Subject: Re: bpf debug info

From: Daniel Borkmann <[email-masked]>
Date: Tue, 13 Dec 2016 20:55:14 +0100

On 12/13/2016 08:38 PM, Alexei Starovoitov wrote:
> On Tue, Nov 29, 2016 at 9:01 AM, Alexei Starovoitov
> <[email-masked]> wrote:
>>>> If I try to run samples/bpf/test_cls_bpf.sh the verifier will complain:
>>>> R0=imm0,min_value=0,max_value=0 R1=pkt(id=0,off=0,r=42) R2=pkt_end
>>>> 112: (0f) r4 += r3
>>>> 113: (0f) r1 += r4
>>>> 114: (b7) r0 = 2
>>>> 115: (69) r2 = *(u16 *)(r1 +2)
>>>> invalid access to packet, off=2 size=2, R1(id=3,off=0,r=0)
>>>>
>>>> Now multiply 115 * 8 and convert to hex. This is address 0x398 in llvm-objdump:
>>>> ; struct udphdr *udp = data + tp_off;
>>>>       388:       r1 += r4
>>>>       390:       r0 = 2
>>>> ; if (udp->dest == htons(DEFAULT_PKTGEN_UDP_PORT) ||
>>>>       398:       r2 = *(u16 *)(r1 + 2)
>>>>       3a0:       if r2 == 2304 goto 16
>>>>
>>>> Now it's clear which line of C code is causing the verifier to reject.
>>> [...]
>>>
>>> Could llvm-objdump switch line numbering for bpf same way as verifier
>>> output, so mapping step is not really needed?
>>
>> you mean that llvm-objdump to print 113,114,115 ?
>> I guess it's doable. Will give it a try.
>
> Hi Daniel,
>
> your feature request turned out to be pretty straightforward
> to implement. Please pull the latest llvm and rebuild llvm-objdump.
> It will be printing instruction numbers instead of absolute addresses.
> No "multiply 115 * 8 and convert to hex" steps necessary anymore.

That's great to hear, thanks for following up on this. Sounds about
right to upgrade.

Thanks,
Daniel