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PObuilding an sk_buff for egress device linux kernel
 

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  1. PObuilding an sk_buff for egress device linux kernel
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    <p>Long story short, I am trying to build a very bare bones UDP SKB just to get something onto the wire. The scenario is as follows:</p> <p>I have a kernel module loading that (among other things) overrides the memory location of the standard udp_sendmsg function in /net/ipv4/udp.c. From here I would like to construct an skb to the point where I can simply put it onto the wire. Normally, udp_sendmsg simply does a little UDP bookkeeping, tacks on a UDP header and sends it down to the IP layer for routing, L3/L2 headers etc. Basically I am bringing some of that functionality up into the sendmsg function. At this time, I am just allocating an skb:</p> <pre><code> skb = alloc_skb(1500, GFP_KERNEL); //skb has 1500 bytes of tail room only skb_reserve(skb, 500); //now has head and tail but no data space data = skb_put(skb, 500); //now we have an skb with 500 head, 500 data sec, 500 tail </code></pre> <p>And then (after some route table seteup) I am trying to add a udp_hdr:</p> <pre><code> struct udphdr *uh; skb-&gt;transport_header = skb_push(skb, sizeof(struct udphdr)); uh = udp_hdr(skb); uh-&gt;source = 5555; uh-&gt;dest = dport; uh-&gt;len = 18; uh-&gt;check = 0; </code></pre> <p>and an ip_hdr (only the basics filled):</p> <pre><code> struct iphdr *iph; skb-&gt;network_header = skb_push(skb, sizeof(struct iphdr)); iph = ip_hdr(skb); iph-&gt;version = 4; iph-&gt;ihl = 5; iph-&gt;tos = inet-&gt;tos; iph-&gt;tot_len = htons(skb-&gt;len); iph-&gt;protocol = IPPROTO_UDP; iph-&gt;saddr = saddr; iph-&gt;daddr = daddr; skb-&gt;dst = dst_clone(&amp;rt-&gt;u.dst); </code></pre> <p>Note: I got most of this stuff from <a href="http://vger.kernel.org/~davem/skb_data.html" rel="nofollow noreferrer">this page</a> but they are using an older kernel (pre 2.6.24) where the network and transport headers were unions and called nh and h respectively. The new way involves using skb->transport_header / skb->network_header and using these helper functions but apparently I am doing something wrong because I get a kernel oops when I try to invoke the udp_sendmsg</p> <p>Note: this ran without an oops and dumped junk to the wire when instead of:</p> <pre><code> skb-&gt;transport_header = skb_push(skb, sizeof(struct udphdr)); </code></pre> <p>I used:</p> <pre><code>skb_reset_transport_header(skb); </code></pre> <p>(and equivalent for network_header. But after reading the link above and looking at the source for the reset function in linux/sk_buff.h, it didn't seem like it was doing what I wanted. </p> <p>Please also note that any assignment statement above with (in this context) undefined variables is simply because I didn't include the entire function. </p> <p>I realize this question might fall into a very specific domain but any guidance on correct usage of the newer skb construction would be greatly helpful. My buddy google is coming up pretty dry.</p> <p>The Oops Call Trace:</p> <blockquote> <pre><code> [&lt;ffffffff813dbf98&gt;] oops_end+0xb9/0xc1 [&lt;ffffffff81030e21&gt;] no_context+0x1f6/0x205 [&lt;ffffffff81030fd3&gt;] __bad_area_nosemaphore+0x1a3/0x1c9 [&lt;ffffffff8101184e&gt;] ? apic_timer_interrupt+0xe/0x20 [&lt;ffffffff8103100c&gt;] bad_area_nosemaphore+0x13/0x15 [&lt;ffffffff813dd30a&gt;] do_page_fault+0x125/0x222 [&lt;ffffffff813db485&gt;] page_fault+0x25/0x30 [&lt;ffffffffa010924f&gt;] ? udp_sendmsg_offload+0x1e3/0x250 [testmodule] [&lt;ffffffffa010922e&gt;] ? udp_sendmsg_offload+0x1c2/0x250 [testmodule] [&lt;ffffffff81390a00&gt;] inet_sendmsg+0x54/0x5d [&lt;ffffffff8132f142&gt;] __sock_sendmsg+0x61/0x6c [&lt;ffffffff8132f8b9&gt;] sock_sendmsg+0xcc/0xe5 </code></pre> </blockquote>
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    1. COPlease show us the Oops.
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    2. COedited post to reflect the oops
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      1. USAndrew G
 

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