fusefrontend: make ListXAttr symlink-safe on Linux

Uses /proc/self/fd.

3 files changed, 47 insertions, 12 deletions

diff --git a/internal/fusefrontend/xattr.go b/internal/fusefrontend/xattr.go
index 8e88a62..816754c 100644
--- a/internal/fusefrontend/xattr.go
+++ b/internal/fusefrontend/xattr.go
@@ -82,21 +82,18 @@ func (fs *FS) RemoveXAttr(relPath string, attr string, context *fuse.Context) fu
 	return fs.removeXAttr(relPath, cAttr, context)
 }
 
-// ListXAttr - FUSE call. Lists extended attributes on the file at "path".
+// ListXAttr - FUSE call. Lists extended attributes on the file at "relPath".
 //
-// TODO: Make symlink-safe. Blocker: package xattr does not provide
-// flistxattr(2).
-func (fs *FS) ListXAttr(path string, context *fuse.Context) ([]string, fuse.Status) {
-	if fs.isFiltered(path) {
+// This function is symlink-safe on Linux.
+// Darwin does not have flistxattr(2) nor /proc/self/fd. How to implement this
+// on Darwin in a symlink-safe way?
+func (fs *FS) ListXAttr(relPath string, context *fuse.Context) ([]string, fuse.Status) {
+	if fs.isFiltered(relPath) {
 		return nil, fuse.EPERM
 	}
-	cPath, err := fs.getBackingPath(path)
-	if err != nil {
-		return nil, fuse.ToStatus(err)
-	}
-	cNames, err := xattr.LList(cPath)
-	if err != nil {
-		return nil, unpackXattrErr(err)
+	cNames, status := fs.listXAttr(relPath, context)
+	if !status.Ok() {
+		return nil, status
 	}
 	names := make([]string, 0, len(cNames))
 	for _, curName := range cNames {
diff --git a/internal/fusefrontend/xattr_darwin.go b/internal/fusefrontend/xattr_darwin.go
index f0b755f..ad81320 100644
--- a/internal/fusefrontend/xattr_darwin.go
+++ b/internal/fusefrontend/xattr_darwin.go
@@ -53,3 +53,17 @@ func (fs *FS) removeXAttr(relPath string, cAttr string, context *fuse.Context) f
 	err = xattr.LRemove(cPath, cAttr)
 	return unpackXattrErr(err)
 }
+
+// This function is NOT symlink-safe because Darwin lacks
+// both flistxattr() and /proc/self/fd.
+func (fs *FS) listXAttr(relPath string, context *fuse.Context) ([]string, fuse.Status) {
+	cPath, err := fs.getBackingPath(relPath)
+	if err != nil {
+		return nil, fuse.ToStatus(err)
+	}
+	cNames, err := xattr.LList(cPath)
+	if err != nil {
+		return nil, unpackXattrErr(err)
+	}
+	return cNames, fuse.OK
+}
diff --git a/internal/fusefrontend/xattr_linux.go b/internal/fusefrontend/xattr_linux.go
index a17b12c..915713d 100644
--- a/internal/fusefrontend/xattr_linux.go
+++ b/internal/fusefrontend/xattr_linux.go
@@ -99,3 +99,27 @@ func (fs *FS) removeXAttr(relPath string, cAttr string, context *fuse.Context) f
 	err := xattr.Remove(procFd(fd), cAttr)
 	return unpackXattrErr(err)
 }
+
+// listXAttr - list encrypted xattr names on plaintext path "relPath".
+//
+// This function is symlink-safe on Linux by using /proc/self/fd.
+func (fs *FS) listXAttr(relPath string, context *fuse.Context) ([]string, fuse.Status) {
+	file, fd, status := fs.getFileFd(relPath, context)
+	if !status.Ok() {
+		// If relPath is a symlink, getFileFd fails with ELOOP. As setXattr()
+		// also fails with ELOOP, there is no way to set xattrs on symlinks,
+		// and we can assume that the file does not have any.
+		if status == fuse.Status(syscall.ELOOP) {
+			return nil, fuse.OK
+		}
+		return nil, status
+	}
+	defer file.Release()
+
+	cNames, err := xattr.List(procFd(fd))
+	if err != nil {
+		status := unpackXattrErr(err)
+		return nil, status
+	}
+	return cNames, fuse.OK
+}