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diff --git a/share/man/man9/mbuf.9 b/share/man/man9/mbuf.9
--- a/share/man/man9/mbuf.9
+++ b/share/man/man9/mbuf.9
@@ -22,7 +22,7 @@
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
-.Dd July 29, 2025
+.Dd August 1, 2025
.Dt MBUF 9
.Os
.\"
@@ -1102,8 +1102,7 @@
.Vt int Va csum_flags
and
.Vt int Va csum_data .
-The meaning of those fields depends on the direction a packet flows in,
-and on whether the packet is fragmented.
+The meaning of those fields depends on whether the packet is fragmented.
Henceforth,
.Va csum_flags
or
@@ -1117,14 +1116,14 @@
.Vt mbuf chain
containing the packet.
.Pp
-On output, the computation of the checksum is delayed until the outgoing
-interface has been determined for a packet.
+When a packet is sent by SCTP, TCP, or UDP, the computation of the checksum
+is delayed until the outgoing interface has been determined for a packet.
The interface-specific field
.Va ifnet.if_data.ifi_hwassist
(see
.Xr ifnet 9 )
-is consulted for the capabilities of the interface to assist in
-computing checksums.
+is consulted by IP for the capabilities of the network interface selected for
+output to assist in computing checksums.
The
.Va csum_flags
field of the packet header is set to indicate which actions the interface
@@ -1163,8 +1162,8 @@
In the case of SCTP, the checksum field will be initially
set by the SCTP implementation to 0.
.Pp
-On input, an interface indicates the actions it has performed
-on a packet by setting one or more of the following flags in
+When a packet is received by an interface, it indicates the actions it has
+performed on a packet by setting one or more of the following flags in
.Va csum_flags
associated with the packet:
.Bl -tag -width ".Dv CSUM_IP_CHECKED" -offset indent
@@ -1215,6 +1214,21 @@
in
.Va csum_flags
is not set, since SCTP does not use a pseudo header checksum.
+.Pp
+If IP delivers a packet with the flags
+.Dv CSUM_SCTP ,
+.Dv CSUM_TCP ,
+or
+.Dv CSUM_UDP
+set in
+.Va csum_flags
+to a local SCTP, TCP, or UDP stack, the packet will be processed without
+computing or validating the checksum, since the packet has not been on the
+wire.
+This can happen if the packet was handled by a virtual interface such as
+.Xr tap 4
+or
+.Xr epair 4 .
.Sh STRESS TESTING
When running a kernel compiled with the option
.Dv MBUF_STRESS_TEST ,
diff --git a/sys/netinet/ip_fastfwd.c b/sys/netinet/ip_fastfwd.c
--- a/sys/netinet/ip_fastfwd.c
+++ b/sys/netinet/ip_fastfwd.c
@@ -69,6 +69,7 @@
#include <sys/cdefs.h>
#include "opt_ipstealth.h"
+#include "opt_sctp.h"
#include <sys/param.h>
#include <sys/systm.h>
@@ -102,6 +103,10 @@
#include <machine/in_cksum.h>
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+#include <netinet/sctp_crc32.h>
+#endif
+
#define V_ipsendredirects VNET(ipsendredirects)
static struct mbuf *
@@ -460,6 +465,23 @@
} else
gw = (const struct sockaddr *)dst;
+ /*
+ * If TCP/UDP header still needs a valid checksum and interface will not
+ * calculate it for us, do it here.
+ */
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_DELAY_DATA &
+ ~nh->nh_ifp->if_hwassist)) {
+ in_delayed_cksum(m);
+ m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA;
+ }
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_IP_SCTP &
+ ~nh->nh_ifp->if_hwassist)) {
+ sctp_delayed_cksum(m, (uint32_t)(ip->ip_hl << 2));
+ m->m_pkthdr.csum_flags &= ~CSUM_IP_SCTP;
+ }
+#endif
+
/* Handle redirect case. */
redest.s_addr = 0;
if (V_ipsendredirects && osrc.s_addr == ip->ip_src.s_addr &&
diff --git a/sys/netinet/sctp_input.c b/sys/netinet/sctp_input.c
--- a/sys/netinet/sctp_input.c
+++ b/sys/netinet/sctp_input.c
@@ -5780,7 +5780,11 @@
goto out;
}
ecn_bits = ip->ip_tos;
- if (m->m_pkthdr.csum_flags & CSUM_SCTP_VALID) {
+ if (m->m_pkthdr.csum_flags & (CSUM_SCTP_VALID | CSUM_IP_SCTP)) {
+ /*
+ * Packet with CSUM_IP_SCTP were sent from local host using
+ * checksum offloading. Checksum not required.
+ */
SCTP_STAT_INCR(sctps_recvhwcrc);
compute_crc = 0;
} else {
diff --git a/sys/netinet/tcp_input.c b/sys/netinet/tcp_input.c
--- a/sys/netinet/tcp_input.c
+++ b/sys/netinet/tcp_input.c
@@ -650,6 +650,12 @@
th->th_sum = in6_cksum_pseudo(ip6, tlen,
IPPROTO_TCP, m->m_pkthdr.csum_data);
th->th_sum ^= 0xffff;
+ } else if (m->m_pkthdr.csum_flags & CSUM_IP6_TCP) {
+ /*
+ * Packet from local host (maybe from a VM).
+ * Checksum not required.
+ */
+ th->th_sum = 0;
} else
th->th_sum = in6_cksum(m, IPPROTO_TCP, off0, tlen);
if (th->th_sum) {
@@ -710,6 +716,12 @@
htonl(m->m_pkthdr.csum_data + tlen +
IPPROTO_TCP));
th->th_sum ^= 0xffff;
+ } else if (m->m_pkthdr.csum_flags & CSUM_IP_TCP) {
+ /*
+ * Packet from local host (maybe from a VM).
+ * Checksum not required.
+ */
+ th->th_sum = 0;
} else {
struct ipovly *ipov = (struct ipovly *)ip;
diff --git a/sys/netinet/udp_usrreq.c b/sys/netinet/udp_usrreq.c
--- a/sys/netinet/udp_usrreq.c
+++ b/sys/netinet/udp_usrreq.c
@@ -560,6 +560,12 @@
ip->ip_dst.s_addr, htonl((u_short)len +
m->m_pkthdr.csum_data + proto));
uh_sum ^= 0xffff;
+ } else if (m->m_pkthdr.csum_flags & CSUM_IP_UDP) {
+ /*
+ * Packet from local host (maybe from a VM).
+ * Checksum not required.
+ */
+ uh_sum = 0;
} else {
char b[offsetof(struct ipovly, ih_src)];
struct ipovly *ipov = (struct ipovly *)ip;
diff --git a/sys/netinet6/ip6_fastfwd.c b/sys/netinet6/ip6_fastfwd.c
--- a/sys/netinet6/ip6_fastfwd.c
+++ b/sys/netinet6/ip6_fastfwd.c
@@ -27,6 +27,7 @@
#include <sys/cdefs.h>
#include "opt_inet6.h"
#include "opt_ipstealth.h"
+#include "opt_sctp.h"
#include <sys/param.h>
#include <sys/systm.h>
@@ -54,6 +55,10 @@
#include <netinet6/ip6_var.h>
#include <netinet6/nd6.h>
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+#include <netinet/sctp_crc32.h>
+#endif
+
static int
ip6_findroute(struct nhop_object **pnh, const struct sockaddr_in6 *dst,
struct mbuf *m)
@@ -277,6 +282,29 @@
ip6->ip6_hlim -= IPV6_HLIMDEC;
}
+ /*
+ * If TCP/UDP header still needs a valid checksum and interface will not
+ * calculate it for us, do it here.
+ */
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_DELAY_DATA_IPV6 &
+ ~nh->nh_ifp->if_hwassist)) {
+ int offset = ip6_lasthdr(m, 0, IPPROTO_IPV6, NULL);
+
+ if (offset < sizeof(struct ip6_hdr) || offset > m->m_pkthdr.len)
+ goto drop;
+ in6_delayed_cksum(m, m->m_pkthdr.len - offset, offset);
+ m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA_IPV6;
+ }
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_IP6_SCTP &
+ ~nh->nh_ifp->if_hwassist)) {
+ int offset = ip6_lasthdr(m, 0, IPPROTO_IPV6, NULL);
+
+ sctp_delayed_cksum(m, offset);
+ m->m_pkthdr.csum_flags &= ~CSUM_IP6_SCTP;
+ }
+#endif
+
m_clrprotoflags(m); /* Avoid confusing lower layers. */
IP_PROBE(send, NULL, NULL, ip6, nh->nh_ifp, NULL, ip6);
diff --git a/sys/netinet6/ip6_forward.c b/sys/netinet6/ip6_forward.c
--- a/sys/netinet6/ip6_forward.c
+++ b/sys/netinet6/ip6_forward.c
@@ -75,6 +75,10 @@
#include <netipsec/ipsec_support.h>
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+#include <netinet/sctp_crc32.h>
+#endif
+
/*
* Forward a packet. If some error occurs return the sender
* an icmp packet. Note we can't always generate a meaningful
@@ -389,6 +393,29 @@
goto bad;
}
+ /*
+ * If TCP/UDP header still needs a valid checksum and interface will not
+ * calculate it for us, do it here.
+ */
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_DELAY_DATA_IPV6 &
+ ~nh->nh_ifp->if_hwassist)) {
+ int offset = ip6_lasthdr(m, 0, IPPROTO_IPV6, NULL);
+
+ if (offset < sizeof(struct ip6_hdr) || offset > m->m_pkthdr.len)
+ goto bad;
+ in6_delayed_cksum(m, m->m_pkthdr.len - offset, offset);
+ m->m_pkthdr.csum_flags &= ~CSUM_DELAY_DATA_IPV6;
+ }
+#if defined(SCTP) || defined(SCTP_SUPPORT)
+ if (__predict_false(m->m_pkthdr.csum_flags & CSUM_IP6_SCTP &
+ ~nh->nh_ifp->if_hwassist)) {
+ int offset = ip6_lasthdr(m, 0, IPPROTO_IPV6, NULL);
+
+ sctp_delayed_cksum(m, offset);
+ m->m_pkthdr.csum_flags &= ~CSUM_IP6_SCTP;
+ }
+#endif
+
/* Currently LLE layer stores embedded IPv6 addresses */
if (IN6_IS_SCOPE_LINKLOCAL(&dst.sin6_addr)) {
in6_set_unicast_scopeid(&dst.sin6_addr, dst.sin6_scope_id);
diff --git a/sys/netinet6/sctp6_usrreq.c b/sys/netinet6/sctp6_usrreq.c
--- a/sys/netinet6/sctp6_usrreq.c
+++ b/sys/netinet6/sctp6_usrreq.c
@@ -139,7 +139,11 @@
goto out;
}
ecn_bits = IPV6_TRAFFIC_CLASS(ip6);
- if (m->m_pkthdr.csum_flags & CSUM_SCTP_VALID) {
+ if (m->m_pkthdr.csum_flags & (CSUM_SCTP_VALID | CSUM_IP6_SCTP)) {
+ /*
+ * Packet with CSUM_IP6_SCTP were sent from local host using
+ * checksum offloading. Checksum not required.
+ */
SCTP_STAT_INCR(sctps_recvhwcrc);
compute_crc = 0;
} else {
diff --git a/sys/netinet6/udp6_usrreq.c b/sys/netinet6/udp6_usrreq.c
--- a/sys/netinet6/udp6_usrreq.c
+++ b/sys/netinet6/udp6_usrreq.c
@@ -434,6 +434,12 @@
uh_sum = in6_cksum_pseudo(ip6, ulen, nxt,
m->m_pkthdr.csum_data);
uh_sum ^= 0xffff;
+ } else if (m->m_pkthdr.csum_flags & CSUM_IP6_UDP) {
+ /*
+ * Packet from local host (maybe from a VM).
+ * Checksum not required.
+ */
+ uh_sum = 0;
} else
uh_sum = in6_cksum_partial(m, nxt, off, plen, ulen);
diff --git a/sys/sys/mbuf.h b/sys/sys/mbuf.h
--- a/sys/sys/mbuf.h
+++ b/sys/sys/mbuf.h
@@ -641,16 +641,15 @@
/*
* Flags indicating checksum, segmentation and other offload work to be
- * done, or already done, by hardware or lower layers. It is split into
- * separate inbound and outbound flags.
+ * done, or already done, by hardware or lower layers.
*
- * Outbound flags that are set by upper protocol layers requesting lower
+ * Flags that are set by upper protocol layers requesting lower
* layers, or ideally the hardware, to perform these offloading tasks.
- * For outbound packets this field and its flags can be directly tested
- * against ifnet if_hwassist. Note that the outbound and the inbound flags do
- * not collide right now but they could be allowed to (as long as the flags are
- * scrubbed appropriately when the direction of an mbuf changes). CSUM_BITS
- * would also have to split into CSUM_BITS_TX and CSUM_BITS_RX.
+ * Before passing packets to a network interface this field and its flags can
+ * be directly tested against ifnet if_hwassist. Note that the flags
+ * CSUM_IP_SCTP, CSUM_IP_TCP, and CSUM_IP_UDP can appear on input processing
+ * of SCTP, TCP, and UDP. In such a case the checksum will not be computed or
+ * validated by SCTP, TCP, or TCP, since the packet has not been on the wire.
*
* CSUM_INNER_<x> is the same as CSUM_<x> but it applies to the inner frame.
* The CSUM_ENCAP_<x> bits identify the outer encapsulation.
@@ -679,7 +678,7 @@
#define CSUM_ENCAP_VXLAN 0x00040000 /* VXLAN outer encapsulation */
#define CSUM_ENCAP_RSVD1 0x00080000
-/* Inbound checksum support where the checksum was verified by hardware. */
+/* Flags used to indicate that the checksum was verified by hardware. */
#define CSUM_INNER_L3_CALC 0x00100000
#define CSUM_INNER_L3_VALID 0x00200000
#define CSUM_INNER_L4_CALC 0x00400000

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