Directory enumeration/search permissions mismatch Unix behavior

[vector-of-bool]

Describe the bug
Certain "pathological" directory modes do not match the native POSIX behaviors. A directory that has S_IRUSR can be enumerated, but not necessarily searched (i.e. one can list the files within it, but cannot access those files). A directory that has S_IXUSR can be searched, but not necessarily enumerated (i.e. one cannot list the files of a directory, but can read those files as long as the entry names are known).

How To Reproduce

Test cases

import os
from pathlib import Path
import pytest


def _enumerate_fails():
    # Prep
    d = Path("testdir")
    d.mkdir(exist_ok=True)
    d.chmod(0o777)
    # Set weird mode:
    d.chmod(0b011_101_101)  # -wxr-xr-x
    # We can add the file, even if the directory is not enumerable
    file = d / "file.txt"
    file.write_text("hey")
    # We can search the directory and read files within it:
    assert file.read_text() == "hey"
    # We cannot enumerate the directory
    with pytest.raises(PermissionError):
        list(d.iterdir())


def _search_fails():
    # Prep
    d = Path("testdir")
    d.mkdir(exist_ok=True)
    d.chmod(0o777)
    file = d / "file.txt"
    file.write_text("hey")
    # Set weird mode:
    d.chmod(0b110_101_101)  # rw-r-xr-x
    # We cannot create any files in the directory, because that requires
    # searching it
    another_file = Path("file.txt")
    another_file.write_text("hey")
    with pytest.raises(PermissionError):
        os.link(another_file, d / "link.txt")
    # We can enumerate the directory:
    assert os.listdir(d) == ["file.txt"]
    # We cannot read files inside of the directory (even if we have read access to the file)
    with pytest.raises(PermissionError):
        file.stat()
    with pytest.raises(PermissionError):
        file.read_text()


def test_enumerate_realfs():
    _enumerate_fails()


def test_enumerate_fakefs(fs):
    _enumerate_fails()


def test_search_fails():
    _search_fails()


def test_search_fakefs(fs):
    _search_fails()

Your environment
Please run the following in the environment where the problem happened and
paste the output.

$ python -c "import platform; print(platform.platform())"
Linux-6.7.6-arch1-1-x86_64-with-glibc2.39
$ python -c "import sys; print('Python', sys.version)"
Python 3.11.7 (main, Jan 29 2024, 16:03:57) [GCC 13.2.1 20230801]
$ python -c "from pyfakefs import __version__; print('pyfakefs', __version__)"
pyfakefs 5.3.5
$ python -c "import pytest; print('pytest', pytest.__version__)"
pytest 7.2.0

---

For directory searching, it looks to be that get_object_from_normpath is responsible for permission checking at each step in path resolution. It incorrectly checks the read bit of the directory objects in _can_read, but it should be checking the execute bit:

@staticmethod
def _can_read(target, owner_can_read):
    if target.st_uid == helpers.get_uid():
        if owner_can_read or target.st_mode & 0o400:
            return True
    if target.st_gid == get_gid():
        if target.st_mode & 0o040:
            return True
    return target.st_mode & 0o004

Additionally, at least to the best of my knowledge, the permission fall-through behavior is not part of POSIX i.e. if get_uid() == st_uid, then only the S_IXUSR bit should be considered, and not the group/other bits.

As for directory enumeration, I believe it would require a separate check since it depends on a different set of mode bits.

[mrbean-bremen]

It incorrectly checks the read bit of the directory objects in _can_read, but it should be checking the execute bit

Right, that seems to be incorrect.

the permission fall-through behavior is not part of POSIX i.e. if get_uid() == st_uid, then only the S_IXUSR bit should be considered, and not the group/other bits

That's also true, I just checked this for the other issue where it also holds.

I actually forgot that there is already a check for uid and gid, so this looks just like a bug fix instead of a refactoring (which I first thought was needed).

[mrbean-bremen]

@vector-of-bool - this (and the other issue) shall be fixed now in main. Please try it out, and check if you see any other permission-related problems. I'll wait a bit if anything else comes up before making a new release.

[vector-of-bool]

Thanks for the quick turnaround! I installed and ran tests on my system and it looks to be behaving properly.

It looks like it may have unveiled issues related to the automatic creation of a /tmp directory, but will require more investigating.