ip_rcv (in ip_input.c for ipv4) behaviour in loopback mode

Question

So I wanted to put a few printk messages in ip_rcv function to see whether after receiving a packet from a particular IP there is a message printed. I am attaching the entire ip_rcv function which has the printk modifications:

int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt, struct net_device *orig_dev)
{
    const struct iphdr *iph;
    struct net *net;
    u32 len;

    /* When the interface is in promisc. mode, drop all the crap
     * that it receives, do not try to analyse it.
     */
    if (skb->pkt_type == PACKET_OTHERHOST)
        goto drop;


    net = dev_net(dev);
    __IP_UPD_PO_STATS(net, IPSTATS_MIB_IN, skb->len);

    skb = skb_share_check(skb, GFP_ATOMIC);
    if (!skb) {
        __IP_INC_STATS(net, IPSTATS_MIB_INDISCARDS);
        goto out;
    }

    if (!pskb_may_pull(skb, sizeof(struct iphdr)))
        goto inhdr_error;

    iph = ip_hdr(skb);

    //**PSK's Modification**
    if (iph->saddr == 0x08080808)
        printk("\n***PSK: %x IP's message recieved: Google***\n", iph->saddr);
    if (iph->saddr == 0x0202000A)
        printk("\n***PSK: %x IP's message recieved: Gateway***\n", iph->saddr);
    if (iph->saddr == 0x010000FF)
        printk("\n***PSK: %x IP's message recieved : Home***\n", iph->saddr);

     /* RFC1122: 3.2.1.2 MUST silently discard any IP frame that fails the checksum.
     *
     *  Is the datagram acceptable?
     *
     *  1.  Length at least the size of an ip header
     *  2.  Version of 4
     *  3.  Checksums correctly. [Speed optimisation for later, skip loopback checksums]
     *  4.  Doesn't have a bogus length
     */

    if (iph->ihl < 5 || iph->version != 4)
        goto inhdr_error;

    BUILD_BUG_ON(IPSTATS_MIB_ECT1PKTS != IPSTATS_MIB_NOECTPKTS + INET_ECN_ECT_1);
    BUILD_BUG_ON(IPSTATS_MIB_ECT0PKTS != IPSTATS_MIB_NOECTPKTS + INET_ECN_ECT_0);
    BUILD_BUG_ON(IPSTATS_MIB_CEPKTS != IPSTATS_MIB_NOECTPKTS + INET_ECN_CE);
    __IP_ADD_STATS(net,
               IPSTATS_MIB_NOECTPKTS + (iph->tos & INET_ECN_MASK),
               max_t(unsigned short, 1, skb_shinfo(skb)->gso_segs));

    if (!pskb_may_pull(skb, iph->ihl*4))
        goto inhdr_error;

    iph = ip_hdr(skb);

    if (unlikely(ip_fast_csum((u8 *)iph, iph->ihl)))
        goto csum_error;

    len = ntohs(iph->tot_len);
    if (skb->len < len) {
        __IP_INC_STATS(net, IPSTATS_MIB_INTRUNCATEDPKTS);
        goto drop;
    } else if (len < (iph->ihl*4))
        goto inhdr_error;

    /* Our transport medium may have padded the buffer out. Now we know it
     * is IP we can trim to the true length of the frame.
     * Note this now means skb->len holds ntohs(iph->tot_len).
     */
    if (pskb_trim_rcsum(skb, len)) {
        __IP_INC_STATS(net, IPSTATS_MIB_INDISCARDS);
        goto drop;
    }

    iph = ip_hdr(skb);
    skb->transport_header = skb->network_header + iph->ihl*4;

    /* Remove any debris in the socket control block */
    memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
    IPCB(skb)->iif = skb->skb_iif;

    /* Must drop socket now because of tproxy. */
    skb_orphan(skb);

    return NF_HOOK(NFPROTO_IPV4, NF_INET_PRE_ROUTING,
               net, NULL, skb, dev, NULL,
               ip_rcv_finish);

csum_error:
    __IP_INC_STATS(net, IPSTATS_MIB_CSUMERRORS);
inhdr_error:
    __IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS);
drop:
    kfree_skb(skb);
out:
    return NET_RX_DROP;
}

I should get a message printed into the kernel buffer after getting a packet from either Google DNS, my local gateway (10.0.2.2) or loop-back address (127.0.0.1). This is working fine for the DNS and the gateway, but not when I ping localhost or try to run a program which involves back and forth from a localhost. Are there some other kernel function calls that specifically handle packets to localhost, or am I missing something very basic? I thought the loopback packets should trace the same path through the stack as any other packet at least until L3. I would also appreciate if someone briefly explains the handling of loopback traffic along with the answer.

System Specs: System- Ubuntu on Virtual Machine Kernel- 4.15.0-70 generic

Answer 1

I think there is something wrong here:

if (iph->saddr == 0x010000FF)

Perhaps you mean:

if (iph->saddr == 0x0100007F)

---

loopback packets should trace the same path

Yep, in general.

Investigate more about the details of loopback device.

---

Also you always can operate with some useful tools like trace-cmd. F.e. to see the function graph you can do:

trace-cmd record -p function_graph -g net_rx_action

Then start ping 127.0.0.1, then stop tracing and watch report like

trace-cmd report | vim -

Here you can see the "path" and ensure that your localhost pinging eventually falls into ip_rcv().