Re: [PATCH 1/2] udp: correct source address for ICMP messages

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

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On Sat, Mar 15, 2025 at 11:32:44AM -0400, Jon Maloy wrote:
> While developing traceroute forwarding tap-to-sock we found that
> struct msghdr.msg_name for the ICMPs in the opposite direction always
> contains the destination address of the original UDP message, and not,
> as one might expect, the one of the host which created the error message.
> 
> Study of the kernel code reveals that this address instead is appended
> as extra data after the received struct sock_extended_err area.
> 
> We now change the ICMP receive code accordingly.
> 
> Fixes: 55431f0077b6 ("udp: create and send ICMPv4 to local peer when applicable")
> Fixes: 68b04182e07d ("udp: create and send ICMPv6 to local peer when applicable")
> 
> Signed-off-by: Jon Maloy <jmaloy@redhat.com>

The idea looks good, a few minor warts in the implementation.

> ---
>  udp.c | 26 +++++++++++++++-----------
>  1 file changed, 15 insertions(+), 11 deletions(-)
> 
> diff --git a/udp.c b/udp.c
> index 80520cb..271e570 100644
> --- a/udp.c
> +++ b/udp.c
> @@ -510,10 +510,13 @@ static void udp_send_conn_fail_icmp6(const struct ctx *c,
>   */
>  static int udp_sock_recverr(const struct ctx *c, union epoll_ref ref)
>  {
> -	const struct sock_extended_err *ee;
> +	struct errhdr {
> +		struct sock_extended_err ee;
> +		union sockaddr_inany saddr;
> +	};
> +	const struct errhdr *eh;
>  	const struct cmsghdr *hdr;
> -	union sockaddr_inany saddr;
> -	char buf[CMSG_SPACE(sizeof(*ee))];
> +	char buf[CMSG_SPACE(sizeof(struct errhdr))];
>  	char data[ICMP6_MAX_DLEN];
>  	int s = ref.fd;
>  	struct iovec iov = {
> @@ -521,11 +524,11 @@ static int udp_sock_recverr(const struct ctx *c, union epoll_ref ref)
>  		.iov_len = sizeof(data)
>  	};
>  	struct msghdr mh = {
> -		.msg_name = &saddr,
> -		.msg_namelen = sizeof(saddr),
> +		.msg_name = 0,
> +		.msg_namelen = 0,

I think you can just omit these and they'll be zero initialized.

>  		.msg_iov = &iov,
>  		.msg_iovlen = 1,
> -		.msg_control = buf,
> +		.msg_control = buf,//(void *)&errhdr,

Looks like you have a leftover comment here.

>  		.msg_controllen = sizeof(buf),
>  	};
>  	ssize_t rc;
> @@ -553,7 +556,7 @@ static int udp_sock_recverr(const struct ctx *c, union epoll_ref ref)
>  		return -1;
>  	}
>  
> -	ee = (const struct sock_extended_err *)CMSG_DATA(hdr);
> +	eh = (const struct errhdr *)CMSG_DATA(hdr);
>  	if (ref.type == EPOLL_TYPE_UDP_REPLY) {
>  		flow_sidx_t sidx = flow_sidx_opposite(ref.flowside);
>  		const struct flowside *toside = flowside_at_sidx(sidx);
> @@ -561,18 +564,19 @@ static int udp_sock_recverr(const struct ctx *c, union epoll_ref ref)
>  
>  		if (hdr->cmsg_level == IPPROTO_IP) {
>  			dlen = MIN(dlen, ICMP4_MAX_DLEN);
> -			udp_send_conn_fail_icmp4(c, ee, toside, saddr.sa4.sin_addr,
> +			udp_send_conn_fail_icmp4(c, &eh->ee, toside,
> +						 eh->saddr.sa4.sin_addr,
>  						 data, dlen);

So, it looks from man ip(7) that you're supposed to use the
SO_EE_OFFENDER() macro to get the socket address, rather than just
assuming it's immediately after the struct sock_extended_err.
Except... without assuming that, I can't really see how you're
supposed to properly size the buffer

>  		} else if (hdr->cmsg_level == IPPROTO_IPV6) {
> -			udp_send_conn_fail_icmp6(c, ee, toside,
> -						 &saddr.sa6.sin6_addr,
> +			udp_send_conn_fail_icmp6(c, &eh->ee, toside,
> +						 &eh->saddr.sa6.sin6_addr,
>  						 data, dlen, sidx.flowi);
>  		}
>  	} else {
>  		trace("Ignoring received IP_RECVERR cmsg on listener socket");
>  	}
>  	debug("%s error on UDP socket %i: %s",
> -	      str_ee_origin(ee), s, strerror_(ee->ee_errno));
> +	      str_ee_origin(&eh->ee), s, strerror_(eh->ee.ee_errno));
>  
>  	return 1;
>  }

-- 
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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