git: fab0a83ff748 - stable/14 - tcp: fix checksum calculation bug

The branch stable/14 has been updated by tuexen:

URL: https://cgit.FreeBSD.org/src/commit/?id=fab0a83ff7484a069f04ec9c6b5657ec9676c7dd

commit fab0a83ff7484a069f04ec9c6b5657ec9676c7dd
Author:     Timo Völker <timo.voelker@fh-muenster.de>
AuthorDate: 2025-12-19 16:50:20 +0000
Commit:     Michael Tuexen <tuexen@FreeBSD.org>
CommitDate: 2026-01-21 12:46:28 +0000

    tcp: fix checksum calculation bug
    
    The new function in_delayed_cksum_o() was introduced to compute
    the checksum in the case the mbuf chain does not start with the
    IP header. The offset of the IP header is specified by the
    parameter iph_offset.
    If iph_offset was positive, the function computed an incorrect
    checksum.
    
    Reviewed by:            sobomax, tuexen
    Fixes:                  5feb38e37847 ("netinet: provide "at offset" variant of the in_delayed_cksum() API")
    Differential Revision:  https://reviews.freebsd.org/D54269
    
    (cherry picked from commit c8b3b605ae854ead6c8804e0400d80cb8fa73fdf)
---
 sys/netinet/ip_output.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/sys/netinet/ip_output.c b/sys/netinet/ip_output.c
index e3ef8e2c7dd9..e97e10db0e6b 100644
--- a/sys/netinet/ip_output.c
+++ b/sys/netinet/ip_output.c
@@ -1068,8 +1068,8 @@ in_delayed_cksum_o(struct mbuf *m, uint16_t iph_offset)
 		if (csum == 0)
 			csum = 0xffff;
 	} else {
-		cklen = ntohs(ip->ip_len);
-		csum = in_cksum_skip(m, cklen, offset);
+		cklen = ntohs(ip->ip_len) - (ip->ip_hl << 2);
+		csum = in_cksum_skip(m, cklen + offset, offset);
 	}
 	offset += m->m_pkthdr.csum_data;	/* checksum offset */