Simplify check for GBR width and height

Tests/test_file_gbr.py

@@ -1,8 +1,10 @@
 from __future__ import annotations
 
+from io import BytesIO
+
 import pytest
 
-from PIL import GbrImagePlugin, Image
+from PIL import GbrImagePlugin, Image, _binary
 
 from .helper import assert_image_equal_tofile
 
@@ -31,8 +33,49 @@ def test_multiple_load_operations() -> None:
         assert_image_equal_tofile(im, "Tests/images/gbr.png")
 
 
+def create_gbr_image(info: dict[str, int] = {}, magic_number=b"") -> BytesIO:
+    return BytesIO(
+        b"".join(
+            _binary.o32be(i)
+            for i in [
+                info.get("header_size", 20),
+                info.get("version", 1),
+                info.get("width", 1),
+                info.get("height", 1),
+                info.get("color_depth", 1),
+            ]
+        )
+        + magic_number
+    )
+
+
 def test_invalid_file() -> None:
-    invalid_file = "Tests/images/flower.jpg"
+    for f in [
+        create_gbr_image({"header_size": 0}),
+        create_gbr_image({"width": 0}),
+        create_gbr_image({"height": 0}),
+    ]:
+        with pytest.raises(SyntaxError, match="not a GIMP brush"):
+            GbrImagePlugin.GbrImageFile(f)
 
-    with pytest.raises(SyntaxError):
+    invalid_file = "Tests/images/flower.jpg"
+    with pytest.raises(SyntaxError, match="Unsupported GIMP brush version"):
         GbrImagePlugin.GbrImageFile(invalid_file)
+
+
+def test_unsupported_gimp_brush() -> None:
+    f = create_gbr_image({"color_depth": 2})
+    with pytest.raises(SyntaxError, match="Unsupported GIMP brush color depth: 2"):
+        GbrImagePlugin.GbrImageFile(f)
+
+
+def test_bad_magic_number() -> None:
+    f = create_gbr_image({"version": 2}, magic_number=b"badm")
+    with pytest.raises(SyntaxError, match="not a GIMP brush, bad magic number"):
+        GbrImagePlugin.GbrImageFile(f)
+
+
+def test_L() -> None:
+    f = create_gbr_image()
+    with Image.open(f) as im:
+        assert im.mode == "L"

src/PIL/GbrImagePlugin.py

@@ -54,7 +54,7 @@ def _open(self) -> None:
         width = i32(self.fp.read(4))
         height = i32(self.fp.read(4))
         color_depth = i32(self.fp.read(4))
-        if width <= 0 or height <= 0:
+        if width == 0 or height == 0:
             msg = "not a GIMP brush"
             raise SyntaxError(msg)
         if color_depth not in (1, 4):
@@ -71,7 +71,7 @@ def _open(self) -> None:
                 raise SyntaxError(msg)
             self.info["spacing"] = i32(self.fp.read(4))
 
-        comment = self.fp.read(comment_length)[:-1]
+        self.info["comment"] = self.fp.read(comment_length)[:-1]
 
         if color_depth == 1:
             self._mode = "L"
@@ -80,8 +80,6 @@ def _open(self) -> None:
 
         self._size = width, height
 
-        self.info["comment"] = comment
-
         # Image might not be small
         Image._decompression_bomb_check(self.size)