netdev - Re: bpf debug info

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Message-ID: <CAADnVQKKjYDCqRGrB2UVKf=-KZBpt+5+M4nXXVuVefEGRv5MYQ@mail.gmail.com>
Date:   Tue, 13 Dec 2016 11:38:32 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Thomas Graf <tgraf@...g.ch>, Wangnan <wangnan0@...wei.com>,
        He Kuang <hekuang@...wei.com>, Kernel Team <kernel-team@...com>
Subject: Re: bpf debug info

On Tue, Nov 29, 2016 at 9:01 AM, Alexei Starovoitov
<alexei.starovoitov@...il.com> 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.

Thanks