On Mon, 5 Dec 2022 19:14:24 +1100 David Gibson <david(a)gibson.dropbear.id.au> wrote:Currently we have special sockets for receiving datagrams from locahost which can use the optimized "splice" path rather than going across the tap interface. We want to loosen this so that sockets can receive sockets that will be forwarded by both the spliced and non-spliced paths. To do this, we alter the meaning of the @splice bit in the reference to mean that packets receieved on this socket *can* be spliced, not that they *will* be spliced. They'll only actually be spliced if they come from 127.0.0.1 or ::1. We can't (for now) remove the splice bit entirely, unlike with TCP. Our gateway mapping means that if the ns initiates communication to the gw address, we'll translate that to target 127.0.0.1 on the host side. Reply packets will therefore have source address 127.0.0.1 when received on the host, but these need to go via the tap path where that will be translated back to the gateway address. We need the @splice bit to distinguish that case from packets going from localhost to a port mapped explicitly with -u which should be spliced. Signed-off-by: David Gibson <david(a)gibson.dropbear.id.au> --- udp.c | 54 +++++++++++++++++++++++++++++++++++------------------- udp.h | 2 +- 2 files changed, 36 insertions(+), 20 deletions(-) diff --git a/udp.c b/udp.c index 6ccfe8c..011a157 100644 --- a/udp.c +++ b/udp.c @@ -513,16 +513,27 @@ static int udp_splice_new_ns(void *arg) } /** - * sa_port() - Determine port from a sockaddr_in or sockaddr_in6 + * udp_mmh_splice_port() - Is source address of message suitable for splicing? * @v6: Is @sa a sockaddr_in6 (otherwise sockaddr_in)? - * @sa: Pointer to either sockaddr_in or sockaddr_in6 + * @mmh: mmsghdr of incoming message + * + * Return: if @sa refers to localhost (127.0.0.1 or ::1) the port from + * @sa, otherwise 0. + * + * NOTE: this relies on the fact that it's not valid to use UDP port 0The port is reserved by IANA indeed, but... it can actually be used. On Linux, you can bind() it and you can connect() to it. As far as I can tell from the new version of udp_sock_handler() we would actually misdirect packets in that case. How bad would it be to use an int here? By the way, I think the comment should also mention that the port is returned in host order. -- Stefano