Xattrs support in reverse mode

Fixes rfjakob#827

internal/fusefrontend/node_xattr.go

@@ -12,9 +12,6 @@ import (
 	"github.com/rfjakob/gocryptfs/v2/internal/tlog"
 )
 
-// -1 as uint32
-const minus1 = ^uint32(0)
-
 // We store encrypted xattrs under this prefix plus the base64-encoded
 // encrypted original name.
 var xattrStorePrefix = "user.gocryptfs."
@@ -50,13 +47,13 @@ func (n *Node) Getxattr(ctx context.Context, attr string, dest []byte) (uint32,
 		var errno syscall.Errno
 		data, errno = n.getXAttr(attr)
 		if errno != 0 {
-			return minus1, errno
+			return 0, errno
 		}
 	} else {
 		// encrypted user xattr
 		cAttr, err := rn.encryptXattrName(attr)
 		if err != nil {
-			return minus1, syscall.EIO
+			return 0, syscall.EIO
 		}
 		cData, errno := n.getXAttr(cAttr)
 		if errno != 0 {
@@ -65,15 +62,11 @@ func (n *Node) Getxattr(ctx context.Context, attr string, dest []byte) (uint32,
 		data, err = rn.decryptXattrValue(cData)
 		if err != nil {
 			tlog.Warn.Printf("GetXAttr: %v", err)
-			return minus1, syscall.EIO
+			return 0, syscall.EIO
 		}
 	}
-	// Caller passes size zero to find out how large their buffer should be
-	if len(dest) == 0 {
-		return uint32(len(data)), 0
-	}
 	if len(dest) < len(data) {
-		return minus1, syscall.ERANGE
+		return uint32(len(data)), syscall.ERANGE
 	}
 	l := copy(dest, data)
 	return uint32(l), 0
@@ -155,12 +148,8 @@ func (n *Node) Listxattr(ctx context.Context, dest []byte) (uint32, syscall.Errn
 		}
 		buf.WriteString(name + "\000")
 	}
-	// Caller passes size zero to find out how large their buffer should be
-	if len(dest) == 0 {
-		return uint32(buf.Len()), 0
-	}
 	if buf.Len() > len(dest) {
-		return minus1, syscall.ERANGE
+		return uint32(buf.Len()), syscall.ERANGE
 	}
 	return uint32(copy(dest, buf.Bytes())), 0
 }

internal/fusefrontend_reverse/node_api_check.go

@@ -11,14 +11,8 @@ var _ = (fs.NodeReaddirer)((*Node)(nil))
 var _ = (fs.NodeReadlinker)((*Node)(nil))
 var _ = (fs.NodeOpener)((*Node)(nil))
 var _ = (fs.NodeStatfser)((*Node)(nil))
-
-/*
-TODO but low prio. reverse mode in gocryptfs v1 did not have xattr support
-either.
-
 var _ = (fs.NodeGetxattrer)((*Node)(nil))
 var _ = (fs.NodeListxattrer)((*Node)(nil))
-*/
 
 /* Not needed
 var _ = (fs.NodeOpendirer)((*Node)(nil))

internal/fusefrontend_reverse/node_xattr.go

@@ -0,0 +1,81 @@
+// Package fusefrontend_reverse interfaces directly with the go-fuse library.
+package fusefrontend_reverse
+
+import (
+	"bytes"
+	"context"
+	"syscall"
+
+	"github.com/rfjakob/gocryptfs/v2/internal/pathiv"
+)
+
+// We store encrypted xattrs under this prefix plus the base64-encoded
+// encrypted original name.
+var xattrStorePrefix = "user.gocryptfs."
+
+// isAcl returns true if the attribute name is for storing ACLs
+//
+// ACLs are passed through without encryption
+func isAcl(attr string) bool {
+	return attr == "system.posix_acl_access" || attr == "system.posix_acl_default"
+}
+
+// GetXAttr - FUSE call. Reads the value of extended attribute "attr".
+//
+// This function is symlink-safe through Fgetxattr.
+func (n *Node) Getxattr(ctx context.Context, attr string, dest []byte) (uint32, syscall.Errno) {
+	rn := n.rootNode()
+	var data []byte
+	// ACLs are passed through without encryption
+	if isAcl(attr) {
+		var errno syscall.Errno
+		data, errno = n.getXAttr(attr)
+		if errno != 0 {
+			return 0, errno
+		}
+	} else {
+		pAttr, err := rn.decryptXattrName(attr)
+		if err != nil {
+			return 0, syscall.EINVAL
+		}
+		pData, errno := n.getXAttr(pAttr)
+		if errno != 0 {
+			return 0, errno
+		}
+		nonce := pathiv.Derive(n.Path()+"\000"+attr, pathiv.PurposeXattrIV)
+		data = rn.encryptXattrValue(pData, nonce)
+	}
+	if len(dest) < len(data) {
+		return uint32(len(data)), syscall.ERANGE
+	}
+	l := copy(dest, data)
+	return uint32(l), 0
+}
+
+// ListXAttr - FUSE call. Lists extended attributes on the file at "relPath".
+//
+// This function is symlink-safe through Flistxattr.
+func (n *Node) Listxattr(ctx context.Context, dest []byte) (uint32, syscall.Errno) {
+	pNames, errno := n.listXAttr()
+	if errno != 0 {
+		return 0, errno
+	}
+	rn := n.rootNode()
+	var buf bytes.Buffer
+	for _, pName := range pNames {
+		// ACLs are passed through without encryption
+		if isAcl(pName) {
+			buf.WriteString(pName + "\000")
+			continue
+		}
+		cName, err := rn.encryptXattrName(pName)
+		if err != nil {
+			continue
+		}
+		buf.WriteString(cName + "\000")
+	}
+	if buf.Len() > len(dest) {
+		return uint32(buf.Len()), syscall.ERANGE
+	}
+	return uint32(copy(dest, buf.Bytes())), 0
+}

internal/fusefrontend_reverse/node_xattr_darwin.go

@@ -0,0 +1,52 @@
+package fusefrontend_reverse
+
+import (
+	"syscall"
+
+	"github.com/hanwen/go-fuse/v2/fs"
+
+	"github.com/rfjakob/gocryptfs/v2/internal/syscallcompat"
+)
+
+func (n *Node) getXAttr(cAttr string) (out []byte, errno syscall.Errno) {
+	d, errno := n.prepareAtSyscall("")
+	if errno != 0 {
+		return
+	}
+	defer syscall.Close(d.dirfd)
+
+	// O_NONBLOCK to not block on FIFOs.
+	fd, err := syscallcompat.Openat(d.dirfd, d.pName, syscall.O_RDONLY|syscall.O_NONBLOCK|syscall.O_NOFOLLOW, 0)
+	if err != nil {
+		return nil, fs.ToErrno(err)
+	}
+	defer syscall.Close(fd)
+
+	cData, err := syscallcompat.Fgetxattr(fd, cAttr)
+	if err != nil {
+		return nil, fs.ToErrno(err)
+	}
+
+	return cData, 0
+}
+
+func (n *Node) listXAttr() (out []string, errno syscall.Errno) {
+	d, errno := n.prepareAtSyscall("")
+	if errno != 0 {
+		return
+	}
+	defer syscall.Close(d.dirfd)
+
+	// O_NONBLOCK to not block on FIFOs.
+	fd, err := syscallcompat.Openat(d.dirfd, d.pName, syscall.O_RDONLY|syscall.O_NONBLOCK|syscall.O_NOFOLLOW, 0)
+	if err != nil {
+		return nil, fs.ToErrno(err)
+	}
+	defer syscall.Close(fd)
+
+	pNames, err := syscallcompat.Flistxattr(fd)
+	if err != nil {
+		return nil, fs.ToErrno(err)
+	}
+	return pNames, 0
+}

internal/fusefrontend_reverse/node_xattr_linux.go

@@ -0,0 +1,40 @@
+package fusefrontend_reverse
+
+import (
+	"fmt"
+	"syscall"
+
+	"github.com/hanwen/go-fuse/v2/fs"
+
+	"github.com/rfjakob/gocryptfs/v2/internal/syscallcompat"
+)
+
+func (n *Node) getXAttr(cAttr string) (out []byte, errno syscall.Errno) {
+	d, errno := n.prepareAtSyscall("")
+	if errno != 0 {
+		return
+	}
+	defer syscall.Close(d.dirfd)
+
+	procPath := fmt.Sprintf("/proc/self/fd/%d/%s", d.dirfd, d.pName)
+	pData, err := syscallcompat.Lgetxattr(procPath, cAttr)
+	if err != nil {
+		return nil, fs.ToErrno(err)
+	}
+	return pData, 0
+}
+
+func (n *Node) listXAttr() (out []string, errno syscall.Errno) {
+	d, errno := n.prepareAtSyscall("")
+	if errno != 0 {
+		return
+	}
+	defer syscall.Close(d.dirfd)
+
+	procPath := fmt.Sprintf("/proc/self/fd/%d/%s", d.dirfd, d.pName)
+	pNames, err := syscallcompat.Llistxattr(procPath)
+	if err != nil {
+		return nil, fs.ToErrno(err)
+	}
+	return pNames, 0
+}

internal/fusefrontend_reverse/root_node.go

@@ -21,7 +21,7 @@ import (
 	"github.com/rfjakob/gocryptfs/v2/internal/syscallcompat"
 	"github.com/rfjakob/gocryptfs/v2/internal/tlog"
 
-	"github.com/sabhiram/go-gitignore"
+	ignore "github.com/sabhiram/go-gitignore"
 )
 
 // RootNode is the root directory in a `gocryptfs -reverse` mount
@@ -182,3 +182,38 @@ func (rn *RootNode) uniqueStableAttr(mode uint32, ino uint64) fs.StableAttr {
 func (rn *RootNode) RootIno() uint64 {
 	return rn.rootIno
 }
+
+// encryptXattrValue encrypts the xattr value "data".
+// The data is encrypted like a file content block, but without binding it to
+// a file location (block number and file id are set to zero).
+// Special case: an empty value is encrypted to an empty value.
+func (rn *RootNode) encryptXattrValue(data []byte, nonce []byte) (cData []byte) {
+	if len(data) == 0 {
+		return []byte{}
+	}
+	return rn.contentEnc.EncryptBlockNonce(data, 0, nil, nonce)
+}
+
+// encryptXattrName transforms "user.foo" to "user.gocryptfs.a5sAd4XAa47f5as6dAf"
+func (rn *RootNode) encryptXattrName(attr string) (string, error) {
+	// xattr names are encrypted like file names, but with a fixed IV.
+	cAttr, err := rn.nameTransform.EncryptXattrName(attr)
+	if err != nil {
+		return "", err
+	}
+	return xattrStorePrefix + cAttr, nil
+}
+
+func (rn *RootNode) decryptXattrName(cAttr string) (attr string, err error) {
+	// Reject anything that does not start with "user.gocryptfs."
+	if !strings.HasPrefix(cAttr, xattrStorePrefix) {
+		return "", syscall.EINVAL
+	}
+	// Strip "user.gocryptfs." prefix
+	cAttr = cAttr[len(xattrStorePrefix):]
+	attr, err = rn.nameTransform.DecryptXattrName(cAttr)
+	if err != nil {
+		return "", err
+	}
+	return attr, nil
+}

internal/pathiv/pathiv.go

@@ -20,6 +20,8 @@ const (
 	PurposeSymlinkIV Purpose = "SYMLINKIV"
 	// PurposeBlock0IV means the value will be used as the IV of ciphertext block #0.
 	PurposeBlock0IV Purpose = "BLOCK0IV"
+	// PurposeXattrIV means the value will be used as a xattr IV
+	PurposeXattrIV Purpose = "XATTRIV"
 )
 
 // Derive derives an IV from an encrypted path by hashing it with sha256