Re: [PATCH 5/6] tcp_splice: Simplify EPOLLRDHUP / eof / FIN handling

[David Gibson <david@gibson.dropbear.id.au>]

[-- Attachment #1: Type: text/plain, Size: 4410 bytes --]

On Wed, May 20, 2026 at 10:30:04PM +0200, Stefano Brivio wrote:
> On Wed, 20 May 2026 23:08:50 +1000
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > There are two ways we can tell one of our sockets has received a FIN.  We
> > can either see an EPOLLRDHUP epoll event, or we can get a zero-length read
> > (EOF) on the socket.  We currently use both, in a mildly confusing way:
> > we only set the FIN_RCVD() flag based on the EPOLLRDHUP event, but then
> > some other close out logic is based on seeing an EOF.
> > 
> > Simplify this by setting the flag based on only the EOF.  To make sure we
> > don't miss an event if we get an EPOLLRDHUP with no data, we trigger the
> > forwarding path for EPOLLRDHUP as well as EPOLLIN.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  tcp_splice.c | 14 +++++---------
> >  1 file changed, 5 insertions(+), 9 deletions(-)
> > 
> > diff --git a/tcp_splice.c b/tcp_splice.c
> > index 8fbd490f..b45f0060 100644
> > --- a/tcp_splice.c
> > +++ b/tcp_splice.c
> > @@ -487,7 +487,6 @@ static int tcp_splice_forward(struct ctx *c, struct
> >  	uint8_t lowat_set_flag = RCVLOWAT_SET(fromsidei);
> >  	uint8_t lowat_act_flag = RCVLOWAT_ACT(fromsidei);
> >  	int never_read = 1;
> > -	int eof = 0;
> >  
> >  	while (1) {
> >  		ssize_t readlen, written;
> > @@ -510,7 +509,7 @@ retry:
> >  		flow_trace(conn, "%zi from read-side call", readlen);
> >  
> >  		if (!readlen) {
> > -			eof = 1;
> > +			conn_event(conn, FIN_RCVD(fromsidei));
> 
> I'm not sure if I really found a concrete issue with this, but it looks
> a bit scary, because it changes the semantics of FIN_RCVD, which used to
> mean that we infer we received a FIN, regardless of whether we're done
> processing all data from that half of the connection.
> 
> Now FIN_RCVD is only set if we actually processed all the data and we
> hit the end of file.

True.  But the only place that tested FIN_RCVD was at the end of
tcp_splice_forward(), conditional on 'eof' anyway.  In a sense, this
was the cause of bug202 - we had FIN_RCVD set, but we didn't process
it and shutdown() on the other side, because we didn't have eof.

> The (potential) issue I see here is that we get EPOLLRDHUP, splice()
> returns -1 with EAGAIN in errno because we had no room in the pipe,
> and it would have returned 0 instead.
> 
> Will we ever get our zero-sized "read" later? If not, we might have
> missed EPOLLRDHUP *and* the end of file. I'm not entirely sure we have
> guarantees in that sense from splice().

It's not really about guarantees from splice.  I'm pretty sure this is
ok, reasoning as follows.

Consider all the exit points from the loop body:
 - Each return is a return -1, so we kill the connection anyway.  They
   don't matter
 - Each continue, goto retry and the end of the body will do the read
   side splice() again, so get another chance to see the EOF
 - That leaves just the breaks

Consider each break (there are three, since patch 2 of this series)
		if (written < 0) {
			if (!conn->pending[fromsidei])
				break;

(1) The pipe is empty and the write-splice returned EAGAIN, so it
didn't remove data from the pipe.  Therefore, the pipe must have been
empty before the write-splice.  Which means the read-splice can't have
blocked on a full pipe.
			conn_event(conn, OUT_WAIT(!fromsidei));
			break;
		}

(2) The pipe is non-empty and the write-splice returned EAGAIN, so it
must have blocked on the output socket.  We've set OUT_WAIT(), so
we'll get an EPOLLOUT at some point which will cause us to read-splice
again, meaning we get another chance to see the EOF.

		[...]
		if (conn->events & FIN_RCVD(fromsidei))
			break;
(3) By the new semantics of FIN_RCVD, we *have* seen the EOF.

> The existing implementation distinguishes between end-of-file we hit in
> a given iteration, and EPOLLRDHUP we might have seen at any time.
> That was actually intended.

It might be intended, but I can't see that we did anything with that
information.

That said the conditions on which we exit / retry this loop are pretty
darn confusing.  I'll see if I can improve them.

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]