The iov_setup() helper in posixmodule.c returns the total size of all buffers. But there is possible an integer overflow because the sequence of buffers can contain the same buffer repeated multiple times.

On 32-bit platform:

>>> import os
>>> f = open('/tmp/temp', 'wb')
>>> os.writev(f.fileno(), [b'x' * 2**16] * 2**15)
-1

Since the overflowed sum is negative, os_writev_impl() returns -1 as a signal of error, but since the exception is not set, -1 is returned as the result of os.writev(). If the overflowed sum is not negative, the sequence of buffers is passed to OS and an OSError is raised:

>>> os.writev(f.fileno(), [b'x' * 2**16] * 2**16)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 22] Invalid argument

I have not tested (because have not installed corresponding 32-bit OSes, and it is harder to reproduce on 64-bit), but seems this can even cause a crash in os.sendfile() on FreeBSD, DragonFly BSD and Mac OS.

This sum is used only in os.sendfile() on Mac OS. In all other cases it is enough to return just an error flag. I can't find the documentation for os.sendfile() on Mac OS for checking if this value actually is needed.

I can reproduce the issue on a i686 GNU/Linux  Debian system:

>>> import os
>>> f = open('/tmp/temp', 'wb')
>>> l = os.writev(f.fileno(), [b'x' * 2**16] * 2**15)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
SystemError: error return without exception set

Docs for sendfile in macOS:

https://www.unix.com/man-page/osx/2/sendfile/

Test case:

import os
fo = open('/tmp/temp', 'wb')
fi = open('/tmp/temp', 'rb')
os.sendfile(fo.fileno(), fi.fileno(), 0, 0, headers=[b'x' * 2**16] * 2**15)

--
run against current master HEAD (2f9cbaa8b2190b6dfd3157ede9b6973523a3b939, as of 2018-06-15)
--with-pydebug
current macOS 10.13.5


64-bit Python

$ ./bin/python3.8 ~/Desktop/test_s.py
Traceback (most recent call last):
 File "/Users/nad/Desktop/test_s.py", line 4, in <module>
   os.sendfile(fo.fileno(), fi.fileno(), 0, 0, headers=[b'x' * 2**16] * 2**15)
OSError: [Errno 38] Socket operation on non-socket
sys:1: ResourceWarning: unclosed file <_io.BufferedWriter name='/tmp/temp'>
sys:1: ResourceWarning: unclosed file <_io.BufferedReader name='/tmp/temp'>


32-bit Python

$ ./bin/python3.8-32 ~/Desktop/test_s.py
Fatal Python error: a function returned NULL without setting an error
SystemError: <built-in function sendfile> returned NULL without setting an error

Current thread 0xa983a1c0 (most recent call first):
 File "/Users/nad/Desktop/test_s.py", line 4 in <module>
Abort trap: 6

Thank you Pablo and Ned!

Seems there is other bug on Mac OS, not related to integer overflow. I am working on it.

New changeset 9d5727326af53ddd91016d98e16ae7cf829caa95 by Serhiy Storchaka in branch 'master':
bpo-33871: Fix os.sendfile(), os.writev(), os.readv(), etc. (GH-7931)
https://github.com/python/cpython/commit/9d5727326af53ddd91016d98e16ae7cf829caa95

New changeset 3e4b68875917a4605b45918f9e3232730fed9399 by Miss Islington (bot) in branch '3.7':
bpo-33871: Fix os.sendfile(), os.writev(), os.readv(), etc. (GH-7931)
https://github.com/python/cpython/commit/3e4b68875917a4605b45918f9e3232730fed9399

New changeset ada5d99306dc8af21c32cefb3d86891e8553dbc6 by Serhiy Storchaka in branch '3.6':
[3.6] bpo-33871: Fix os.sendfile(), os.writev(), os.readv(), etc. (GH-7931) (GH-8584)
https://github.com/python/cpython/commit/ada5d99306dc8af21c32cefb3d86891e8553dbc6