math between pkt pointer and register with unbounded min value is not allowed #verifier


Simon
 

I'm playing with bcc to prototype an UDP load balancer.

I'm facing an issue that I didn't succeed to understand...

In my code I tried to validate my UDP packet using code like this :

    struct udphdr *udp;
    udp = iph + 1;
    if (udp + 1 > data_end)
        return XDP_DROP;
    __u16 udp_len = bpf_ntohs(udp->len);
    //__u16 udp_len = 8;
    if (udp_len < 8)
        return XDP_DROP;
    if (udp_len > 512) // TODO use a more approriate max value
        return XDP_DROP;
    if ((void *) udp + udp_len > data_end)
        return XDP_DROP;
And the verifier does not like it ..
28: (71) r2 = *(u8 *)(r7 +23)
29: (b7) r0 = 2
30: (55) if r2 != 0x11 goto pc+334
 R0=inv2 R1=pkt_end(id=0,off=0,imm=0) R2=inv17 R3=inv5 R6=ctx(id=0,off=0,imm=0) R7=pkt(id=0,off=0,r=34,imm=0) R8=pkt(id=0,off=34,r=34,imm=0) R9=pkt(id=0,off=14,r=34,imm=0) R10=fp0,call_-1
31: (bf) r2 = r8
32: (07) r2 += 8
33: (b7) r0 = 1
34: (2d) if r2 > r1 goto pc+330
 R0=inv1 R1=pkt_end(id=0,off=0,imm=0) R2=pkt(id=0,off=42,r=42,imm=0) R3=inv5 R6=ctx(id=0,off=0,imm=0) R7=pkt(id=0,off=0,r=42,imm=0) R8=pkt(id=0,off=34,r=42,imm=0) R9=pkt(id=0,off=14,r=42,imm=0) R10=fp0,call_-1
35: (69) r3 = *(u16 *)(r7 +38)
36: (dc) r3 = be16 r3
37: (bf) r2 = r3
38: (07) r2 += -8
39: (57) r2 &= 65535
40: (b7) r0 = 1
41: (25) if r2 > 0x1f8 goto pc+323
 R0=inv1 R1=pkt_end(id=0,off=0,imm=0) R2=inv(id=0,umax_value=504,var_off=(0x0; 0x1ff)) R3=inv(id=0) R6=ctx(id=0,off=0,imm=0) R7=pkt(id=0,off=0,r=42,imm=0) R8=pkt(id=0,off=34,r=42,imm=0) R9=pkt(id=0,off=14,r=42,imm=0) R10=fp0,call_-1
42: (bf) r2 = r7
43: (0f) r2 += r3
math between pkt pointer and register with unbounded min value is not allowed

I'm pretty sure the issue is about udp_len, that's why I tried to validate its value before to use it ... but without success...
When I set udp_len to 8 (just for testing) this seems to works. Any idea about that ?

Full code is available here : https://gist.github.com/sbernard31/d4fee7518a1ff130452211c0d355b3f7

(I'm using python-bpfcc  0.8.0-4 from debian sid with a 4.19.12 kernel)
(I don't know if this is the right place for this kind of question, )

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