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Sun Aug 21 23:45:01 EDT 2016 · https://mail.python.org/pipermail/python-dev/2016-August/145875.html

On 21 August 2016 at 12:37, Christian Heimes <christian at python.org> wrote:
> the socket.setsockopt(level, optname, value) method has two calling
> variants. When it is called with a buffer-like object as value, it calls
> the C API function setsockopt() with optval=buffer.buf and
> optlen=buffer.len. When value is an integer, setsockopt() packs it as
> int32 and sends it with optlen=4.
>
> ---
> # example.py
> import socket
> sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
> sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR,
>     b'\x00\x00\x00\x00')
> sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
> ---
>
> $ strace -e setsockopt ./python example.py
> setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [0], 4) = 0
> setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
>
>
> For AF_ALG (Linux Kernel crypto) I need a way to call the C API function
> setsockopt() with optval=NULL and optlen as any arbitrary number. I have
> been playing with multiple ideas. So far I liked the idea of
> value=(None, int) most.
>
> setsockopt(socket.SOL_ALG, socket.ALG_SET_AEAD_AUTHSIZE, (None, taglen))

Would this new functionality be open-ended? What would happen if you did

sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, (None, 4))

On Linux, this seems to fail with errno = EFAULT, which would be fine.
But this does not seem to be specified by Posix, and I imagine other
platforms might crash. On the other hand, these sort of holes are
already present in the socket module. E.g. with MSG_TRUNC
<https://bugs.python.org/issue24933> you can apparently get Python to
copy from unallocated memory.

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[Python-Dev] socket.setsockopt() with optval=NULL