[Windows] ntpath.realpath() of bytes root directory may raise TypeError in some cases

[9001]

BPO	43847
Nosy	@pfmoore, @tjguk, @zware, @eryksun, @zooba

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-04-14.12:51:02.772>
labels = ['easy', 'type-bug', '3.8', '3.9', '3.10', 'library', 'OS-windows']
title = '[Windows] ntpath.realpath() of bytes root directory may raise TypeError in some cases'
updated_at = <Date 2021-04-28.15:53:40.393>
user = 'https://github.com/9001'

bugs.python.org fields:

activity = <Date 2021-04-28.15:53:40.393>
actor = 'steve.dower'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'Windows']
creation = <Date 2021-04-14.12:51:02.772>
creator = '9001'
dependencies = []
files = []
hgrepos = []
issue_num = 43847
keywords = ['easy']
message_count = 2.0
messages = ['391074', '391119']
nosy_count = 6.0
nosy_names = ['paul.moore', 'tim.golden', "Antoine d'Otreppe", 'zach.ware', 'eryksun', 'steve.dower']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43847'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

Linked PRs

• gh-88013: ntpath.realpath may raise TypeError in some cases #102813
• [3.11] GH-88013: Fix TypeError raised by ntpath.realpath in some cases (GH-102813) #103343

[9001]

some win7sp1 and win10:20H2 boxes cannot realpath a networked drive letter such as b"n:" (also affects b"n:\\")

• observed with 3.8.7 and 3.9.1
• 3.7.9 is fine

requirements to trigger:

• bytestring (not unicode str)
• just the drive letter (subfolders are ok)
• networked drive (regular disks and vmhgfs are ok)
• enterprise/AD network? (doesn't seem to happen with samba)

hits the following exceptions in succession:

• access denied at L601: "path = _getfinalpathname(path)"
• "cant concat str to bytes" at L621: "return path + tail"

[eryksun]

In ntpath._getfinalpathname_nonstrict(), tail should be initialized to path[:0].

Currently tail is initialized to the empty string value ''. If an error occurs that's allowed and path is a root directory that's passed as bytes, then joining path + tail will fail if tail still has its initial value.

To reproduce this issue, create a substitute drive for a directory that grants no access. For example:

    import os
    os.mkdir('spam')
    os.system('icacls spam /inheritance:r')
    os.system('subst N: spam')

    >>> os.path.realpath(b'N:/')
    Traceback (most recent call last):
      File "C:\Program Files\Python39\lib\ntpath.py", line 647, in realpath
        path = _getfinalpathname(path)
    PermissionError: [WinError 5] Access is denied: b'N:\\'

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "C:\Program Files\Python39\lib\ntpath.py", line 601, in _getfinalpathname_nonstrict
        path = _getfinalpathname(path)
    PermissionError: [WinError 5] Access is denied: b'N:\\'

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "C:\Program Files\Python39\lib\ntpath.py", line 651, in realpath
        path = _getfinalpathname_nonstrict(path)
      File "C:\Program Files\Python39\lib\ntpath.py", line 621, in _getfinalpathname_nonstrict
        return path + tail
    TypeError: can't concat str to bytes