replace MBR extraction with python-native implementation

udib/modiso.py

@@ -246,17 +246,9 @@ def extract_mbr_from_iso(path_to_output_file, path_to_source_iso):
             f"Input file is not an image file: '{path_to_source_iso}'.")
 
     # extract the MBR (first 432 Bytes) of the source ISO file
-    # FIXME do this in python, dd is too dangerous
-    try:
-        subprocess.run(
-            ["dd", f"if={path_to_source_iso.resolve()}", "bs=1", "count=432",
-             f"of={path_to_output_file.resolve()}"],
-            shell=True,
-            check=True)
-
-    except subprocess.CalledProcessError:
-        raise RuntimeError(f"Failed while extracting MBR from source file: "
-                           f"'{path_to_source_iso}'.")
+    with open(path_to_source_iso, mode="r+b") as iso_file:
+        with open(path_to_output_file, mode="w+b") as mbr_file:
+            mbr_file.write(iso_file.read(432))
 
 
 def repack_iso(path_to_output_iso,

udib/udib.py

@@ -200,6 +200,9 @@ def main():
     else:
         # modify image file using specified preseed file
         path_to_preseed_file = Path(args.existing_preseed_file)
+        if not path_to_preseed_file.is_file():
+            p.error(f"No such file: '{path_to_preseed_file}'.")
+            sys.exit(1)
 
         if args.path_to_existing_image:
             # user has specified an existing image file
@@ -227,15 +230,14 @@ def main():
         # extract image MBR to a temporary directory
         p.info("Extracting master boot record...")
         path_to_mbr = Path(tempfile.mkdtemp())/"mbr.bin"
-        # FIXME fix MBR extraction
-        #modiso.extract_mbr_from_iso(
-        #    path_to_mbr,
-        #    path_to_image_file)
+        modiso.extract_mbr_from_iso(
+            path_to_mbr,
+            path_to_image_file)
 
         # append preseed file to extracted initrd
         p.info("Appending preseed file...")
         modiso.append_file_contents_to_initrd_archive(
-            path_to_extracted_iso_dir,
+            path_to_extracted_iso_dir/"install.amd"/"initrd.gz",
             path_to_preseed_file)
 
         # regenerate md5sum.txt