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

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Date:   Tue, 20 Dec 2022 09:31:14 +0800
From:   Changbin Du <changbin.du@...il.com>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Changbin Du <changbin.du@...il.com>,
        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 Mon, Dec 19, 2022 at 11:45:08AM +0800, Leo Yan wrote:
> Hi Changbin,
> 
> On Sun, Dec 18, 2022 at 06:35:08AM +0800, Changbin Du wrote:
> > Show the real problem instead of just saying "No such file or directory".
> > 
> > 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.

> > 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.

> Thanks,
> Leo
> 
> > Signed-off-by: Changbin Du <changbin.du@...il.com>
> > ---
> >  tools/lib/bpf/btf.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> > index 71e165b09ed5..dd2badf1a54e 100644
> > --- a/tools/lib/bpf/btf.c
> > +++ b/tools/lib/bpf/btf.c
> > @@ -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;
> >  		goto done;
> >  	}
> > -- 
> > 2.37.2
> > 

-- 
Cheers,
Changbin Du