3 files changed, 130 insertions, 4 deletions
diff --git a/internal/fusefrontend/node.go b/internal/fusefrontend/node.go
index fde6a1a..f246408 100644
--- a/internal/fusefrontend/node.go
+++ b/internal/fusefrontend/node.go
@@ -2,13 +2,18 @@ package fusefrontend
 
 import (
 	"context"
+	"path/filepath"
 	"syscall"
 
+	"golang.org/x/sys/unix"
+
 	"github.com/hanwen/go-fuse/v2/fs"
 	"github.com/hanwen/go-fuse/v2/fuse"
 
 	"github.com/rfjakob/gocryptfs/internal/contentenc"
+	"github.com/rfjakob/gocryptfs/internal/inomap"
 	"github.com/rfjakob/gocryptfs/internal/nametransform"
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
 )
 
 // Node is a file or directory in the filesystem tree
@@ -20,25 +25,66 @@ type Node struct {
 // RootNode is the root of the filesystem tree of Nodes.
 type RootNode struct {
 	Node
-
-	// This flag is set to zero each time fs.isFiltered() is called
+	// args stores configuration arguments
+	args Args
+	// Filename encryption helper
+	nameTransform nametransform.NameTransformer
+	// Content encryption helper
+	contentEnc *contentenc.ContentEnc
+	// IsIdle flag is set to zero each time fs.isFiltered() is called
 	// (uint32 so that it can be reset with CompareAndSwapUint32).
 	// When -idle was used when mounting, idleMonitor() sets it to 1
 	// periodically.
 	IsIdle uint32
+
+	// inoMap translates inode numbers from different devices to unique inode
+	// numbers.
+	inoMap *inomap.InoMap
 }
 
 func NewRootNode(args Args, c *contentenc.ContentEnc, n nametransform.NameTransformer) *RootNode {
 	// TODO
-	return &RootNode{}
+	return &RootNode{
+		args:          args,
+		nameTransform: n,
+		contentEnc:    c,
+		inoMap:        inomap.New(),
+	}
 }
 
+// path returns the relative plaintext path of this node
 func (n *Node) path() string {
 	return n.Path(n.Root())
 }
 
+func (n *Node) rootNode() *RootNode {
+	return n.Root().Operations().(*RootNode)
+}
+
 func (n *Node) Lookup(ctx context.Context, name string, out *fuse.EntryOut) (*fs.Inode, syscall.Errno) {
-	return nil, 1
+	rn := n.rootNode()
+	p := filepath.Join(n.path(), name)
+	dirfd, cName, err := rn.openBackingDir(p)
+	if err != nil {
+		return nil, fs.ToErrno(err)
+	}
+	// Get device number and inode number into `st`
+	st, err := syscallcompat.Fstatat2(dirfd, cName, unix.AT_SYMLINK_NOFOLLOW)
+	if err != nil {
+		return nil, fs.ToErrno(err)
+	}
+	// Get unique inode number
+	rn.inoMap.TranslateStat(st)
+	out.Attr.FromStat(st)
+	// Create child node
+	id := fs.StableAttr{
+		Mode: uint32(st.Mode),
+		Gen:  1,
+		Ino:  st.Ino,
+	}
+	node := &Node{}
+	ch := n.NewInode(ctx, node, id)
+	return ch, 0
 }
 
 func (n *Node) Getattr(ctx context.Context, f fs.FileHandle, out *fuse.AttrOut) syscall.Errno {
diff --git a/internal/fusefrontend/node_openbackingdir.go b/internal/fusefrontend/node_openbackingdir.go
new file mode 100644
index 0000000..733c239
--- /dev/null
+++ b/internal/fusefrontend/node_openbackingdir.go
@@ -0,0 +1,68 @@
+package fusefrontend
+
+import (
+	"path/filepath"
+	"strings"
+	"syscall"
+
+	"github.com/rfjakob/gocryptfs/internal/nametransform"
+	"github.com/rfjakob/gocryptfs/internal/syscallcompat"
+)
+
+// openBackingDir opens the parent ciphertext directory of plaintext path
+// "relPath". It returns the dirfd (opened with O_PATH) and the encrypted
+// basename.
+//
+// The caller should then use Openat(dirfd, cName, ...) and friends.
+// For convenience, if relPath is "", cName is going to be ".".
+//
+// openBackingDir is secure against symlink races by using Openat and
+// ReadDirIVAt.
+func (rn *RootNode) openBackingDir(relPath string) (dirfd int, cName string, err error) {
+	dirRelPath := nametransform.Dir(relPath)
+	// With PlaintextNames, we don't need to read DirIVs. Easy.
+	if rn.args.PlaintextNames {
+		dirfd, err = syscallcompat.OpenDirNofollow(rn.args.Cipherdir, dirRelPath)
+		if err != nil {
+			return -1, "", err
+		}
+		// If relPath is empty, cName is ".".
+		cName = filepath.Base(relPath)
+		return dirfd, cName, nil
+	}
+	// Open cipherdir (following symlinks)
+	dirfd, err = syscall.Open(rn.args.Cipherdir, syscall.O_DIRECTORY|syscallcompat.O_PATH, 0)
+	if err != nil {
+		return -1, "", err
+	}
+	// If relPath is empty, cName is ".".
+	if relPath == "" {
+		return dirfd, ".", nil
+	}
+	// Walk the directory tree
+	parts := strings.Split(relPath, "/")
+	for i, name := range parts {
+		iv, err := nametransform.ReadDirIVAt(dirfd)
+		if err != nil {
+			syscall.Close(dirfd)
+			return -1, "", err
+		}
+		cName, err = rn.nameTransform.EncryptAndHashName(name, iv)
+		if err != nil {
+			syscall.Close(dirfd)
+			return -1, "", err
+		}
+		// Last part? We are done.
+		if i == len(parts)-1 {
+			break
+		}
+		// Not the last part? Descend into next directory.
+		dirfd2, err := syscallcompat.Openat(dirfd, cName, syscall.O_NOFOLLOW|syscall.O_DIRECTORY|syscallcompat.O_PATH, 0)
+		syscall.Close(dirfd)
+		if err != nil {
+			return -1, "", err
+		}
+		dirfd = dirfd2
+	}
+	return dirfd, cName, nil
+}
diff --git a/internal/syscallcompat/sys_common.go b/internal/syscallcompat/sys_common.go
index b6bbdff..d5e3251 100644
--- a/internal/syscallcompat/sys_common.go
+++ b/internal/syscallcompat/sys_common.go
@@ -107,6 +107,18 @@ func Fstatat(dirfd int, path string, stat *unix.Stat_t, flags int) (err error) {
 	return unix.Fstatat(dirfd, path, stat, flags)
 }
 
+// Fstatat2 is a more convenient version of Fstatat. It allocates a Stat_t
+// for you and also handles the Unix2syscall conversion.
+func Fstatat2(dirfd int, path string, flags int) (*syscall.Stat_t, error) {
+	var stUnix unix.Stat_t
+	err := Fstatat(dirfd, path, &stUnix, flags)
+	if err != nil {
+		return nil, err
+	}
+	st := Unix2syscall(stUnix)
+	return &st, nil
+}
+
 const XATTR_SIZE_MAX = 65536
 
 // Make the buffer 1kB bigger so we can detect overflows