fileno argument to socket.socket is not validated

[dimaqq]

BPO	35415
Nosy	@dimaqq, @miss-islington
PRs	bpo-35415: validate fileno argument to socket.socket #10917

^{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 = <Date 2018-12-17.13:08:34.186>
created_at = <Date 2018-12-05.08:41:12.281>
labels = ['3.8']
title = 'fileno argument to socket.socket is not validated'
updated_at = <Date 2018-12-17.13:08:34.186>
user = 'https://github.com/dimaqq'

bugs.python.org fields:

activity = <Date 2018-12-17.13:08:34.186>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2018-12-17.13:08:34.186>
closer = 'asvetlov'
components = []
creation = <Date 2018-12-05.08:41:12.281>
creator = 'Dima.Tisnek'
dependencies = []
files = []
hgrepos = []
issue_num = 35415
keywords = ['patch']
message_count = 2.0
messages = ['331096', '332001']
nosy_count = 2.0
nosy_names = ['Dima.Tisnek', 'miss-islington']
pr_nums = ['10917']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue35415'
versions = ['Python 3.8']

[dimaqq]

socket.socket gained a fileno= kwarg the value of which is not checked if address family and socket type are both provided.

For example, following is accepted:

>> socket.socket(socket.AF_INET, socket.SOCK_STREAM, fileno=-1234)
>> socket.socket(socket.AF_INET, socket.SOCK_STREAM, fileno=1234)
>> socket.socket(socket.AF_INET, socket.SOCK_STREAM, fileno=0.999)

Resulting in a socket object that will fail at runtime.

One of the implications is that it's possible to "steal" file descriptor, i.e. create a socket for an fd that doesn't exist; then some other function/thread happens to create e.g. socket with this specific fd, which can be "unexpectedly" used (or closed or modified, e.g. non-blocking changed) through the first socket object.

Additionally if the shorthand is used, the exception raised in these cases has odd text, at least it was misleading for me.

>>> socket.socket(fileno=get_wrong_fd_from_somewhere())
[snip]
OSError: [Errno 9] Bad file descriptor: 'family'

I thought that I had a bug whereby a string was passed in instead of an int fd;
Ultimately I had to look in cpython source code to understand what the "family" meant.

I volunteer to submit a patch!

[miss-islington]

New changeset e991270 by Miss Islington (bot) (Dima Tisnek) in branch 'master':
bpo-35415: validate fileno argument to socket.socket (GH-10917)
e991270