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

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Date:   Tue, 20 Dec 2022 19:34:21 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Changbin Du <changbin.du@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Quentin Monnet <quentin@...valent.com>,
        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 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.

Thanks,
Leo