_posixsubprocess.fork_exec() takes a sequence of file descriptors. It first validates it, and since the validation is passed uses it without checking for errors. But since __len__, __getitem__ and __int__ can execute user code and release GIL, errors can occur after the validation. This can cause a crash.

Proposed patch fixes this by the simplest way -- it restricts the type of a sequence to tuple and types of elements to int. Since _posixsubprocess is private module this shouldn't break third-party code.

Other issue with _posixsubprocess.fork_exec() was that it converts args to a tuple or a list and iterate it without checking if the size is changed.