linux-kernel - Re: [PATCH v3 1/2] libbpf: show error info about missing ".BTF" section

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Date:   Wed, 21 Dec 2022 11:55:24 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Quentin Monnet <quentin@...valent.com>,
        Changbin Du <changbin.du@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Mykola Lysenko <mykolal@...com>,
        linux-perf-users@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 1/2] libbpf: show error info about missing ".BTF"
 section

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?

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.