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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Mon, 9 May 2022 17:24:33 -0700
Message-ID: <CAEf4BzZ=_C6pOjoLKpbZaWB0=28YhbPvU3f9sS5P1W3Lc_NQHQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2] 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>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Quentin Monnet <quentin@isovalent.com>,
        Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Alexander Lobakin <alexandr.lobakin@intel.com>
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On Thu, May 5, 2022 at 6:17 AM Larysa Zaremba <larysa.zaremba@intel.com> wrote:
>
> Currently, dumping almost all BTFs specified by id requires
> using the -B option to pass the base BTF. For kernel module
> BTFs 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 and the ID corresponds to a kernel
> module BTF.

It feels sloppy to first try without base BTF and then fallback to
base BTF. When specified ID of BTF object, let's just get its struct
bpf_btf_info with bpf_obj_get_info_by_fd() and then check that
kernel_btf is set and name isn't "vmlinux". This will mean it's kernel
module, so load base BTF from /sys/kernel/btf/vmlinux. If that fails,
there is no way that kernel module BTF will be successfully loaded, so
there is no point in trying.

>
> Signed-off-by: Larysa Zaremba <larysa.zaremba@intel.com>
> Reviewed-by: Alexander Lobakin <alexandr.lobakin@intel.com>
> ---
>  tools/bpf/bpftool/btf.c | 67 +++++++++++++++++++++++++++++++++++------
>  1 file changed, 58 insertions(+), 9 deletions(-)
>

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