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

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Date:   Tue, 29 Nov 2022 16:59:22 +0800
From:   Changbin Du <changbin.du@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
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 1/2] libbpf: show more info about missing ".BTF" section

On Mon, Nov 28, 2022 at 09:59:00PM -0800, Andrii Nakryiko wrote:
> On Sat, Nov 26, 2022 at 3:13 AM Changbin Du <changbin.du@...il.com> wrote:
> >
> > Show more information about why failed instead of just saying "No such file or
> > directory".
> >
> > Now will print below info:
> > libbpf: can not find '.BTF' section
> > libbpf: is CONFIG_DEBUG_INFO_BTF enabled for kernel?
> > Error: failed to load BTF from /home/changbin/work/linux/vmlinux: No such file or directory
> >
> > Signed-off-by: Changbin Du <changbin.du@...il.com>
> > ---
> >  tools/lib/bpf/btf.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> >
> > diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> > index d88647da2c7f..3f661d991808 100644
> > --- a/tools/lib/bpf/btf.c
> > +++ b/tools/lib/bpf/btf.c
> > @@ -906,6 +906,15 @@ struct btf *btf__new(const void *data, __u32 size)
> >         return libbpf_ptr(btf_new(data, size, NULL));
> >  }
> >
> > +static bool is_vmlinux(const char *path)
> > +{
> > +       size_t path_len = strlen(path);
> > +       size_t suffix_len = strlen("vmlinux");
> > +
> > +       return (path_len >= suffix_len) &&
> > +              (!memcmp(path + path_len - suffix_len, "vmlinux", suffix_len));
> > +}
> > +
> >  static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
> >                                  struct btf_ext **btf_ext)
> >  {
> > @@ -990,6 +999,9 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
> >         err = 0;
> >
> >         if (!btf_data) {
> > +               pr_warn("can not find '%s' section\n", BTF_ELF_SEC);
> > +               if (is_vmlinux(path))
> > +                       pr_warn("is CONFIG_DEBUG_INFO_BTF enabled for kernel?\n");
> 
> this is generic piece of BTF loading code in libbpf, it knows nothing
> and should know nothing about vmlinux and CONFIG_DEBUG_INFO_BTF, this
> is not the right place to add such suggestions.
> 
> Check bpf_object__load_vmlinux_btf(), libbpf emits vmlinux-specific
> error there: "Error loading vmlinux BTF". If we need to mention
> CONFIG_DEBUG_INFO_BTF anywhere, that would be the place to do that.
>
Agreed. I think adding "can not find '.BTF' section" is enough to diagnose the problem.
The standalone error msg "No such file or directory" is really weird.

> >                 err = -ENOENT;
> >                 goto done;
> >         }
> > --
> > 2.37.2
> >

-- 
Cheers,
Changbin Du