Re: [PATCH v3 04/11] fwd: Check all configured addresses in guest accessibility functions

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

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On Fri, Jan 30, 2026 at 04:44:40PM -0500, Jon Maloy wrote:
> As a preparation for handling multiple addresses, we update
> fwd_guest_accessible4() and fwd_guest_accessible6() to check
> against all addresses in the unified addrs[] array using the
> for_each_addr() macro.
> 
> This ensures that when multiple addresses are configured via -a options,
> inbound traffic for any of them is correctly detected as having no valid
> forwarding path, and subsequently dropped. This occurs when a peer
> address collides with an address the guest is using, and we have no
> translation for it.
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>
> 
> ---
> v2: Updated commit log to make it clearer
> v3: Adapted to changes earlier in the series
> ---
>  fwd.c | 25 +++++++++++++++++--------
>  1 file changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/fwd.c b/fwd.c
> index 54248a3..20d581d 100644
> --- a/fwd.c
> +++ b/fwd.c
> @@ -502,6 +502,8 @@ static bool is_dns_flow(uint8_t proto, const struct flowside *ini)
>  static bool fwd_guest_accessible4(const struct ctx *c,
>  				    const struct in_addr *addr)

With the changes to this point, there's no longer a point to having
separate fwd_guest_accessible4() and fwd_guest_accessible6()
functions.  fwd_guest_accessible() can check against the combined list
in a single pass.

This is an example of the simplfications that I think will outweigh
the complications introduced by using merged list for v4 and v6
addresses.

>  {
> +	const struct inany_addr_entry *e;
> +
>  	if (IN4_IS_ADDR_LOOPBACK(addr))
>  		return false;
>  
> @@ -513,12 +515,15 @@ static bool fwd_guest_accessible4(const struct ctx *c,
>  	if (IN4_IS_ADDR_UNSPECIFIED(addr))
>  		return false;
>  
> -	/* For IPv4, addr_seen is initialised to addr, so is always a valid
> -	 * address
> +	/* Check against all configured guest addresses */
> +	for_each_addr(c, e, AF_INET)
> +		if (IN4_ARE_ADDR_EQUAL(addr, inany_v4(&e->addr)))
> +			return false;
> +
> +	/* Also check addr_seen: it tracks the address the guest is actually
> +	 * using, which may differ from configured addresses.
>  	 */
> -	if ((first_v4(c) &&
> -	     IN4_ARE_ADDR_EQUAL(addr, inany_v4(&first_v4(c)->addr))) ||
> -	    IN4_ARE_ADDR_EQUAL(addr, &c->ip4.addr_seen))
> +	if (IN4_ARE_ADDR_EQUAL(addr, &c->ip4.addr_seen))
>  		return false;
>  
>  	return true;
> @@ -535,11 +540,15 @@ static bool fwd_guest_accessible4(const struct ctx *c,
>  static bool fwd_guest_accessible6(const struct ctx *c,
>  				  const struct in6_addr *addr)
>  {
> +	const struct inany_addr_entry *e;
> +
>  	if (IN6_IS_ADDR_LOOPBACK(addr))
>  		return false;
>  
> -	if (first_v6(c) && IN6_ARE_ADDR_EQUAL(addr, &first_v6(c)->addr.a6))
> -		return false;
> +	/* Check against all configured guest addresses */
> +	for_each_addr(c, e, AF_INET6)
> +		if (IN6_ARE_ADDR_EQUAL(addr, &e->addr.a6))
> +			return false;
>  
>  	/* For IPv6, addr_seen starts unspecified, because we don't know what LL
>  	 * address the guest will take until we see it.  Only check against it
> @@ -714,7 +723,7 @@ bool nat_inbound(const struct ctx *c, const union inany_addr *addr,
>  		   first_v4(c) && inany_equals(addr, &first_v4(c)->addr)) {
>  		*translated = inany_from_v4(c->ip4.map_guest_addr);
>  	} else if (!IN6_IS_ADDR_UNSPECIFIED(&c->ip6.map_guest_addr) &&
> -		   first_v6(c) && inany_equals6(addr, &first_v6(c)->addr.a6)) {
> +		   first_v6(c) && inany_equals(addr, &first_v6(c)->addr)) {
>  		translated->a6 = c->ip6.map_guest_addr;
>  	} else if (fwd_guest_accessible(c, addr)) {
>  		*translated = *addr;
> -- 
> 2.52.0
> 

-- 
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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