netdev - Re: debug annotations for bpf progs. Was: [PATCH bpf-next 1/3] bpf: preserve command of the process that loaded the program

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Date:   Thu, 17 Oct 2019 23:19:47 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...ichev.me>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>, Yonghong Song <yhs@...com>
Subject: Re: debug annotations for bpf progs. Was: [PATCH bpf-next 1/3] bpf:
 preserve command of the process that loaded the program

On Thu, Oct 17, 2019 at 9:28 AM Stanislav Fomichev <sdf@...ichev.me> wrote:
> Tried to do the following:
>
> 1. Add: static volatile const char __annotate_source1[] = __FILE__;
>    to test_rdonly_maps.c and I think it got optimized away :-/
>    At least I don't see it in the 'bpftool btf dump' output.
>
> 2. Add: char __annotate_source2[] SEC(".meta") = __FILE__;
>    to test_rdonly_maps.c and do all the required plumbing in libbpf
>    to treat .meta like .rodata. I think it works, but the map
>    disappears after bpftool exits because this data is not referenced
>    in the prog and the refcount drops to zero :-(
>
> Am I missing something?

"Some assembly required".

I think first variant should work at the end.
I don't think extra section should be a requirement.
Sounds like with 2 it's pretty close. Just need to make sure
that prog side sees the map as referenced.
And more importantly bpftool can find that map from prog later.