[openssl-users] Graceful shutdown of TLS connection for blocking sockets

Kyle Hamilton aerowolf at gmail.com
Mon Oct 9 00:58:56 UTC 2017

Do you have a reference to what should be done instead?

My understanding of what happens with blocking sockets is that
SSL_read() will return SSL_ERROR_WANT_READ if it needs additional data
read from a socket that doesn't have it available (and will return
SSL_ERROR_WANT_WRITE if it needs to write for a handful of reasons,
but can't).  I had thought that the appropriate response would be to
add that descriptor to the appropriate set to query on the next call
to select(), and then call the same function with the same parameters
so the library can advance its state machine.

write() and read() have the means to tell you how much data was
written or read, and that's what you're supposed to use to keep
blocking descriptors from hanging your application, I thought.

-Kyle H

On Sun, Oct 8, 2017 at 6:48 AM, Thomas J. Hruska
<shinelight at shininglightpro.com> wrote:
> On 10/8/2017 4:17 AM, Kyle Hamilton wrote:
>> The way to handle this situation is simply to never enter SSL_read() if
>> there isn't anything to read on the socket.  select() or pselect() are
>> your
>> friends, here, because they'll tell you if there's data to read from the
>> underlying file descriptor.
>> I hope this helps!
>> -Kyle H
> Since the OP is talking about blocking sockets, I'm going to reiterate
> something someone pointed out to me on this very list many years ago and
> save someone a LOT of headaches:
> select() should NEVER, EVER be used for blocking sockets.
> Just because select() returns any given descriptor doesn't mean that a call
> won't still block when working with blocking sockets.  select() is for
> non-blocking descriptors ONLY.  The amount of extra code involved for
> handling non-blocking sockets is actually quite minimal when a state engine
> is adopted.
> I'd love to see select() implementations raise an exception and kill the
> whole application off when passing it a blocking descriptor.  Then we would
> discover how much broken software is floating around out there. Since I
> still see lots of recommendations for using select() with blocking
> descriptors and all of the official system-level documentation for select()
> is silent on this issue, I'm guessing a lot.
> --
> Thomas Hruska
> Shining Light Productions
> Home of BMP2AVI and Win32 OpenSSL.
> http://www.slproweb.com/
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