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From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Tue, 3 Jan 2023 15:46:44 -0800
Message-ID: <CAEf4BzZMJGrRhNeQeWB0fRsuRYUv01aZGhvDeFV2o5zdpRbR-w@mail.gmail.com>
Subject: Re: [PATCH v3 1/2] libbpf: show error info about missing ".BTF" section
To:     Quentin Monnet <quentin@isovalent.com>
Cc:     Leo Yan <leo.yan@linaro.org>, 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, Jan 3, 2023 at 7:03 AM Quentin Monnet <quentin@isovalent.com> wrote:
>
> 2022-12-20 16:13 UTC-0800 ~ Andrii Nakryiko <andrii.nakryiko@gmail.com>
> > On Tue, Dec 20, 2022 at 3:34 AM Leo Yan <leo.yan@linaro.org> wrote:
> >>
> >> On Tue, Dec 20, 2022 at 09:31:14AM +0800, Changbin Du wrote:
> >>
> >> [...]
> >>
> >>>>> Now will print below info:
> >>>>> libbpf: failed to find '.BTF' ELF section in /home/changbin/work/linux/vmlinux
> >>>>
> >>>> Recently I encountered the same issue, it could be caused by:
> >>>> either missing to install tool pahole or missing to enable kernel
> >>>> configuration CONFIG_DEBUG_INFO_BTF.
> >>>>
> >>>> Could we give explict info for reasoning failure?  Like:
> >>>>
> >>>> "libbpf: failed to find '.BTF' ELF section in /home/changbin/work/linux/vmlinux,
> >>>> please install pahole and enable CONFIG_DEBUG_INFO_BTF=y for kernel building".
> >>>>
> >>> This is vmlinux special information and similar tips are removed from
> >>> patch V2. libbpf is common for all ELFs.
> >>
> >> Okay, I see.  Sorry for noise.
> >>
> >>>>> Error: failed to load BTF from /home/changbin/work/linux/vmlinux: No such file or directory
> >>>>
> >>>> This log is confusing when we can find vmlinux file but without BTF
> >>>> section.  Consider to use a separate patch to detect vmlinux not
> >>>> found case and print out "No such file or directory"?
> >>>>
> >>> I think it's already there. If the file doesn't exist, open will fail.
> >>
> >> [...]
> >>
> >>>>> @@ -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.
> >
> > 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?
>
> Apologies for the delay.
> What you're proposing is to replace all messages currently looking like
> this:
>
>         $ bpftool prog
>         Error: can't get next program: Operation not permitted
>
> by:
>
>         $ bpftool prog
>         Error: can't get next program: -EPERM
>
> Do I understand correctly?

yep, that's what I had in mind

>
> I think the strerror() messages are helpful in some occasions (they
> _are_ more human-friendly to many users), but it's also true that
> they're not always precise. With bpftool, "Invalid argument" is a
> classic when the program doesn't load, and may lead to confusion with
> the args passed to bpftool on the command line. Then there are the other
> corner cases like the one discussed in this thread. So, why not.

maybe the right approach would be to have both symbolic error name and
its human-readable representation, so for example above

Error: can't get next program: [-EPERM] Operation not permitted

or something like that? And if error value is unknown, just keep it as
integer: "[-5555]" ?

>
> If we do change, yeah I'd rather have as much of this handling in libbpf
> itself, and then adjust bpftool to handle the remaining cases, for
> consistency.

we can teach libbpf_strerror_r() to do this and if bpftool is going to
use it consistently then it would get the benefit automatically

>
> Quentin