chmod -R: cannot access ...: Permission denied when gnuchmod gives Read-only file system

[danielzgtg]

My downstream report for rust-coreutils=0.8.0-0ubuntu3: https://bugs.launchpad.net/ubuntu/+source/rust-coreutils/+bug/2152057 . Also tested 055a66b

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At https://askubuntu.com/q/1566581/1004020 , with ReadonlyRoot enabled, I tried

# chmod -R ugo+rX /lib/python3/dist-packages/
chmod: cannot access '/lib/python3/dist-packages/Onboard/__pycache__/OnboardGtk.pypy311.pyc': Permission denied
# chmod -R ugo+rX /lib/python3/dist-packages/Onboard/__pycache__/OnboardGtk.pypy311.pyc
# # See there's no error

It's similar for other subdirectories. Whereas with GNU coreutils:

root@daniel-desktop3:/lib/python3/dist-packages# chmod -R ugo+rX /lib/python3/dist-packages/zstandard
chmod: cannot access '/lib/python3/dist-packages/zstandard/__pycache__/__init__.pypy311.pyc': Permission denied
root@daniel-desktop3:/lib/python3/dist-packages# gnuchmod -R ugo+rX /lib/python3/dist-packages/zstandard
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/py.typed': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/_cffi.cpython-314-x86_64-linux-gnu.so': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/backend_cffi.py': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/__init__.py': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/backend_c.cpython-314-x86_64-linux-gnu.so': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/__init__.pyi': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/__pycache__': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/__pycache__/backend_cffi.cpython-314.pyc': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/__pycache__/__init__.cpython-314.pyc': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/__pycache__/backend_cffi.pypy311.pyc': Read-only file system
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/__pycache__/__init__.pypy311.pyc': Read-only file system
root@daniel-desktop3:/lib/python3/dist-packages# gnuchmod -R ugo+rX /lib/python3/dist-packages/zstandard/py.typed
gnuchmod: changing permissions of '/lib/python3/dist-packages/zstandard/py.typed': Read-only file system

The error message should be "Read-only file system" not "Permission denied".