fs: allow inode number 1

The root directory always get nodeid 1, and the v2 api
originally had "nodeid == inode number".

But since 1d0096e5,
inode number and node id are independent, and we can allow
inode number 1.

Why does this matter? MacOS actually uses inode number 1 on ExFAT
mounts, triggering a panic on overlay filesystems like gocryptfs:

    panic: using reserved ID 1 for inode number

Fixes: https://github.com/rfjakob/gocryptfs/issues/585
Change-Id: I5e3476da65400d32efdb3dac1fe4901c64c00a89

fs/bridge.go

@@ -270,9 +270,6 @@ func NewNodeFS(root InodeEmbedder, opts *Options) fuse.RawFileSystem {
 		nextNodeId:   2, // the root node has nodeid 1
 		stableAttrs:  make(map[StableAttr]*Inode),
 	}
-	if bridge.automaticIno == 1 {
-		bridge.automaticIno++
-	}
 
 	if bridge.automaticIno == 0 {
 		bridge.automaticIno = 1 << 63

fs/bridge_test.go

@@ -201,3 +201,40 @@ func (n *testDeletedIno) Getattr(ctx context.Context, f FileHandle, out *fuse.At
 	// Otherwise EILSEQ
 	return syscall.EILSEQ
 }
+
+// TestIno1 tests that inode number 1 is allowed.
+//
+// We used to panic like this because inode number 1 was special:
+//
+//    panic: using reserved ID 1 for inode number
+//
+func TestIno1(t *testing.T) {
+	rootNode := testIno1{}
+	mnt, _, clean := testMount(t, &rootNode, nil)
+	defer clean()
+
+	var st syscall.Stat_t
+	err := syscall.Stat(mnt+"/ino1", &st)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if st.Ino != 1 {
+		t.Errorf("wrong inode number: want=1 have=%d", st.Ino)
+	}
+}
+
+type testIno1 struct {
+	Inode
+}
+
+func (fn *testIno1) Lookup(ctx context.Context, name string, out *fuse.EntryOut) (*Inode, syscall.Errno) {
+	if name != "ino1" {
+		return nil, syscall.ENOENT
+	}
+	stable := StableAttr{
+		Mode: syscall.S_IFREG,
+		Ino:  1,
+	}
+	child := fn.NewInode(ctx, &testIno1{}, stable)
+	return child, 0
+}

fs/inode.go

@@ -27,8 +27,8 @@ type StableAttr struct {
 
 	// The inode number must be unique among the currently live
 	// objects in the file system. It is used to communicate to
-	// the kernel about this file object. The values uint64(-1),
-	// and 1 are reserved. When using Ino==0, a unique, sequential
+	// the kernel about this file object. The value uint64(-1)
+	// is reserved. When using Ino==0, a unique, sequential
 	// number is assigned (starting at 2^63 by default) on Inode creation.
 	Ino uint64
 
@@ -41,7 +41,7 @@ type StableAttr struct {
 
 // Reserved returns if the StableAttr is using reserved Inode numbers.
 func (i *StableAttr) Reserved() bool {
-	return i.Ino == 1 || i.Ino == ^uint64(0)
+	return i.Ino == ^uint64(0) // fuse.pollHackInode = ^uint64(0)
 }
 
 // Inode is a node in VFS tree.  Inodes are one-to-one mapped to