如何从Linux内核发送UDP数据包

我正在修改UDP协议，以便在UDP套接字上调用connect（）时，除了查找路由外，还会将“Hello”数据包发送到目标。

从UDP协议结构中，我发现ip4_datagram_connect函数ip4_datagram_connectfind到达目的地的路由。 现在在这个函数的最后，我需要发送Hello数据包。

我不认为我可以使用udp_sendmsg()因为它用于从用户空间复制和发送数据。

我认为udp_send_skb()应该用来发送你好。 我的问题是我不知道如何创build一个适当的skbuff来存储hello消息（它应该是一个适当的udp数据报）传递给udp_send_skb() 。 我已经试过了

int quic_connect(struct sock *sk,struct flowi4 *fl4,struct rtable *rt){ struct sk_buff *skb; char *hello; int err = 0,exthdrlen,hh_len,datalen,trailerlen; char *data; hh_len = LL_RESERVED_SPACE(rt->dst.dev); exthdrlen = rt->dst.header_len; trailerlen = rt->dst.trailer_len; datalen = 200; //Create a buffer to be send without fragmentation skb = sock_alloc_send_skb(sk,exthdrlen + datalen + hh_len + trailerlen + 15,MSG_DONTWAIT,&err); if (skb == NULL) goto out; skb->ip_summed = CHECKSUM_PARTIAL; // Use hardware checksum skb->csum = 0; skb_reserve(skb,hh_len); skb_shinfo(skb)->tx_flags = 1; //Time stamp the packet /* * Find where to start putting bytes. */ data = skb_put(skb,datalen + exthdrlen); skb_set_network_header(skb,exthdrlen); skb->transport_header = (skb->network_header + sizeof(struct iphdr)); err = udp_send_skb(skb,fl4);

但是，这给我在内核日志中的错误

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BUG: unable to handle kernel NULL pointer dereference at 0000000000000018 IP: [<ffffffff81686555>] __ip_local_out+0x45/0x80 PGD 4f4dd067 PUD 4f4df067 PMD 0 Oops: 0000 [#1] SMP Modules linked in: cpu: 0 PID: 3019 Comm: client Not tainted 3.13.11-ckt39-test006 #28 Hardware name: innotek GmbH VirtualBox/VirtualBox,BIOS VirtualBox 12/01/2006 task: ffff8800598df6b0 ti: ffff880047022000 task.ti: ffff880047022000 RIP: 0010:[<ffffffff81686555>] [<ffffffff81686555>] __ip_local_out+0x45/0x80 RSP: 0018:ffff880047023d78 EFLAGS: 00010287 RAX: 0000000000000001 RBX: ffff880047008a00 RCX: 0000000020000000 RDX: 0000000000000000 RSI: ffff880047008a00 RDI: ffff8800666fde40 RBP: ffff880047023d88 R08: 0000000000003200 R09: 0000000000000001 R10: 0000000000000000 R11: 00000000000001f9 R12: ffff880047008a00 R13: ffff8800666fde80 R14: ffff880059aec380 R15: ffff880059aec690 FS: 00007f5508b04740(0000) GS:ffff88007fc00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b CR2: 0000000000000018 CR3: 000000004f561000 CR4: 00000000000406f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 Stack: ffff880047023d80 ffff880047008a00 ffff880047023da0 ffffffff8168659d ffffffff81c8f8c0 ffff880047023db8 ffffffff81687810 0000000000000000 ffff880047023df8 ffffffff816ac6be 0000000000000020 ffff880047008a00 Call Trace: [<ffffffff8168659d>] ip_local_out+0xd/0x30 [<ffffffff81687810>] ip_send_skb+0x10/0x40 [<ffffffff816ac6be>] udp_send_skb+0x14e/0x3d0 [<ffffffff816b0e9e>] quic_connect+0x6e/0x80 [<ffffffff816aa3ff>] __ip4_datagram_connect+0x2bf/0x2d0 [<ffffffff816aa437>] ip4_datagram_connect+0x27/0x40 [<ffffffff816b8748>] inet_dgram_connect+0x38/0x80 [<ffffffff8161fd97>] SYSC_connect+0xc7/0x100 [<ffffffff817ed471>] ? __schedule+0x341/0x8c0 [<ffffffff816206e9>] SyS_connect+0x9/0x10 [<ffffffff817f8d42>] system_call_fastpath+0x16/0x1b Code: c8 00 00 00 66 c1 c0 08 66 89 47 02 e8 d5 e0 ff ff 48 8b 53 58 b8 01 00 00 00 48 83 e2 fe 48 81 3d 9d 0e 64 00 f0 73 cc 81 74 26 <4c> 8b 42 18 49 c7 c1 f0 45 68 81 c7 04 24 00 00 00 80 31 c9 48 RIP [<ffffffff81686555>] __ip_local_out+0x45/0x80 RSP <ffff880047023d78> CR2: 0000000000000018 ---[ end trace 474c5db1b9b19a03 ]---

所以我的问题是，还有什么需要填写我的skbuff之前，它可以由udp_send_skb正确处理。 还是我在这里错过了别的东西？

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您的代码中存在一个错误。

if (skb_tailroom(hbuff) > 30) { printk(" Enough room for QUIC connect messagen"); hello = kmalloc(30,GFP_ATOMIC); //You allocate slub memory hello = "Hello from QUIC connect"; //You let 'hello' point to a string,//which is stored somewhere else. //At this point,your slub memory //allocated is lost. memcpy(__skb_put(hbuff,30),hello,30); kfree(hello); //You try to free the memory pointed by //hello as slub memory,I think this is // why you get mm/slub.c bug message. } else

你可以像这样改变你的代码：

if (skb_tailroom(hbuff) > 30) { printk(" Enough room for QUIC connect messagen"); memcpy(__skb_put(hbuff,"Hello from QUIC connect",30); } else

我刚刚使用函数ip_make_skb其次是ip_send_skb 。 由于ip_make_skb用于从用户空间复制数据，我不需要这样做，所以我只是使用了一个虚拟指针，并提供了被复制为零的长度（+ sizeof udphdr，按照此函数的要求）。 这是一个肮脏的方式来做到这一点，但现在起作用。 我的代码：

int quic_connect(struct sock *sk,struct rtable *rt,int oif){ struct sk_buff *skb; struct ipcm_cookie ipc; void *hello = "Hello"; int err = 0; ipc.opt = NULL; ipc.tx_flags = 1; ipc.ttl = 0; ipc.tos = -1; ipc.oif = oif; ipc.addr = fl4->daddr; skb = ip_make_skb(sk,fl4,ip_generic_getfrag,sizeof(struct udphdr),&ipc,&rt,0); err = PTR_ERR(skb); if (!IS_ERR_OR_NULL(skb)){ err = udp_send_skb(skb,fl4); } return err; }