It still fails under AMD64 OpenIndiana:

======================================================================
ERROR: test_offset_overflow (test.test_os.TestSendfile)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/export/home/buildbot/64bits/3.x.cea-indiana-amd64/build/Lib/test/test_os.py", line 1473, in test_offset_overflow
    sent = os.sendfile(self.sockno, self.fileno, offset, 4096)
OSError: [Errno 22] Invalid argument

======================================================================
ERROR: test_send_at_certain_offset (test.test_os.TestSendfile)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/export/home/buildbot/64bits/3.x.cea-indiana-amd64/build/Lib/test/test_os.py", line 1456, in test_send_at_certain_offset
    sent = self.sendfile_wrapper(self.sockno, self.fileno, offset, nbytes)
  File "/export/home/buildbot/64bits/3.x.cea-indiana-amd64/build/Lib/test/test_os.py", line 1419, in sendfile_wrapper
    return os.sendfile(sock, file, offset, nbytes)
OSError: [Errno 22] Invalid argument

======================================================================
ERROR: test_send_whole_file (test.test_os.TestSendfile)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/export/home/buildbot/64bits/3.x.cea-indiana-amd64/build/Lib/test/test_os.py", line 1436, in test_send_whole_file
    sent = self.sendfile_wrapper(self.sockno, self.fileno, offset, nbytes)
  File "/export/home/buildbot/64bits/3.x.cea-indiana-amd64/build/Lib/test/test_os.py", line 1419, in sendfile_wrapper
    return os.sendfile(sock, file, offset, nbytes)
OSError: [Errno 22] Invalid argument


According to the man page, EINVAL means:

     EINVAL          The offset  cannot  be  represented  by  the
                     off_t  structure,  or the length is negative
                     when cast to ssize_t.