fuse: fix ListXAttr out of range slice

ListXAttr called with an empty buffer returns the current size of
the list but does not touch the buffer (see man 2 listxattr).

Fixes the xfstests generic/020 crash.
Fixes https://github.com/hanwen/go-fuse/issues/337 .

Add test
Co-authored-by: Han-Wen Nienhuys <hanwen@google.com>
Change-Id: I94c94bc56fdf506d2eb924b246336b787fa2976a

fs/loopback_linux_test.go

@@ -124,7 +124,30 @@ func TestXAttr(t *testing.T) {
 	if err := syscall.Setxattr(tc.mntDir+"/file", attr, value, 0); err != nil {
 		t.Fatalf("Setxattr: %v", err)
 	}
-	sz, err := syscall.Getxattr(tc.mntDir+"/file", attr, buf)
+
+	sz, err := syscall.Listxattr(tc.mntDir+"/file", nil)
+	if err != nil {
+		t.Fatalf("Listxattr: %v", err)
+	}
+	buf = make([]byte, sz)
+	if _, err := syscall.Listxattr(tc.mntDir+"/file", buf); err != nil {
+		t.Fatalf("Listxattr: %v", err)
+	} else {
+		attributes := bytes.Split(buf[:sz], []byte{0})
+		found := false
+		for _, a := range attributes {
+			if string(a) == attr {
+				found = true
+				break
+			}
+		}
+
+		if !found {
+			t.Fatalf("Listxattr: %q (not found: %q", buf[:sz], attr)
+		}
+	}
+
+	sz, err = syscall.Getxattr(tc.mntDir+"/file", attr, buf)
 	if err != nil {
 		t.Fatalf("Getxattr: %v", err)
 	}

fuse/opcode.go

@@ -271,7 +271,11 @@ func doGetXAttr(server *Server, req *request) {
 		req.status = OK
 		out.Size = n
 	} else if req.status.Ok() {
-		req.flatData = req.flatData[:n]
+		// ListXAttr called with an empty buffer returns the current size of
+		// the list but does not touch the buffer (see man 2 listxattr).
+		if len(req.flatData) > 0 {
+			req.flatData = req.flatData[:n]
+		}
 		out.Size = n
 	} else {
 		req.flatData = req.flatData[:0]