linux-kernel - Re: [PATCH bpf-next 1/2] bpf: Remove bpf_jit

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Date:   Fri, 14 May 2021 08:34:07 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Quentin Monnet <quentin@...valent.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Ian Rogers <irogers@...gle.com>, Song Liu <songliubraving@...com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Zi Shen Lim <zlim.lnx@...il.com>,
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        Paul Mackerras <paulus@...ba.org>,
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        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH bpf-next 1/2] bpf: Remove bpf_jit_enable=2 debugging mode



Le 23/04/2021 à 12:26, Quentin Monnet a écrit :
> 2021-04-23 09:19 UTC+0200 ~ Christophe Leroy <christophe.leroy@...roup.eu>
> 
> [...]
> 
>> I finally managed to cross compile bpftool with libbpf, libopcodes,
>> readline, ncurses, libcap, libz and all needed stuff. Was not easy but I
>> made it.
> 
> Libcap is optional and bpftool does not use readline or ncurses. May I
> ask how you tried to build it?
> 
>>
>> Now, how do I use it ?
>>
>> Let say I want to dump the jitted code generated from a call to
>> 'tcpdump'. How do I do that with 'bpftool prog dump jited' ?
>>
>> I thought by calling this line I would then get programs dumped in a way
>> or another just like when setting 'bpf_jit_enable=2', but calling that
>> line just provides me some bpftool help text.
> 
> Well the purpose of this text is to help you find the way to call
> bpftool to do what you want :). For dumping your programs' instructions,
> you need to tell bpftool what program to dump: Bpftool isn't waiting
> until you load a program to dump it, instead you need to load your
> program first and then tell bpftool to retrieve the instructions from
> the kernel. To reference your program you could use a pinned path, or
> first list the programs on your system with "bpftool prog show":
> 
> 
>      # bpftool prog show
>      138: tracing  name foo  tag e54c922dfa54f65f  gpl
>              loaded_at 2021-02-25T01:32:30+0000  uid 0
>              xlated 256B  jited 154B  memlock 4096B  map_ids 64
>              btf_id 235

Got the following error:

root@...ip:~# ./bpftool prog show
libbpf: elf: endianness mismatch in pid_iter_bpf.
libbpf: failed to initialize skeleton BPF object 'pid_iter_bpf': -4003
Error: failed to open PID iterator skeleton


Christophe