问xdp运行中寄存器R2 offset is outside of the packet？

提问于 2024-07-09 22:38:17

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我开始用ebpf和XDP编写代码。我正在使用python bcc将XDP程序加载到端口上，想使用xdp提取传输层udp协议上层中dns协议的发送数据中的查询域名。代码如下：


#include <uapi/linux/bpf.h>
#include <linux/in.h>
#include <linux/if_ether.h>
#include <linux/if_packet.h>
#include <linux/if_vlan.h>
#include <linux/ip.h>
#include <linux/udp.h>
#include <linux/ipv6.h>
// BPF_TABLE(MAPTYPE, uint32_t, long, dropcnt, 256);
#define ETH_HLEN 14
#define UDP_HLEN 8
#define TLS2_PRE_LEN 43
#define DNS_HD_LEN 12
int xdp_prog1(struct xdp_md *ctx) {
    void* data_end = (void*)(long)ctx->data_end;
    void* data = (void*)(long)ctx->data;
    char sni[64]="";
    struct ethhdr *eth = data;
    // drop packets
    int rc = XDP_PASS; // let pass XDP_PASS or redirect to tx via XDP_TX
    long *value;
    uint16_t h_proto;
    uint8_t iph_proto;
    uint64_t nh_off = 0;
    uint32_t index;
    nh_off = sizeof(*eth);
    if (data + nh_off  > data_end)
        return rc;
    h_proto = eth->h_proto;
    // parse double vlans
    if (h_proto != htons(ETH_P_IP)){
        return rc;
    }
    else{
        //bpf_trace_printk("in ip\\n");
        struct iphdr *iph = data + nh_off;
        nh_off += sizeof(*iph);
        if (data + nh_off  > data_end)
            return rc;
        if (iph->protocol != IPPROTO_UDP)
            return rc;
        bpf_trace_printk("in udp\\n");
        struct udphdr *udph = data + nh_off;
        nh_off += sizeof(*udph);
        if (data + nh_off  > data_end)
            return rc;
        if (htons(udph->dest) == 53){
            bpf_trace_printk("in dns\\n");
            int payload_length = htons(udph->len) - UDP_HLEN;
            int server_name_len = payload_length - DNS_HD_LEN - 4;
            if (server_name_len<=0){
                return rc;
            }
            unsigned int len = payload_length - DNS_HD_LEN - 4;
            unsigned int server_name_offset = nh_off + DNS_HD_LEN;
            void *data_need = data+server_name_offset+len;
            if ( data_need > data_end){
                return rc;
            }
            //char *to_ptr = (char *)bpf_map_alloc_skb_bytes(ctx, len, 0);
            //if (!to_ptr) {
            //     return XDP_DROP; // 分配内存失败则丢弃数据包
            //}
            char *data_off = (char *)data+server_name_offset;
            //bpf_skb_store_bytes(ctx, 0, data_off, len, 0);
            char c;
            for(unsigned char i = 0;i<len &&i<64;i++){
                if((void *)&data_off[i]>data_end){
                    break;
                }
                c = *(data_off+i);
                sni[i] = c;
            }  
            bpf_trace_printk("in done %d\\n", c);
            //bpf_map_free_skb_bytes(ctx, to_ptr, 0);
        }
        else{
            return rc;
        }
        bpf_trace_printk("udp captured  ip port: %d, %d\\n", htons(udph->source),htons(udph->dest));
    }
    return rc;
}

使用的思路如下，首先分析完udp层的数据，使用udp中字段的length长度减去udp头的长度得到载荷的大小，然后根据dns查询数据包的结构计算出域名的大小以及偏移off位置，最后使用循环读取域名所在的地址值。

但是在读取数据过程中出现了验证器未能通过的现象，但是我在过程对于值的情况是进行了安全检查的。。。

验证器提供的信息如下：

; void* data_end = (void*)(long)ctx->data_end;
0: (61) r7 = *(u32 *)(r1 +4)
; void* data = (void*)(long)ctx->data;
1: (61) r6 = *(u32 *)(r1 +0)
; if (data + nh_off  > data_end)
2: (bf) r1 = r6
3: (07) r1 += 14
; if (data + nh_off  > data_end)
4: (2d) if r1 > r7 goto pc+86
 R1_w=pkt(id=0,off=14,r=14,imm=0) R6_w=pkt(id=0,off=0,r=14,imm=0) R7_w=pkt_end(id=0,off=0,imm=0) R10=fp0
5: (bf) r1 = r6
6: (07) r1 += 34
; if (h_proto != htons(ETH_P_IP)){
7: (2d) if r1 > r7 goto pc+83
 R1_w=pkt(id=0,off=34,r=34,imm=0) R6_w=pkt(id=0,off=0,r=34,imm=0) R7_w=pkt_end(id=0,off=0,imm=0) R10=fp0
8: (71) r1 = *(u8 *)(r6 +13)
9: (67) r1 <<= 8
10: (71) r2 = *(u8 *)(r6 +12)
11: (4f) r1 |= r2
12: (57) r1 &= 65535
13: (55) if r1 != 0x8 goto pc+77
 R1=inv8 R2=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R6=pkt(id=0,off=0,r=34,imm=0) R7=pkt_end(id=0,off=0,imm=0) R10=fp0
; if (iph->protocol != IPPROTO_UDP)
14: (71) r1 = *(u8 *)(r6 +23)
; if (iph->protocol != IPPROTO_UDP)
15: (55) if r1 != 0x11 goto pc+75
 R1_w=inv17 R2=inv(id=0,umax_value=255,var_off=(0x0; 0xff)) R6=pkt(id=0,off=0,r=34,imm=0) R7=pkt_end(id=0,off=0,imm=0) R10=fp0
16: (18) r1 = 0xa706475206e69
; ({ char _fmt[] = "in udp\n"; bpf_trace_printk_(_fmt, sizeof(_fmt)); });
18: (7b) *(u64 *)(r10 -32) = r1
19: (bf) r1 = r10
; 
20: (07) r1 += -32
; ({ char _fmt[] = "in udp\n"; bpf_trace_printk_(_fmt, sizeof(_fmt)); });
21: (b7) r2 = 8
22: (85) call bpf_trace_printk#6
last_idx 22 first_idx 13
regs=4 stack=0 before 21: (b7) r2 = 8
; if (data + nh_off  > data_end)
23: (bf) r8 = r6
24: (07) r8 += 42
; if (data + nh_off  > data_end)
25: (2d) if r8 > r7 goto pc+65
 R0=inv(id=0) R6=pkt(id=0,off=0,r=42,imm=0) R7=pkt_end(id=0,off=0,imm=0) R8_w=pkt(id=0,off=42,r=42,imm=0) R10=fp0 fp-32=mmmmmmmm
; if (htons(udph->dest) == 53){
26: (69) r1 = *(u16 *)(r6 +36)
; if (htons(udph->dest) == 53){
27: (55) if r1 != 0x3500 goto pc+63
 R0=inv(id=0) R1_w=inv13568 R6=pkt(id=0,off=0,r=42,imm=0) R7=pkt_end(id=0,off=0,imm=0) R8_w=pkt(id=0,off=42,r=42,imm=0) R10=fp0 fp-32=mmmmmmmm
28: (18) r1 = 0xa736e64206e69
; ({ char _fmt[] = "in dns\n"; bpf_trace_printk_(_fmt, sizeof(_fmt)); });
30: (7b) *(u64 *)(r10 -32) = r1
31: (bf) r1 = r10
; 
32: (07) r1 += -32
; ({ char _fmt[] = "in dns\n"; bpf_trace_printk_(_fmt, sizeof(_fmt)); });
33: (b7) r2 = 8
34: (85) call bpf_trace_printk#6
last_idx 34 first_idx 23
regs=4 stack=0 before 33: (b7) r2 = 8
; int payload_length = htons(udph->len) - UDP_HLEN;
35: (69) r1 = *(u16 *)(r6 +38)
36: (dc) r1 = be16 r1
37: (b7) r2 = 25
; if ( data_need > data_end){
38: (2d) if r2 > r1 goto pc+52
 R0=inv(id=0) R1_w=inv(id=0,umin_value=25) R2_w=inv25 R6=pkt(id=0,off=0,r=42,imm=0) R7=pkt_end(id=0,off=0,imm=0) R8=pkt(id=0,off=42,r=42,imm=0) R10=fp0 fp-32=mmmmmmmm
; 
39: (bf) r2 = r1
40: (07) r2 += -8
41: (67) r2 <<= 32
42: (77) r2 >>= 32
43: (0f) r8 += r2
last_idx 43 first_idx 35
regs=4 stack=0 before 42: (77) r2 >>= 32
regs=4 stack=0 before 41: (67) r2 <<= 32
regs=4 stack=0 before 40: (07) r2 += -8
regs=4 stack=0 before 39: (bf) r2 = r1
regs=2 stack=0 before 38: (2d) if r2 > r1 goto pc+52
regs=6 stack=0 before 37: (b7) r2 = 25
regs=2 stack=0 before 36: (dc) r1 = be16 r1
regs=2 stack=0 before 35: (69) r1 = *(u16 *)(r6 +38)
; if ( data_need > data_end){
44: (2d) if r8 > r7 goto pc+46
 R0=inv(id=0) R1_w=inv(id=1,umin_value=25) R2_w=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R6=pkt(id=0,off=0,r=42,imm=0) R7=pkt_end(id=0,off=0,imm=0) R8_w=pkt(id=2,off=42,r=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0 fp-32=mmmmmmmm
; for(unsigned char i = 0;i<len &&i<64;i++){
45: (07) r1 += -25
46: (bf) r2 = r1
47: (67) r2 <<= 32
48: (77) r2 >>= 32
49: (b7) r3 = 63
50: (2d) if r3 > r2 goto pc+1
 R0=inv(id=0) R1=inv(id=3) R2=inv(id=0,umin_value=63,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R3=inv63 R6=pkt(id=0,off=0,r=42,imm=0) R7=pkt_end(id=0,off=0,imm=0) R8=pkt(id=2,off=42,r=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0 fp-32=mmmmmmmm
51: (b7) r1 = 63
; c = *(data_off+i);
52: (67) r1 <<= 32
53: (77) r1 >>= 32
; char *data_off = (char *)data+server_name_offset;
54: (bf) r2 = r6
55: (0f) r2 += r1
last_idx 55 first_idx 50
regs=2 stack=0 before 54: (bf) r2 = r6
regs=2 stack=0 before 53: (77) r1 >>= 32
regs=2 stack=0 before 52: (67) r1 <<= 32
regs=2 stack=0 before 51: (b7) r1 = 63
; c = *(data_off+i);
56: (71) r3 = *(u8 *)(r2 +54)
invalid access to packet, off=117 size=1, R2(id=0,off=117,r=42)
R2 offset is outside of the packet

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