Re: [PATCH] tcp_splice, flow: Add socket to epoll set before connect(), drop assert

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

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On Tue, Dec 09, 2025 at 12:53:33AM +0100, Stefano Brivio wrote:
> ...otherwise, if we have a real error on connect() (that is, not
> EINPROGRESS), we'll return early from tcp_splice_connect() and later
> try to fetch the epoll file descriptor:
> 
>   ASSERTION FAILED in flow_epollfd (flow.c:362): f->epollid < ((1 << 8) - 1)
> 
> which is still (correctly) EPOLLFD_ID_INVALID.
> 
> Replace the ASSERT() in flow_epollfd() with a warning, as it looks
> like there might be harmless cases where the socket is not in the
> epoll set yet, and we'll just crash for nothing. We can turn this back
> to an ASSERT() once we audit these paths in more detail.
> 
> Link: https://bodhi.fedoraproject.org/updates/FEDORA-2025-93b4eb64c3#comment-4473411
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
> I might merge this in a bit even without review as we might now have
> broken distribution packages around.
> 
>  flow.c       | 7 ++++++-
>  tcp_splice.c | 4 ++--
>  2 files changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/flow.c b/flow.c
> index 8d72965..4f53486 100644
> --- a/flow.c
> +++ b/flow.c
> @@ -359,7 +359,12 @@ bool flow_in_epoll(const struct flow_common *f)
>   */
>  int flow_epollfd(const struct flow_common *f)
>  {
> -	ASSERT(f->epollid < EPOLLFD_ID_MAX);
> +	if (f->epollid >= EPOLLFD_ID_MAX) {
> +		flow_log_(f, true, LOG_WARNING,
> +			  "Invalid epollid %i for flow, assuming default",
> +			  f->epollid);
> +		return epoll_id_to_fd[EPOLLFD_ID_DEFAULT];
> +	}

This LGTM for safety's sake, although it's conceptually ugly.

>  
>  	return epoll_id_to_fd[f->epollid];
>  }
> diff --git a/tcp_splice.c b/tcp_splice.c
> index 717766a..4405224 100644
> --- a/tcp_splice.c
> +++ b/tcp_splice.c
> @@ -381,14 +381,14 @@ static int tcp_splice_connect(const struct ctx *c, struct tcp_splice_conn *conn)
>  
>  	pif_sockaddr(c, &sa, tgtpif, &tgt->eaddr, tgt->eport);
>  
> +	conn_event(c, conn, SPLICE_CONNECT);
> +
>  	if (connect(conn->s[1], &sa.sa, socklen_inany(&sa))) {
>  		if (errno != EINPROGRESS) {
>  			flow_trace(conn, "Couldn't connect socket for splice: %s",
>  				   strerror_(errno));
>  			return -errno;
>  		}
> -
> -		conn_event(c, conn, SPLICE_CONNECT);

I don't really understand the rationale for this.

>  	} else {
>  		conn_event(c, conn, SPLICE_ESTABLISHED);
>  		return tcp_splice_connect_finish(c, conn);

I think the true fix for this specific failure on the connect-error
path is to check flow_in_epoll() before calling flow_epollfd() /
epoll_del() in the CLOSING path of conn_flag_do().

-- 
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

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