radarhere: Could you add a test that fails without your fix and passes with it?
diff --git a/src/PIL/TiffImagePlugin.py b/src/PIL/TiffImagePlugin.py
index dabcf8eb4..bb86c53aa 100644
--- a/src/PIL/TiffImagePlugin.py
+++ b/src/PIL/TiffImagePlugin.py
@@ -1164,7 +1164,7 @@ class TiffImageFile(ImageFile.ImageFile):
             if DEBUG:
                 print("have getvalue. just sending in a string from getvalue")
             n, err = decoder.decode(self.fp.getvalue())
-        elif hasattr(self.fp, "fileno"):
+        elif fp:
             # we've got a actual file on disk, pass in the fp.
             if DEBUG:
                 print("have fileno, calling fileno version of the decoder.")
@@ -1175,6 +1175,7 @@ class TiffImageFile(ImageFile.ImageFile):
             # we have something else.
             if DEBUG:
                 print("don't have fileno or getvalue. just reading")
+            self.fp.seek(0)
             # UNDONE -- so much for that buffer size thing.
             n, err = decoder.decode(self.fp.read())
 

BufferedReader wrapping BytesIO breaks tiff loader
I struck an edge case trying to open a TIFF file from a BufferedReader that wraps a BytesIO object. The problem is that when the wrapped object is not a file, BufferedReader has a "fileno" attribute but calling it returns an error. This case is handled for BytesIO itself, and sidestepped by using "getvalue" rather than trying to open by fileno. I'll open a PR but documenting the issue here.

* OS: macOS 10.14.3
* Python: 3.7.3
* Pillow: 6.1.0

```python
import io
from PIL import Image

bytesio = io.BytesIO()
with open(tiff_filename, 'rb') as fp:
    bytesio.write(fp.read())

reader = io.BufferedReader(bytesio)
im = Image.open(reader)
im.load()
```
Result:
```
tempfile.tif: Cannot read TIFF header.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.7/site-packages/PIL/TiffImagePlugin.py", line 1099, in load
    return self._load_libtiff()
  File "/usr/local/lib/python3.7/site-packages/PIL/TiffImagePlugin.py", line 1191, in _load_libtiff
    raise IOError(err)
OSError: -2
```
diff --git a/Tests/test_file_libtiff.py b/Tests/test_file_libtiff.py
index bc5003f5f..6339d878a 100644
--- a/Tests/test_file_libtiff.py
+++ b/Tests/test_file_libtiff.py
@@ -81,6 +81,19 @@ class TestFileLibTiff(LibTiffTestCase):
         self.assertEqual(im.size, (500, 500))
         self._assert_noerr(im)
 
+    def test_g4_non_disk_file_object(self):
+        """Testing loading from non-disk non-BytesIO file object"""
+        test_file = "Tests/images/hopper_g4_500.tif"
+        s = io.BytesIO()
+        with open(test_file, "rb") as f:
+            s.write(f.read())
+            s.seek(0)
+        r = io.BufferedReader(s)
+        im = Image.open(r)
+
+        self.assertEqual(im.size, (500, 500))
+        self._assert_noerr(im)
+
     def test_g4_eq_png(self):
         """ Checking that we're actually getting the data that we expect"""
         png = Image.open("Tests/images/hopper_bw_500.png")

pip install -e .[dev]
parse_log_pytest
requirements.txt
apt-get update apt-get install -y gcc libjpeg-dev zlib1g-dev libtiff5-dev libfreetype6-dev liblcms2-dev libwebp-dev tcl8.6-dev tk8.6-dev libharfbuzz-dev libfribidi-dev libxcb1-dev
3.9
requirements.txt
pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
alabaster==0.7.16
babel==2.17.0
black==25.1.0
blessed==1.20.0
build==1.2.2.post1
certifi==2025.1.31
charset-normalizer==3.4.1
check-manifest==0.50
click==8.1.8
coverage==7.8.0
coveralls==4.0.1
docopt==0.6.2
docutils==0.21.2
exceptiongroup==1.2.2
idna==3.10
imagesize==1.4.1
importlib_metadata==8.6.1
iniconfig==2.1.0
jarn.viewdoc==2.7
Jinja2==3.1.6
MarkupSafe==3.0.2
mypy-extensions==1.0.0
olefile==0.47
packaging==24.2
pathspec==0.12.1
-e git+https://github.com/python-pillow/Pillow.git@6de118abd333d5de1679947bd75898b3468ea519#egg=Pillow
platformdirs==4.3.7
pluggy==1.5.0
pycodestyle==2.13.0
pyflakes==3.3.1
Pygments==2.19.1
pyproject_hooks==1.2.0
pyroma==4.2
pytest==8.3.5
pytest-cov==6.0.0
requests==2.32.3
six==1.17.0
snowballstemmer==2.2.0
Sphinx==7.4.7
sphinx-rtd-theme==3.0.2
sphinxcontrib-applehelp==2.0.0
sphinxcontrib-devhelp==2.0.0
sphinxcontrib-htmlhelp==2.1.0
sphinxcontrib-jquery==4.1
sphinxcontrib-jsmath==1.0.1
sphinxcontrib-qthelp==2.0.0
sphinxcontrib-serializinghtml==2.0.0
tomli==2.2.1
trove-classifiers==2025.3.19.19
typing_extensions==4.13.0
urllib3==2.3.0
wcwidth==0.2.13
zipp==3.21.0

name: Pillow
channels:
  - defaults
  - https://repo.anaconda.com/pkgs/main
  - https://repo.anaconda.com/pkgs/r
  - conda-forge
dependencies:
  - _libgcc_mutex=0.1=main
  - _openmp_mutex=5.1=1_gnu
  - ca-certificates=2025.2.25=h06a4308_0
  - ld_impl_linux-64=2.40=h12ee557_0
  - libffi=3.4.4=h6a678d5_1
  - libgcc-ng=11.2.0=h1234567_1
  - libgomp=11.2.0=h1234567_1
  - libstdcxx-ng=11.2.0=h1234567_1
  - ncurses=6.4=h6a678d5_0
  - openssl=3.0.16=h5eee18b_0
  - pip=25.0=py39h06a4308_0
  - python=3.9.21=he870216_1
  - readline=8.2=h5eee18b_0
  - setuptools=75.8.0=py39h06a4308_0
  - sqlite=3.45.3=h5eee18b_0
  - tk=8.6.14=h39e8969_0
  - tzdata=2025a=h04d1e81_0
  - wheel=0.45.1=py39h06a4308_0
  - xz=5.6.4=h5eee18b_1
  - zlib=1.2.13=h5eee18b_1
  - pip:
      - alabaster==0.7.16
      - babel==2.17.0
      - black==25.1.0
      - blessed==1.20.0
      - build==1.2.2.post1
      - certifi==2025.1.31
      - charset-normalizer==3.4.1
      - check-manifest==0.50
      - click==8.1.8
      - coverage==7.8.0
      - coveralls==4.0.1
      - docopt==0.6.2
      - docutils==0.21.2
      - exceptiongroup==1.2.2
      - idna==3.10
      - imagesize==1.4.1
      - importlib-metadata==8.6.1
      - iniconfig==2.1.0
      - jarn-viewdoc==2.7
      - jinja2==3.1.6
      - markupsafe==3.0.2
      - mypy-extensions==1.0.0
      - olefile==0.47
      - packaging==24.2
      - pathspec==0.12.1
      - platformdirs==4.3.7
      - pluggy==1.5.0
      - pycodestyle==2.13.0
      - pyflakes==3.3.1
      - pygments==2.19.1
      - pyproject-hooks==1.2.0
      - pyroma==4.2
      - pytest==8.3.5
      - pytest-cov==6.0.0
      - requests==2.32.3
      - six==1.17.0
      - snowballstemmer==2.2.0
      - sphinx==7.4.7
      - sphinx-rtd-theme==3.0.2
      - sphinxcontrib-applehelp==2.0.0
      - sphinxcontrib-devhelp==2.0.0
      - sphinxcontrib-htmlhelp==2.1.0
      - sphinxcontrib-jquery==4.1
      - sphinxcontrib-jsmath==1.0.1
      - sphinxcontrib-qthelp==2.0.0
      - sphinxcontrib-serializinghtml==2.0.0
      - tomli==2.2.1
      - trove-classifiers==2025.3.19.19
      - typing-extensions==4.13.0
      - urllib3==2.3.0
      - wcwidth==0.2.13
      - zipp==3.21.0
prefix: /opt/conda/envs/Pillow

Tests/test_file_libtiff.py::TestFileLibTiff::test_g4_non_disk_file_object
Tests/test_file_libtiff.py::TestFileLibTiff::test_custom_metadata
Tests/test_file_libtiff.py::TestFileLibTiff::test_12bit_rawmode Tests/test_file_libtiff.py::TestFileLibTiff::test_16bit_RGB_tiff Tests/test_file_libtiff.py::TestFileLibTiff::test_16bit_RGBa_tiff Tests/test_file_libtiff.py::TestFileLibTiff::test_4bit Tests/test_file_libtiff.py::TestFileLibTiff::test__next Tests/test_file_libtiff.py::TestFileLibTiff::test_additional_metadata Tests/test_file_libtiff.py::TestFileLibTiff::test_adobe_deflate_tiff Tests/test_file_libtiff.py::TestFileLibTiff::test_big_endian Tests/test_file_libtiff.py::TestFileLibTiff::test_blur Tests/test_file_libtiff.py::TestFileLibTiff::test_cmyk_save Tests/test_file_libtiff.py::TestFileLibTiff::test_compressions Tests/test_file_libtiff.py::TestFileLibTiff::test_crashing_metadata Tests/test_file_libtiff.py::TestFileLibTiff::test_fd_duplication Tests/test_file_libtiff.py::TestFileLibTiff::test_fp_leak Tests/test_file_libtiff.py::TestFileLibTiff::test_g3_compression Tests/test_file_libtiff.py::TestFileLibTiff::test_g4_eq_png Tests/test_file_libtiff.py::TestFileLibTiff::test_g4_fillorder_eq_png Tests/test_file_libtiff.py::TestFileLibTiff::test_g4_large Tests/test_file_libtiff.py::TestFileLibTiff::test_g4_string_info Tests/test_file_libtiff.py::TestFileLibTiff::test_g4_tiff Tests/test_file_libtiff.py::TestFileLibTiff::test_g4_tiff_bytesio Tests/test_file_libtiff.py::TestFileLibTiff::test_g4_tiff_file Tests/test_file_libtiff.py::TestFileLibTiff::test_g4_write Tests/test_file_libtiff.py::TestFileLibTiff::test_gimp_tiff Tests/test_file_libtiff.py::TestFileLibTiff::test_gray_semibyte_per_pixel Tests/test_file_libtiff.py::TestFileLibTiff::test_int_dpi Tests/test_file_libtiff.py::TestFileLibTiff::test_little_endian Tests/test_file_libtiff.py::TestFileLibTiff::test_lzw Tests/test_file_libtiff.py::TestFileLibTiff::test_multipage Tests/test_file_libtiff.py::TestFileLibTiff::test_multipage_compression Tests/test_file_libtiff.py::TestFileLibTiff::test_multipage_nframes Tests/test_file_libtiff.py::TestFileLibTiff::test_old_style_jpeg Tests/test_file_libtiff.py::TestFileLibTiff::test_page_number_x_0 Tests/test_file_libtiff.py::TestFileLibTiff::test_quality Tests/test_file_libtiff.py::TestFileLibTiff::test_read_icc Tests/test_file_libtiff.py::TestFileLibTiff::test_sampleformat Tests/test_file_libtiff.py::TestFileLibTiff::test_save_bytesio Tests/test_file_libtiff.py::TestFileLibTiff::test_save_tiff_with_jpegtables Tests/test_file_libtiff.py::TestFileLibTiff::test_strip_cmyk_16l_jpeg Tests/test_file_libtiff.py::TestFileLibTiff::test_strip_cmyk_jpeg Tests/test_file_libtiff.py::TestFileLibTiff::test_strip_ycbcr_jpeg_1x1_sampling Tests/test_file_libtiff.py::TestFileLibTiff::test_strip_ycbcr_jpeg_2x2_sampling Tests/test_file_libtiff.py::TestFileLibTiff::test_tiled_cmyk_jpeg Tests/test_file_libtiff.py::TestFileLibTiff::test_tiled_ycbcr_jpeg_1x1_sampling Tests/test_file_libtiff.py::TestFileLibTiff::test_tiled_ycbcr_jpeg_2x2_sampling Tests/test_file_libtiff.py::TestFileLibTiff::test_write_metadata
src/PIL/TiffImagePlugin.py
src/PIL/TiffImagePlugin.py