From: David Gibson <david@gibson.dropbear.id.au>
To: Jon Maloy <jmaloy@redhat.com>
Cc: passt-dev@passt.top, sbrivio@redhat.com, lvivier@redhat.com,
dgibson@redhat.com
Subject: Re: [PATCH 1/2] udp: correct source address for ICMP messages
Date: Sun, 16 Mar 2025 15:47:41 +1100 [thread overview]
Message-ID: <Z9ZX7Z6VXpGZ71nU@zatzit> (raw)
In-Reply-To: <20250315153245.435293-2-jmaloy@redhat.com>
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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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next prev parent reply other threads:[~2025-03-16 11:41 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-03-15 15:32 [PATCH 0/2] udp: add guest-to-remote traceroute for IPv4 Jon Maloy
2025-03-15 15:32 ` [PATCH 1/2] udp: correct source address for ICMP messages Jon Maloy
2025-03-16 4:47 ` David Gibson [this message]
2025-03-15 15:32 ` [PATCH 2/2] udp: support traceroute for IPv4 Jon Maloy
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