From 527609266386890fe1657c225c643ca448bcd71d Mon Sep 17 00:00:00 2001
From: Jakob Unterwurzacher
Date: Sat, 8 Aug 2020 18:45:47 +0200
Subject: v2api/reverse: implement Readlink

---
 internal/fusefrontend_reverse/node.go           | 28 +++++++++++++++----
 internal/fusefrontend_reverse/node_api_check.go | 37 +++++++++++++++++++++++++
 internal/fusefrontend_reverse/node_helpers.go   | 31 +++++++++++++++++++--
 3 files changed, 88 insertions(+), 8 deletions(-)
 create mode 100644 internal/fusefrontend_reverse/node_api_check.go

diff --git a/internal/fusefrontend_reverse/node.go b/internal/fusefrontend_reverse/node.go
index 713beee..884a97e 100644
--- a/internal/fusefrontend_reverse/node.go
+++ b/internal/fusefrontend_reverse/node.go
@@ -2,6 +2,7 @@ package fusefrontend_reverse
 
 import (
 	"context"
+	"path/filepath"
 	"syscall"
 
 	"golang.org/x/sys/unix"
@@ -20,16 +21,16 @@ type Node struct {
 }
 
 // Lookup - FUSE call for discovering a file.
-func (n *Node) Lookup(ctx context.Context, name string, out *fuse.EntryOut) (ch *fs.Inode, errno syscall.Errno) {
+func (n *Node) Lookup(ctx context.Context, cName string, out *fuse.EntryOut) (ch *fs.Inode, errno syscall.Errno) {
 	dirfd := int(-1)
 	pName := ""
-	t := n.lookupFileType(name)
+	t := n.lookupFileType(cName)
 	if t == typeDiriv {
 		// gocryptfs.diriv
 		return n.lookupDiriv(ctx, out)
 	} else if t == typeName {
 		// gocryptfs.longname.*.name
-		return n.lookupLongnameName(ctx, name, out)
+		return n.lookupLongnameName(ctx, cName, out)
 	} else if t == typeConfig {
 		// gocryptfs.conf
 		var err error
@@ -43,7 +44,7 @@ func (n *Node) Lookup(ctx context.Context, name string, out *fuse.EntryOut) (ch
 		defer syscall.Close(dirfd)
 	} else if t == typeReal {
 		// real file
-		dirfd, pName, errno = n.prepareAtSyscall(name)
+		dirfd, pName, errno = n.prepareAtSyscall(cName)
 		if errno != 0 {
 			return
 		}
@@ -58,7 +59,7 @@ func (n *Node) Lookup(ctx context.Context, name string, out *fuse.EntryOut) (ch
 	ch = n.newChild(ctx, st, out)
 	// Translate ciphertext size in `out.Attr.Size` to plaintext size
 	if t == typeReal {
-		n.translateSize(dirfd, pName, &out.Attr)
+		n.translateSize(dirfd, cName, pName, &out.Attr)
 	}
 	return ch, 0
 }
@@ -89,10 +90,25 @@ func (n *Node) Getattr(ctx context.Context, f fs.FileHandle, out *fuse.AttrOut)
 	out.Attr.FromStat(st)
 
 	// Translate ciphertext size in `out.Attr.Size` to plaintext size
-	n.translateSize(dirfd, pName, &out.Attr)
+	cName := filepath.Base(n.Path())
+	n.translateSize(dirfd, cName, pName, &out.Attr)
 
 	if rn.args.ForceOwner != nil {
 		out.Owner = *rn.args.ForceOwner
 	}
 	return 0
 }
+
+// Readlink - FUSE call.
+//
+// Symlink-safe through openBackingDir() + Readlinkat().
+func (n *Node) Readlink(ctx context.Context) (out []byte, errno syscall.Errno) {
+	dirfd, pName, errno := n.prepareAtSyscall("")
+	if errno != 0 {
+		return
+	}
+	defer syscall.Close(dirfd)
+
+	cName := filepath.Base(n.Path())
+	return n.readlink(dirfd, cName, pName)
+}
diff --git a/internal/fusefrontend_reverse/node_api_check.go b/internal/fusefrontend_reverse/node_api_check.go
new file mode 100644
index 0000000..e926fc3
--- /dev/null
+++ b/internal/fusefrontend_reverse/node_api_check.go
@@ -0,0 +1,37 @@
+package fusefrontend_reverse
+
+import (
+	"github.com/hanwen/go-fuse/v2/fs"
+)
+
+// Check that we have implemented the fs.Node* interfaces
+var _ = (fs.NodeGetattrer)((*Node)(nil))
+var _ = (fs.NodeLookuper)((*Node)(nil))
+var _ = (fs.NodeReaddirer)((*Node)(nil))
+var _ = (fs.NodeReadlinker)((*Node)(nil))
+
+/* TODO
+var _ = (fs.NodeOpener)((*Node)(nil))
+var _ = (fs.NodeStatfser)((*Node)(nil))
+var _ = (fs.NodeMknoder)((*Node)(nil))
+var _ = (fs.NodeGetxattrer)((*Node)(nil))
+var _ = (fs.NodeListxattrer)((*Node)(nil))
+*/
+
+/* Not needed
+var _ = (fs.NodeOpendirer)((*Node)(nil))
+*/
+
+/* Will not implement these - reverse mode is read-only!
+var _ = (fs.NodeCreater)((*Node)(nil))
+var _ = (fs.NodeMkdirer)((*Node)(nil))
+var _ = (fs.NodeRmdirer)((*Node)(nil))
+var _ = (fs.NodeUnlinker)((*Node)(nil))
+var _ = (fs.NodeSetattrer)((*Node)(nil))
+var _ = (fs.NodeLinker)((*Node)(nil))
+var _ = (fs.NodeSymlinker)((*Node)(nil))
+var _ = (fs.NodeRenamer)((*Node)(nil))
+var _ = (fs.NodeSetxattrer)((*Node)(nil))
+var _ = (fs.NodeRemovexattrer)((*Node)(nil))
+var _ = (fs.NodeCopyFileRanger)((*Node)(nil))
+*/
diff --git a/internal/fusefrontend_reverse/node_helpers.go b/internal/fusefrontend_reverse/node_helpers.go
index df9de9b..7669a17 100644
--- a/internal/fusefrontend_reverse/node_helpers.go
+++ b/internal/fusefrontend_reverse/node_helpers.go
@@ -26,12 +26,13 @@ const (
 )
 
 // translateSize translates the ciphertext size in `out` into plaintext size.
-func (n *Node) translateSize(dirfd int, cName string, out *fuse.Attr) {
+func (n *Node) translateSize(dirfd int, cName string, pName string, out *fuse.Attr) {
 	if out.IsRegular() {
 		rn := n.rootNode()
 		out.Size = rn.contentEnc.PlainSizeToCipherSize(out.Size)
 	} else if out.IsSymlink() {
-		panic("todo: call readlink once it is implemented")
+		cLink, _ := n.readlink(dirfd, cName, pName)
+		out.Size = uint64(len(cLink))
 	}
 }
 
@@ -151,3 +152,29 @@ func (n *Node) lookupDiriv(ctx context.Context, out *fuse.EntryOut) (ch *fs.Inod
 	ch = n.NewInode(ctx, vf, id)
 	return
 }
+
+// readlink reads and encrypts a symlink. Used by Readlink, Getattr, Lookup.
+func (n *Node) readlink(dirfd int, cName string, pName string) (out []byte, errno syscall.Errno) {
+	plainTarget, err := syscallcompat.Readlinkat(dirfd, pName)
+	if err != nil {
+		errno = fs.ToErrno(err)
+		return
+	}
+	rn := n.rootNode()
+	if rn.args.PlaintextNames {
+		return []byte(plainTarget), 0
+	}
+	// Nonce is derived from the relative *ciphertext* path
+	p := filepath.Join(n.Path(), cName)
+	nonce := pathiv.Derive(p, pathiv.PurposeSymlinkIV)
+	// Symlinks are encrypted like file contents and base64-encoded
+	cBinTarget := rn.contentEnc.EncryptBlockNonce([]byte(plainTarget), 0, nil, nonce)
+	cTarget := rn.nameTransform.B64EncodeToString(cBinTarget)
+	// The kernel will reject a symlink target above 4096 chars and return
+	// and I/O error to the user. Better emit the proper error ourselves.
+	if len(cTarget) > syscallcompat.PATH_MAX {
+		errno = syscall.ENAMETOOLONG
+		return
+	}
+	return []byte(cTarget), 0
+}
-- 
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