Re: [PATCH v2 16/16] udp: Correct splice forwarding when receiving from multiple sources

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

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On Thu, Nov 24, 2022 at 12:16:59PM +1100, David Gibson wrote:
> udp_sock_handler_splice() reads a whole batch of datagrams at once with
> recvmmsg().  It then forwards them all via a single socket on the other
> side, based on the source port.
> 
> However, it's entirely possible that the datagrams in the set have
> different source ports, and thus ought to be forwarded via different
> sockets on the destination side.  In fact this situation arises with the
> iperf -P4 throughput tests in our own test suite.  AFAICT we only get away
> with this because iperf3 is strictly one way and doesn't send reply packets
> which would be misdirected because of the incorrect source ports.
> 
> Alter udp_sock_handler_splice() to split the packets it receives into
> batches with the same source address and send each batch with a separate
> sendmmsg().
> 
> For now we only look for already contiguous batches, which means that if
> there are multiple active flows interleaved this is likely to degenerate
> to batches of size 1.  For now this is the simplest way to correct the
> behaviour and we can try to optimize later.
> 
> Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

This has a nasty bug, do not apply.  I'll send a v3 shortly.


Specifically:

> ---
>  udp.c | 27 +++++++++++++++++++--------
>  1 file changed, 19 insertions(+), 8 deletions(-)
> 
> diff --git a/udp.c b/udp.c
> index 2311e7d..ee5c2c5 100644
> --- a/udp.c
> +++ b/udp.c
> @@ -572,9 +572,9 @@ static void udp_splice_sendfrom(const struct ctx *c, struct mmsghdr *mmh, int n,
>  static void udp_sock_handler_splice(const struct ctx *c, union epoll_ref ref,
>  				    uint32_t events, const struct timespec *now)
>  {
> -	in_port_t src, dst = ref.r.p.udp.udp.port;
> +	in_port_t dst = ref.r.p.udp.udp.port;
> +	int v6 = ref.r.p.udp.udp.v6, n, i, m;
>  	struct mmsghdr *mmh_recv, *mmh_send;
> -	int v6 = ref.r.p.udp.udp.v6, n, i;
>  
>  	if (!(events & EPOLLIN))
>  		return;
> @@ -610,12 +610,23 @@ static void udp_sock_handler_splice(const struct ctx *c, union epoll_ref ref,
>  		});
>  	}
>  
> -	for (i = 0; i < n; i++)
> -		mmh_send[i].msg_hdr.msg_iov->iov_len = mmh_recv[i].msg_len;
> -
> -	src = sa_port(v6, mmh_recv[0].msg_hdr.msg_name);
> -	udp_splice_sendfrom(c, mmh_send, n, src, ref.r.p.udp.udp.port,
> -			    v6, ref.r.p.udp.udp.ns, ref.r.p.udp.udp.orig, now);
> +	for (i = 0; i < n; i += m) {
> +		const struct mmsghdr *mmh = &mmh_recv[i];
> +		in_port_t src = sa_port(v6, mmh->msg_hdr.msg_name);
> +
> +		m = 0;
> +		do {
> +			mmh_send[i + m].msg_hdr.msg_iov->iov_len = mmh->msg_len;
> +			mmh++;
> +			m++;
> +		} while (sa_port(v6, mmh->msg_hdr.msg_name) == src);

I missed a bounds check here, so this inner loop can overrun the mmh array.

-- 
David Gibson			| 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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