[Bug 194697] New: incoming IP TOS bits get zeroed on mbufs that need checksumming

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=194697

            Bug ID: 194697
           Summary: incoming IP TOS bits get zeroed on mbufs that need
                    checksumming
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: Needs Triage
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: seb at f5.com

Created attachment 148780
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=148780&action=edit
save & restore IP TOS field when computing TCP checksum

On mbufs that don't have the CSUM_DATA_VALID flag set, tcp_input() needs to
compute the TCP checksum itself.

The TCP checksum includes some but not all fields from the IP header.

tcp_input() zeroes the fields of the IP header that are not to be included in
the checksum, then checksums the entire packet (IP header, TCP header, and TCP
data), then restores the IP header fields it will need later.

The bug is that the IP TOS field was not restored, so the later read of it
returned a zeroed byte instead of the incoming packet's actual TOS value.

The attached patch contains a suggested fix that resolves the issue for me.

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