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

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Date:   Fri, 23 Apr 2021 11:26:16 +0100
From:   Quentin Monnet <quentin@...valent.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Ian Rogers <irogers@...gle.com>, Song Liu <songliubraving@...com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.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

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

Then you can use for example the program id displayed on the first line
to reference and dump your program:

    # bpftool prog dump jited id 138

You should find additional documentation under
tools/bpf/bpftool/Documentation.

> 
> By the way, I would be nice to have a kernel OPTION that selects all
> OPTIONS required for building bpftool. Because you discover them one by
> one at every build failure. I had to had CONFIG_IPV6, CONFIG_DEBUG_BTF,
> CONFIG_CGROUPS, ... If there could be an option like "Build a 'bpftool'
> ready kernel" that selected all those, it would be great.
> 
> Christophe

I do not believe any of these are required to build bpftool.

Quentin