Matt Caswell

As a separate issue, this fix needs changing because the ktls code in master has been moved around and refactored a lot.

> Can the CI be triggered for my pull request? Done

**OTC Question: Should zero-copy sendfile be the default on Linux? This is already the default on FreeBSD but isn't currently on Linux - so there is an inconsistency. Changing this...

**OTC: We should not change the default for Linux in 3.x. We might reconsider for 4.0. This PR would be ok if it was an opt-in runtime configuration.**

> Please advise whether my adaptation in ssl/record/methods/ktls_meth.c is valid. Looks fine aside from my minor comments above.

Ping @openssl/committers for second review

> Anything left to do from my end? No. Thanks for the ping. I'll merge shortly.

Squashed and merged to maser and 3.0. I resolved a trivial conflict when backporting to 3.0.

Thanks for the reproducer. I can replicate this in master too.

The problem is that we are issuing a TLSv1.2 format ticket in this case. In TLSv1.2 if the callback returns 0, then we just issue a 0 length ticket. However,...