`posixpath.realpath('secretlink')` raises

[nineteendo]

Bug report

Bug description:

GNU coreutils realpath -m doesn't raise an error for secret symlinks (no read permission):

wannes@Stefans-iMac dirs % sudo ls -l secret-symlink
l---------  1 wannes  staff  44 Jun 30  2023 secret-symlink -> /Users/wannes/path-picker/link-test/dirs/dir
wannes@Stefans-iMac dirs % grealpath -m secret-symlink
/Users/wannes/path-picker/link-test/dirs/secret-symlink

But posixpath.realpath() does:

>>> import posixpath
>>> posixpath.realpath("secret-symlink")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<frozen posixpath>", line 435, in realpath
  File "<frozen posixpath>", line 495, in _joinrealpath
PermissionError: [Errno 13] Permission denied: 'secret-symlink'

CPython versions tested on:

3.12

Operating systems tested on:

macOS

Linked PRs

• GH-118447: Fix handling of unreadable symlinks in os.path.realpath() #118489
• [3.13] GH-118447: Fix handling of unreadable symlinks in os.path.realpath() (GH-118489) #119163
• GH-118447: Fix FreeBSD test failures. #119170
• [3.13] GH-118447: Fix FreeBSD test failures. (GH-119170) #119181

[eryksun]

Setting permissions on a symlink is a BSD (including macOS) feature. Linux doesn't allow it. Testing this should be limited to platforms with os.chmod in os.supports_follow_symlinks.

[nineteendo]

Upon closer inspection, this is a bug in coreutils:

wannes@Stefans-iMac dirs % sudo ls -l secret-recursive-symlink
l---------  1 wannes  staff  40 Jun 30  2023 secret-recursive-symlink -> /Users/wannes/path-picker/link-test/dirs
wannes@Stefans-iMac dirs % sudo realpath secret-recursive-symlink/..
/Users/wannes/path-picker/link-test
wannes@Stefans-iMac dirs % grealpath -m secret-recursive-symlink/..
/Users/wannes/path-picker/link-test/dirs

[barneygale]

I wouldn't expect os.path.realpath(..., strict=False) to raise OSError, no matter what coreutils does!

[nineteendo]

That's probably a reason which this is not part of POSIX... I expected you needed permission to follow it, not to determine the real location.

Return the canonical path of the specified filename, eliminating any symbolic links encountered in the path.

We can't eliminate this symlink, because we don't know where it points to, which we need to know for determining the parent directory. Also, coreutils doesn't even raise an error in strict mode:

wannes@Stefans-iMac ~ % ln -s . src        
wannes@Stefans-iMac ~ % grealpath -e src/..
/Users
wannes@Stefans-iMac ~ % chmod -h 000 src
wannes@Stefans-iMac ~ % grealpath -e src/..
/Users/wannes

Raising an error here will lead to the fewest bugs.

[eryksun]

On macOS and NetBSD, fcntl() supports F_GETPATH. In that case, you could try to open "secret-symlink" and query the resolved path, e.g. target = os.fsdecode(fcntl.fcntl(fd, fcntl.F_GETPATH, bytes(1024))). You may have to revisit the design of fcntl.fcntl() to make it use an internal buffer bigger than 1024 bytes, though I think that's currently the maximum path length supported by macOS.

[nineteendo]

That works:

>>> import fcntl
>>> import os
>>> os.symlink('.', 'src')
>>> os.chmod('src', 0o000, follow_symlinks=False)
>>> os.readlink('src')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
PermissionError: [Errno 13] Permission denied: 'src'
>>> fd = os.open('src', os.O_RDONLY)
>>> os.fsdecode(fcntl.fcntl(fd, fcntl.F_GETPATH, bytes(1024))).rstrip('\x00')
'/Users/wannes'

But I think it might be cleaner to fix this in readlink...

[nineteendo]

But it doesn't work for broken symlinks:

>>> import fcntl
>>> import os
>>> open('tmp', 'w', encoding='utf-8')
<_io.TextIOWrapper name='tmp' mode='w' encoding='utf-8'>
>>> os.symlink('tmp', 'dst')
>>> os.unlink('tmp')
>>> os.chmod('dst', 0o000, follow_symlinks=False)
>>> os.readlink('dst')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
PermissionError: [Errno 13] Permission denied: 'dst'
>>> fd = os.open('dst', os.O_RDONLY)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
FileNotFoundError: [Errno 2] No such file or directory: 'dst'