Re: [PATCH v3 12/15] icmp: Populate and use host side flow information

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

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On Wed, Jan 17, 2024 at 08:59:41PM +0100, Stefano Brivio wrote:
> On Thu, 21 Dec 2023 18:02:34 +1100
> David Gibson <david@gibson.dropbear.id.au> wrote:
> 
> > Complete filling out the common flow information for "ping" flows by
> > storing the host side information for the ping socket.  We can only
> > retrieve this from the socket after we send the echo-request, because
> > that's when the kernel will assign an ID.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > ---
> >  icmp.c | 36 +++++++++++++++++++++++++++++++++---
> >  1 file changed, 33 insertions(+), 3 deletions(-)
> > 
> > diff --git a/icmp.c b/icmp.c
> > index 53ad087..917c810 100644
> > --- a/icmp.c
> > +++ b/icmp.c
> > @@ -310,11 +310,41 @@ int icmp_tap_handler(const struct ctx *c, uint8_t pif, int af,
> >  	if (sendto(pingf->sock, pkt, plen, MSG_NOSIGNAL, &sa.sa, sl) < 0) {
> >  		debug("%s: failed to relay request to socket: %s",
> >  		      pname, strerror(errno));
> > -	} else {
> > -		debug("%s: echo request to socket, ID: %"PRIu16", seq: %"PRIu16,
> > -		      pname, id, seq);
> > +		if (flow)
> > +			goto cancel;
> > +	}
> > +
> > +	debug("%s: echo request to socket, ID: %"PRIu16", seq: %"PRIu16,
> > +	      pname, id, seq);
> > +
> > +	if (!flow)
> > +		/* Nothing more to do for an existing flow */
> > +		return 1;
> > +
> > +	/* We need to wait until after the sendto() to fill in the SOCKSIDE
> > +	 * information, so that we can find out the host side id the kernel
> > +	 * assigned.  If there's no bind address specified, this will still have
> > +	 * 0.0.0.0 or :: as the host side forwarding address.  There's not
> > +	 * really anything we can do to fill that in, which means we can never
> > +	 * insert the SOCKSIDE of a ping flow into the hash table.
> > +	 */
> 
> Well, if it was just because of that we could argue at some point that,
> with wildcards or suchlike, we could insert that in the hash table as
> well.

If we allow wildcards, it's probably not a hash table any more, or at
least not a normal one.  Hashing is pretty inherently tied to a single
value lookup.

> But that wouldn't make sense anyway, right? That is, unless I'm missing
> something, we would have no use for a hash table lookup of the SOCKSIDE
> of a ping flow anyway.

That's correct, which is why we can get away with this.  Likewise
SOCKSIDE of tcp flows is also never hashed.  However in some cases we
may want to hash the host sides of flows: I think we'll want to for
UDP, for example, because we can receive multiple flows via the same
bound socket.  That's why I want to be very clear about when and why
we're constructing a flowside that can't be hashed.

-- 
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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