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References: <20220428110839.111042-1-larysa.zaremba@intel.com>
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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Thu, 28 Apr 2022 21:58:58 -0700
Message-ID: <CAEf4BzZRioYpgsUFP1TLsqtjtvA3WLyuWjSyq12ctUoMqkUorg@mail.gmail.com>
Subject: Re: [PATCH] bpftool: Use sysfs vmlinux when dumping BTF by ID
To:     Larysa Zaremba <larysa.zaremba@intel.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Alexander Lobakin <alexandr.lobakin@intel.com>
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On Thu, Apr 28, 2022 at 4:17 AM Larysa Zaremba <larysa.zaremba@intel.com> wrote:
>
> Currently, dumping almost all BTFs specified by id requires

It should and will work only for kernel modules. It won't and
shouldn't work for BTFs coming from BPF programs. We shouldn't blindly
guess and substitute vmlinux BTF as base BTF, let's fetch
bpf_btf_info, check that BTF is from kernel and is not vmlinux, and
only in such case substitute vmlinux BTF as base BTF.

> using the -B option to pass the base BTF. For most cases
> the vmlinux BTF sysfs path should work.
>
> This patch simplifies dumping by ID usage by attempting to
> use vmlinux BTF from sysfs, if the first try of loading BTF by ID
> fails with certain conditions.
>
> Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
> Reviewed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
> ---
>  tools/bpf/bpftool/btf.c | 35 ++++++++++++++++++++++++++---------
>  1 file changed, 26 insertions(+), 9 deletions(-)
>

[...]