貌似发现一个FreeBSD内核的小bug

代码中只判断了IP头是否在mbuf的空间内，而实际上ipv4和ipv6头里的IP_TOS有可能在mbuf之外。不过二层头一般应该不会大到把ip头挤到mbuf最后一个字节，从而使ip_tos在另一个mbuf内。

/*
 * read and write diffserv field in IPv4 or IPv6 header
 */
u_int8_t
read_dsfield(m, pktattr)
    struct mbuf *m;
    struct altq_pktattr *pktattr;
{
    struct mbuf *m0;
    u_int8_t ds_field = 0;

    if (pktattr == NULL ||
     (pktattr->pattr_af != AF_INET && pktattr->pattr_af != AF_INET6))
        return ((u_int8_t)0);

    /* verify that pattr_hdr is within the mbuf data */
    for (m0 = m; m0 != NULL; m0 = m0->m_next)
        if ((pktattr->pattr_hdr >= m0->m_data) &&
         (pktattr->pattr_hdr < m0->m_data + m0->m_len))
            break;
    if (m0 == NULL) {
        /* ick, pattr_hdr is stale */
        pktattr->pattr_af = AF_UNSPEC;
#ifdef ALTQ_DEBUG
        printf("read_dsfield: can't locate header!\n");
#endif
        return ((u_int8_t)0);
    }

    if (pktattr->pattr_af == AF_INET) {
        struct ip *ip = (struct ip *)pktattr->pattr_hdr;

        if (ip->ip_v != 4)
            return ((u_int8_t)0);    /* version mismatch! */
        ds_field = ip->ip_tos;
    }
#ifdef INET6
    else if (pktattr->pattr_af == AF_INET6) {
        struct ip6_hdr *ip6 = (struct ip6_hdr *)pktattr->pattr_hdr;
        u_int32_t flowlabel;

        flowlabel = ntohl(ip6->ip6_flow);
        if ((flowlabel >> 28) != 6)
            return ((u_int8_t)0);    /* version mismatch! */
        ds_field = (flowlabel >> 20) & 0xff;
    }
#endif
    return (ds_field);
}

void
write_dsfield(m, pktattr, dsfield)
    struct mbuf *m;
    struct altq_pktattr *pktattr;
    u_int8_t dsfield;
{
    struct mbuf *m0;

    if (pktattr == NULL ||
     (pktattr->pattr_af != AF_INET && pktattr->pattr_af != AF_INET6))
        return;

    /* verify that pattr_hdr is within the mbuf data */
    for (m0 = m; m0 != NULL; m0 = m0->m_next)
        if ((pktattr->pattr_hdr >= m0->m_data) &&
         (pktattr->pattr_hdr < m0->m_data + m0->m_len))
            break;
    if (m0 == NULL) {
        /* ick, pattr_hdr is stale */
        pktattr->pattr_af = AF_UNSPEC;
#ifdef ALTQ_DEBUG
        printf("write_dsfield: can't locate header!\n");
#endif
        return;
    }

    if (pktattr->pattr_af == AF_INET) {
        struct ip *ip = (struct ip *)pktattr->pattr_hdr;
        u_int8_t old;
        int32_t sum;

        if (ip->ip_v != 4)
            return;        /* version mismatch! */
        old = ip->ip_tos;
        dsfield |= old & 3;    /* leave CU bits */
        if (old == dsfield)
            return;
        ip->ip_tos = dsfield;
        /*
         * update checksum (from RFC1624)
         *     HC' = ~(~HC + ~m + m')
         */
        sum = ~ntohs(ip->ip_sum) & 0xffff;
        sum += 0xff00 + (~old & 0xff) + dsfield;
        sum = (sum >> 16) + (sum & 0xffff);
        sum += (sum >> 16); /* add carry */

        ip->ip_sum = htons(~sum & 0xffff);
    }
#ifdef INET6
    else if (pktattr->pattr_af == AF_INET6) {
        struct ip6_hdr *ip6 = (struct ip6_hdr *)pktattr->pattr_hdr;
        u_int32_t flowlabel;

        flowlabel = ntohl(ip6->ip6_flow);
        if ((flowlabel >> 28) != 6)
            return;        /* version mismatch! */
        flowlabel = (flowlabel & 0xf03fffff) | (dsfield << 20);
        ip6->ip6_flow = htonl(flowlabel);
    }
#endif
    return;
}