netdev - Re: [PATCH bpf-next 2/2] bpf: Add support to detect and dedup instances of same structs

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Message-ID: <CAEf4BzY70kx8VhK5R-ianrQMZJPPynGV_3+Xc2-9Q5JRE3QKUw@mail.gmail.com>
Date:   Mon, 25 Oct 2021 21:59:19 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next 2/2] bpf: Add support to detect and dedup
 instances of same structs

On Sat, Oct 23, 2021 at 5:05 AM Jiri Olsa <jolsa@...hat.com> wrote:
>
> The s390x compiler generates multiple definitions of the same struct
> and dedup algorithm does not seem to handle this at the moment.
>
> I found code in dedup that seems to handle such situation for arrays,
> and added btf_dedup_is_equiv call for structs.
>
> With this change I can no longer see vmlinux's structs in kernel
> module BTF data, but I have no idea if that breaks anything else.
>

I'm pretty sure this is not the right way to handle this. Let's figure
out what is causing a difference in types (i.e., why they are not
equivalent), before attempting to patch up BTF dedup algorithm.

> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  tools/lib/bpf/btf.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 3a01c4b7f36a..ec164d0cee30 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -3920,8 +3920,16 @@ static int btf_dedup_is_equiv(struct btf_dedup *d, __u32 cand_id,
>                  * types within a single CU. So work around that by explicitly
>                  * allowing identical array types here.
>                  */
> -               return hypot_type_id == cand_id ||
> -                      btf_dedup_identical_arrays(d, hypot_type_id, cand_id);
> +               struct btf_type *t;
> +
> +               if (hypot_type_id == cand_id)
> +                       return 1;
> +               t = btf_type_by_id(d->btf, hypot_type_id);
> +               if (btf_is_array(t))
> +                       return btf_dedup_identical_arrays(d, hypot_type_id, cand_id);
> +               if (btf_is_struct(t))
> +                       return btf_dedup_is_equiv(d, hypot_type_id, cand_id);
> +               return 0;
>         }
>
>         if (btf_dedup_hypot_map_add(d, canon_id, cand_id))
> --
> 2.31.1
>