RE: Bug in short splice to socket?

From: David Howells

Sent: 30 May 2023 23:27

Jakub Kicinski [off-list ref] wrote:

quoted

Will the TLS selftests under tools/.../net/tls.c exercise this?

Interesting.  Now that you've pointed me at it, I've tried running it.  Mostly
it passes, but I'm having some problems with the multi_chunk_sendfile tests
that time out.  I think that splice_direct_to_actor() has a bug.  The problem
is this bit of code:

		/*
		 * If more data is pending, set SPLICE_F_MORE
		 * If this is the last data and SPLICE_F_MORE was not set
		 * initially, clears it.
		 */
		if (read_len < len)
			sd->flags |= SPLICE_F_MORE;
		else if (!more)
			sd->flags &= ~SPLICE_F_MORE;

When used with sendfile(), it sets SPLICE_F_MORE (which causes MSG_MORE to be
passed to the network protocol) if we haven't yet read everything that the
user requested and clears it if we fulfilled what the user requested.

This has the weird effect that MSG_MORE gets kind of inverted.  It's never
seen by the actor if we can read the entire request into the pipe - except if
we hit the EOF first.  If we hit the EOF before we fulfil the entire request,
we get a short read and SPLICE_F_MORE and thus MSG_MORE *is* set.  The
upstream TLS code ignores it - but I'm changing this with my patches as
sendmsg() then uses it to mark the EOR.

Isn't MSG_MORE supposed to be just a hint that more data will follow.
So you'd expect a final send with MSG_MORE to get sent, but possibly
after a short timeout.

Using it as a record marker seems wrong.

I'm not sure how to clear 'Oh bugger I set MSG_MORE but have no data'
to avoid the timeout.
A zero length semdmsg() won't DTRT with protocols like SCTP.
(Does splice even do anything sensible with SCTP?)

	David

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