netdev - Re: [PATCH bpf] tools/bpftool: Add/Fix support for modules btf dump

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Date:   Mon, 7 Dec 2020 19:14:42 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Saeed Mahameed <saeed@...nel.org>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...dia.com>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH bpf] tools/bpftool: Add/Fix support for modules btf dump

On Sun, Dec 6, 2020 at 9:21 PM <saeed@...nel.org> wrote:
>
> From: Saeed Mahameed <saeedm@...dia.com>
>
> While playing with BTF for modules, i noticed that executing the command:
> $ bpftool btf dump id <module's btf id>
>
> Fails due to lack of information in the BTF data.
>
> Maybe I am missing a step but actually adding the support for this is
> very simple.

yes, bpftool takes -B <path> argument for specifying base BTF. So if
you added -B /sys/kernel/btf/vmlinux, it should have worked. I've
added auto-detection logic for the case of `btf dump file
/sys/kernel/btf/<module>` (see [0]), and we can also add it for when
ID corresponds to a module BTF. But I think it's simplest to re-use
the logic and just open /sys/kernel/btf/vmlinux, instead of adding
narrowly-focused libbpf API for that.

>
> To completely parse modules BTF data, we need the vmlinux BTF as their
> "base btf", which can be easily found by iterating through the btf ids and
> looking for btf.name == "vmlinux".
>
> I am not sure why this hasn't been added by the original patchset

because I never though of dumping module BTF by id, given there is
nicely named /sys/kernel/btf/<module> :)

> "Integrate kernel module BTF support", as adding the support for
> this is very trivial. Unless i am missing something, CCing Andrii..
>
> Signed-off-by: Saeed Mahameed <saeedm@...dia.com>
> CC: Andrii Nakryiko <andrii@...nel.org>
> ---
>  tools/lib/bpf/btf.c      | 57 ++++++++++++++++++++++++++++++++++++++++
>  tools/lib/bpf/btf.h      |  2 ++
>  tools/lib/bpf/libbpf.map |  1 +
>  3 files changed, 60 insertions(+)
>

[...]