Use checkpath from the linux source tree.

pkgs/checkpatch/default.nix

-{ stdenv, perl }:
+{ stdenv, src, perl }:
 stdenv.mkDerivation {
-    src = ./checkpatch.pl;
+    inherit src;
     name = "checkpatch";
-    unpackPhase = "true";
+    buildPhase = "true";
     installPhase = ''
-    chmod +x $src
-    mkdir -p $out/bin && cp $src $out/bin
+      mkdir -p $out/bin $out/share
+      substituteInPlace scripts/checkpatch.pl --replace 'dirname(abs_path($P))' \"$out/share\";
+      cp scripts/checkpatch.pl $out/bin
+      cp scripts/spelling.txt scripts/const_structs.checkpatch $out/share
     '';
     buildInputs = [perl];
   }

pkgs/default.nix

@@ -95,7 +95,7 @@ let
     openmc-data = fetched openmc/data.json;
 
     # tools
-    checkpatch = callPackage ./checkpatch {};
+    checkpatch = callPackage ./checkpatch {src=pkgs.linux_testing.src;};
     zymake = callPackage ./zymake {
       inherit (pkgs.ocamlPackages) findlib ocamlbuild topkg batteries;
     };