fusefrontend: add xattr support

At the moment, only for reverse mode.

https://github.com/rfjakob/gocryptfs/issues/217

1 files changed, 157 insertions, 0 deletions

diff --git a/internal/fusefrontend/xattr.go b/internal/fusefrontend/xattr.go
new file mode 100644
index 0000000..357e889
--- /dev/null
+++ b/internal/fusefrontend/xattr.go
@@ -0,0 +1,157 @@
+// Package fusefrontend interfaces directly with the go-fuse library.
+package fusefrontend
+
+// FUSE operations on paths
+
+import (
+	"strings"
+	"syscall"
+
+	"github.com/hanwen/go-fuse/fuse"
+	xattr "github.com/rfjakob/pkg-xattr"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+// xattr names are encrypted like file names, but with a fixed IV.
+var xattrNameIV = []byte("xattr_name_iv_xx")
+
+// Only allow the "user" namespace, block "trusted" and "security", as
+// these may be interpreted by the system, and we don't want to cause
+// trouble with our encrypted garbage.
+var xattrUserPrefix = "user."
+
+// We store encrypted xattrs under this prefix plus the base64-encoded
+// encrypted original name.
+var xattrStorePrefix = "user.gocryptfs."
+
+// GetXAttr: read the value of extended attribute "attr".
+// Implements pathfs.Filesystem.
+func (fs *FS) GetXAttr(path string, attr string, context *fuse.Context) ([]byte, fuse.Status) {
+	if fs.isFiltered(path) {
+		return nil, fuse.EPERM
+	}
+	cAttr, err := fs.encryptXattrName(attr)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	cPath, err := fs.getBackingPath(path)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	cData64, err := xattr.Get(cPath, cAttr)
+	if err != nil {
+		return nil, unpackXattrErr(err)
+	}
+	// xattr data is decrypted like a symlink target
+	data, err := fs.decryptSymlinkTarget(string(cData64))
+	if err != nil {
+		tlog.Warn.Printf("GetXAttr: %v", err)
+		return nil, fuse.EIO
+	}
+	return []byte(data), fuse.OK
+}
+
+// SetXAttr implements pathfs.Filesystem.
+func (fs *FS) SetXAttr(path string, attr string, data []byte, flags int, context *fuse.Context) fuse.Status {
+	if fs.isFiltered(path) {
+		return fuse.EPERM
+	}
+	if flags != 0 {
+		return fuse.EPERM
+	}
+	cPath, err := fs.getBackingPath(path)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	cAttr, err := fs.encryptXattrName(attr)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	// xattr data is encrypted like a symlink target
+	cData64 := []byte(fs.encryptSymlinkTarget(string(data)))
+	return unpackXattrErr(xattr.Set(cPath, cAttr, cData64))
+}
+
+// RemoveXAttr implements pathfs.Filesystem.
+func (fs *FS) RemoveXAttr(path string, attr string, context *fuse.Context) fuse.Status {
+	if fs.isFiltered(path) {
+		return fuse.EPERM
+	}
+	cPath, err := fs.getBackingPath(path)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	cAttr, err := fs.encryptXattrName(attr)
+	if err != nil {
+		return fuse.ToStatus(err)
+	}
+	return unpackXattrErr(xattr.Remove(cPath, cAttr))
+}
+
+// ListXAttr implements pathfs.Filesystem.
+func (fs *FS) ListXAttr(path string, context *fuse.Context) ([]string, fuse.Status) {
+	if fs.isFiltered(path) {
+		return nil, fuse.EPERM
+	}
+	cPath, err := fs.getBackingPath(path)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	cNames, err := xattr.List(cPath)
+	if err != nil {
+		return nil, unpackXattrErr(err)
+	}
+	names := make([]string, 0, len(cNames))
+	for _, curName := range cNames {
+		if !strings.HasPrefix(curName, xattrStorePrefix) {
+			continue
+		}
+		name, err := fs.decryptXattrName(curName)
+		if err != nil {
+			tlog.Warn.Printf("ListXAttr: invalid xattr name %q: %v", curName, err)
+			continue
+		}
+		names = append(names, name)
+	}
+	return names, fuse.OK
+}
+
+// encryptXattrName transforms "user.foo" to "user.gocryptfs.a5sAd4XAa47f5as6dAf"
+func (fs *FS) encryptXattrName(attr string) (cAttr string, err error) {
+	// Reject anything that does not start with "user."
+	if !strings.HasPrefix(attr, xattrUserPrefix) {
+		return "", syscall.EPERM
+	}
+	// xattr names are encrypted like file names, but with a fixed IV.
+	cAttr = xattrStorePrefix + fs.nameTransform.EncryptName(attr, xattrNameIV)
+	return cAttr, nil
+}
+
+func (fs *FS) 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 = fs.nameTransform.DecryptName(cAttr, xattrNameIV)
+	if err != nil {
+		return "", err
+	}
+	return attr, nil
+}
+
+// unpackXattrErr unpacks an error value that we got from xattr.Get/Set/etc
+// and converts it to a fuse status.
+func unpackXattrErr(err error) fuse.Status {
+	if err == nil {
+		return fuse.OK
+	}
+	err2, ok := err.(*xattr.Error)
+	if !ok {
+		tlog.Warn.Printf("unpackXattrErr: cannot unpack err=%v", err)
+		return fuse.EIO
+	}
+	return fuse.ToStatus(err2.Err)
+}