Allow for different palette entry sizes when correcting BMP pixel data offset

Tests/test_file_bmp.py

@@ -42,7 +42,7 @@ def test_fallback_if_mmap_errors() -> None:
     # This image has been truncated,
     # so that the buffer is not large enough when using mmap
     with Image.open("Tests/images/mmap_error.bmp") as im:
-        assert_image_equal_tofile(im, "Tests/images/pal8_offset.bmp")
+        assert_image_equal_tofile(im, "Tests/images/bmp/g/pal8.bmp")
 
 
 def test_save_to_bytes() -> None:
@@ -238,11 +238,21 @@ def test_unsupported_bmp_bitfields_layout() -> None:
         Image.open(fp)
 
 
-def test_offset() -> None:
-    # This image has been hexedited
-    # to exclude the palette size from the pixel data offset
-    with Image.open("Tests/images/pal8_offset.bmp") as im:
-        assert_image_equal_tofile(im, "Tests/images/bmp/g/pal8.bmp")
+@pytest.mark.parametrize(
+    "offset, path",
+    (
+        (26, "pal8os2.bmp"),
+        (54, "pal8.bmp"),
+    ),
+)
+def test_offset(offset: int, path: str) -> None:
+    image_path = "Tests/images/bmp/g/" + path
+    # Exclude the palette size from the pixel data offset
+    with open(image_path, "rb") as fp:
+        data = fp.read()
+        data = data[:10] + o32(offset) + data[14:]
+        with Image.open(io.BytesIO(data)) as im:
+            assert_image_equal_tofile(im, image_path)
 
 
 def test_use_raw_alpha(monkeypatch: pytest.MonkeyPatch) -> None:

src/PIL/BmpImagePlugin.py

@@ -179,14 +179,12 @@ def _bitmap(self, header: int = 0, offset: int = 0) -> None:
 
         # ------- If color count was not found in the header, compute from bits
         assert isinstance(file_info["bits"], int)
-        file_info["colors"] = (
-            file_info["colors"]
-            if file_info.get("colors", 0)
-            else (1 << file_info["bits"])
-        )
+        if not file_info.get("colors", 0):
+            file_info["colors"] = 1 << file_info["bits"]
+        assert isinstance(file_info["palette_padding"], int)
         assert isinstance(file_info["colors"], int)
         if offset == 14 + file_info["header_size"] and file_info["bits"] <= 8:
-            offset += 4 * file_info["colors"]
+            offset += file_info["palette_padding"] * file_info["colors"]
 
         # ---------------------- Check bit depth for unusual unsupported values
         self._mode, raw_mode = BIT2MODE.get(file_info["bits"], ("", ""))
@@ -265,7 +263,6 @@ def _bitmap(self, header: int = 0, offset: int = 0) -> None:
                 msg = f"Unsupported BMP Palette size ({file_info['colors']})"
                 raise OSError(msg)
             else:
-                assert isinstance(file_info["palette_padding"], int)
                 padding = file_info["palette_padding"]
                 palette = read(padding * file_info["colors"])
                 grayscale = True