netdev - Re: [RFC bpf-next 2/3] libbpf: bpf__find_by_name[_kind] should use btf__get_nr

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Date:   Fri, 13 Nov 2020 22:46:42 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alan Maguire <alan.maguire@...cle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Hao Luo <haoluo@...gle.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Quentin Monnet <quentin@...valent.com>,
        open list <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [RFC bpf-next 2/3] libbpf: bpf__find_by_name[_kind] should use btf__get_nr_types()

On Fri, Nov 13, 2020 at 10:11 AM Alan Maguire <alan.maguire@...cle.com> wrote:
>
> When operating on split BTF, btf__find_by_name[_kind] will not
> iterate over all types since they use btf->nr_types to show
> the number of types to iterate over.  For split BTF this is
> the number of types _on top of base BTF_, so it will
> underestimate the number of types to iterate over, especially
> for vmlinux + module BTF, where the latter is much smaller.
>
> Use btf__get_nr_types() instead.
>
> Signed-off-by: Alan Maguire <alan.maguire@...cle.com>
> ---

Good catch. I'm amazed I didn't fix it up when I implemented split BTF
support, I distinctly remember looking at these two APIs...

Can you please add Fixes tag and post this as a separate patch? There
is no need to wait on all the other changes.

Fixes: ba451366bf44 ("libbpf: Implement basic split BTF support")

>  tools/lib/bpf/btf.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 2d0d064..0fccf4b 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -679,7 +679,7 @@ __s32 btf__find_by_name(const struct btf *btf, const char *type_name)
>         if (!strcmp(type_name, "void"))
>                 return 0;
>
> -       for (i = 1; i <= btf->nr_types; i++) {
> +       for (i = 1; i <= btf__get_nr_types(btf); i++) {

I think it's worthwhile to cache the result of btf__get_nr_types(btf)
in a local variable instead of re-calculating it thousands of times.

>                 const struct btf_type *t = btf__type_by_id(btf, i);
>                 const char *name = btf__name_by_offset(btf, t->name_off);
>
> @@ -698,7 +698,7 @@ __s32 btf__find_by_name_kind(const struct btf *btf, const char *type_name,
>         if (kind == BTF_KIND_UNKN || !strcmp(type_name, "void"))
>                 return 0;
>
> -       for (i = 1; i <= btf->nr_types; i++) {
> +       for (i = 1; i <= btf__get_nr_types(btf); i++) {

same as above


>                 const struct btf_type *t = btf__type_by_id(btf, i);
>                 const char *name;
>
> --
> 1.8.3.1
>