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From: "E. Madison Bray" <erik DOT m DOT bray AT gmail DOT com>
Date: Tue, 23 Apr 2019 14:28:21 +0200
Message-ID: <CAOTD34Yc=3023bXgKEpKkYehQPyiPdNn5SK9ZQR2rCCWqETU0w@mail.gmail.com>
Subject: Re: EPERM on bind() ?
To: cygwin AT cygwin DOT com
Content-Type: text/plain; charset="UTF-8"

On Tue, Apr 23, 2019 at 2:17 PM E. Madison Bray wrote:
>
> Hello,
>
> I have had some users of the Jupyter Notebook [1] on Cygwin report an
> crash on startup where, when the Notebook server tries to bind() to
> the port it will listen on (TCP 8888) the bind() fails and errno is
> set to EPERM, which is not an expected errno from bind().
>
> Looking at the Cygwin sources, in net.cc I see that in
> set_winsock_errno, EPERM is returned by default if there is some WSA
> error for which there is no POSIX equivalent mapped.  Fine--EPERM is
> as good as any other fallback I suppose (?) in that it unambiguously
> indicates some unknown WSA error.
>
> I'm just wondering if anyone has any idea what might cause such an
> error.  Some third-party firewall or BLODA?  I can't reproduce it
> myself.  Trying to bind to a port already in use correctly returns
> EADDRINUSE.
>
>
> [1] https://jupyter.org/

Answering my own question after comparing the list on
https://docs.microsoft.com/en-us/windows/desktop/winsock/windows-sockets-error-codes-2
to Cygwin's wsock_errmap table this is a likely culprit:

> WSAEACCES
> 10013
> Permission denied.An attempt was made to access a socket in a way forbidden by its access permissions.
> An example is using a broadcast address for sendto without broadcast permission being set using
> setsockopt(SO_BROADCAST).
> Another possible reason for the WSAEACCES error is that when the bind function is called (on Windows NT
> 4.0 with SP4 and later), another application, service, or kernel mode driver is bound to the same address
> with exclusive access. Such exclusive access is a new feature of Windows NT 4.0 with SP4 and later, and is
> implemented by using the SO_EXCLUSIVEADDRUSE option.

This appears to be missing from the wsock_errmap table, but should
obviously map to EACCES.  I'll supply a patch.

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