fuse-hash

fuse: Mix inode hashes in a non-symmetric way

changelog/unreleased/issue-4253

@@ -0,0 +1,18 @@
+Bugfix: Mount command should no longer create spurious filesystem loops
+
+When a backup contains a directory that has the same name as its parent,
+say, a/b/b, and the GNU find command were run on this backup in a restic
+mount, find command would refuse to traverse the lowest "b" directory,
+instead printing "File system loop detected". This is due to the way the
+restic mount command generates inode numbers for directories in the mount
+point.
+
+The rule for generating these inode numbers was changed in 0.15.0. It has
+now been changed again to avoid this issue. A perfect rule does not exist,
+but the probability of this behavior occurring is now extremely small.
+When it does occur, the mount point is not broken, and scripts that traverse
+the mount point should work as long as they don't rely on inode numbers for
+detecting filesystem loops.
+
+https://github.com/restic/restic/issues/4253
+https://github.com/restic/restic/pull/4255

internal/fuse/fuse_test.go

@@ -258,6 +258,17 @@ func TestInodeFromNode(t *testing.T) {
 	ino1 = inodeFromNode(1, node)
 	ino2 = inodeFromNode(2, node)
 	rtest.Assert(t, ino1 != ino2, "same inode %d but different parent", ino1)
+
+	// Regression test: in a path a/b/b, the grandchild should not get the
+	// same inode as the grandparent.
+	a := &restic.Node{Name: "a", Type: "dir", Links: 2}
+	ab := &restic.Node{Name: "b", Type: "dir", Links: 2}
+	abb := &restic.Node{Name: "b", Type: "dir", Links: 2}
+	inoA := inodeFromNode(1, a)
+	inoAb := inodeFromNode(inoA, ab)
+	inoAbb := inodeFromNode(inoAb, abb)
+	rtest.Assert(t, inoA != inoAb, "inode(a/b) = inode(a)")
+	rtest.Assert(t, inoA != inoAbb, "inode(a/b/b) = inode(a)")
 }
 
 var sink uint64

internal/fuse/inode.go

@@ -10,9 +10,11 @@ import (
 	"github.com/restic/restic/internal/restic"
 )
 
+const prime = 11400714785074694791 // prime1 from xxhash.
+
 // inodeFromName generates an inode number for a file in a meta dir.
 func inodeFromName(parent uint64, name string) uint64 {
-	inode := parent ^ xxhash.Sum64String(cleanupNodeName(name))
+	inode := prime*parent ^ xxhash.Sum64String(cleanupNodeName(name))
 
 	// Inode 0 is invalid and 1 is the root. Remap those.
 	if inode < 2 {
@@ -33,7 +35,7 @@ func inodeFromNode(parent uint64, node *restic.Node) (inode uint64) {
 	} else {
 		// Else, use the name and the parent inode.
 		// node.{DeviceID,Inode} may not even be reliable.
-		inode = parent ^ xxhash.Sum64String(cleanupNodeName(node.Name))
+		inode = prime*parent ^ xxhash.Sum64String(cleanupNodeName(node.Name))
 	}
 
 	// Inode 0 is invalid and 1 is the root. Remap those.