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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Thu, 22 Dec 2022 10:51:28 -0800
Message-ID: <CAEf4BzaiNOo_qqo16qLACfBe6esg2w+GafqU0TtwZMprpuDBEQ@mail.gmail.com>
Subject: Re: [PATCH v3 1/2] libbpf: show error info about missing ".BTF" section
To:     Leo Yan <leo.yan@linaro.org>
Cc:     Quentin Monnet <quentin@isovalent.com>,
        Changbin Du <changbin.du@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Shuah Khan <shuah@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Mykola Lysenko <mykolal@fb.com>,
        linux-perf-users@vger.kernel.org, linux-kselftest@vger.kernel.org
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On Tue, Dec 20, 2022 at 7:55 PM Leo Yan <leo.yan@linaro.org> wrote:
>
> On Tue, Dec 20, 2022 at 04:13:13PM -0800, Andrii Nakryiko wrote:
>
> [...]
>
> > > > > > @@ -990,6 +990,7 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
> > > > > >   err = 0;
> > > > > >
> > > > > >   if (!btf_data) {
> > > > > > +         pr_warn("failed to find '%s' ELF section in %s\n", BTF_ELF_SEC, path);
> > > > > >           err = -ENOENT;
> > >
> > > btf_parse_elf() returns -ENOENT when ELF file doesn't contain BTF
> > > section, therefore, bpftool dumps error string "No such file or
> > > directory".  It's confused that actually vmlinux is existed.
> > >
> > > I am wondering if we can use error -LIBBPF_ERRNO__FORMAT (or any
> > > better choice?) to replace -ENOENT at here, this can avoid bpftool to
> > > outputs "No such file or directory" in this case.
> >
> > The only really meaningful error code would be -ESRCH, which
> > strerror() will translate to "No such process", which is also
> > completely confusing.
>
> Or maybe -ENODATA (No data available) is a better choice?

-ENODATA sounds good to me, yep.

>
> Thanks,
> Leo
>
> > In general, I always found these strerror() messages extremely
> > unhelpful and confusing. I wonder if we should make an effort to
> > actually emit symbolic names of errors instead (literally, "-ENOENT"
> > in this case). This is all tooling for engineers, I find -ENOENT or
> > -ESRCH much more meaningful as an error message, compared to "No such
> > file" seemingly human-readable interpretation.
> >
> > Quenting, what do you think about the above proposal for bpftool? We
> > can have some libbpf helper internally and do it in libbpf error
> > messages as well and just reuse the logic in bpftool, perhaps?
> >
> > Anyways, I've applied this patch set to bpf-next. Thanks.