git: ba49ad50c0b9 - stable/15 - tcp: fix checksum calculation bug

The branch stable/15 has been updated by tuexen:

URL: https://cgit.FreeBSD.org/src/commit/?id=ba49ad50c0b92dfc2efd97d45bb6e2931dd9aaa3

commit ba49ad50c0b92dfc2efd97d45bb6e2931dd9aaa3
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 19:49:46 +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 ef08b9cfd3d6..bd87e9f2d6a6 100644
--- a/sys/netinet/ip_output.c
+++ b/sys/netinet/ip_output.c
@@ -1067,8 +1067,8 @@ 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 */