netdev - Re: [PATCH bpf-next 4/5] btf: fix bug with resolving STRUCT/UNION into corresponding FWD

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Date:   Thu, 28 Feb 2019 10:29:02 -0800
From:   Song Liu <liu.song.a23@...il.com>
To:     Andrii Nakryiko <andriin@...com>
Cc:     Andrii Nakryiko <andrii.nakryiko@...il.com>, kernel-team@...com,
        ast@...com, Arnaldo Carvalho de Melo <acme@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf-next 4/5] btf: fix bug with resolving STRUCT/UNION
 into corresponding FWD

On Wed, Feb 27, 2019 at 2:48 PM Andrii Nakryiko <andriin@...com> wrote:
>
> When checking available canonical candidates for struct/union algorithm
> utilizes btf_dedup_is_equiv to determine if candidate is suitable. This
> check is not enough when candidate is corresponding FWD for that
> struct/union, because according to equivalence logic they are
> equivalent. When it so happens that FWD and STRUCT/UNION end in hashing
> to the same bucket, it's possible to create remapping loop from FWD to
> STRUCT and STRUCT to same FWD, which will cause btf_dedup() to loop
> forever.
>
> This patch fixes the issue by additionally checking that type and
> canonical candidate are strictly equal (utilizing btf_equal_struct).
>
> Fixes: d5caef5b5655 ("btf: add BTF types deduplication algorithm")
> Reported-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> Signed-off-by: Andrii Nakryiko <andriin@...com>

Acked-by: Song Liu <songliubraving@...com>

> ---
>  tools/lib/bpf/btf.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
> index 6bbb710216e6..53db26d158c9 100644
> --- a/tools/lib/bpf/btf.c
> +++ b/tools/lib/bpf/btf.c
> @@ -2255,7 +2255,7 @@ static void btf_dedup_merge_hypot_map(struct btf_dedup *d)
>  static int btf_dedup_struct_type(struct btf_dedup *d, __u32 type_id)
>  {
>         struct btf_dedup_node *cand_node;
> -       struct btf_type *t;
> +       struct btf_type *cand_type, *t;
>         /* if we don't find equivalent type, then we are canonical */
>         __u32 new_id = type_id;
>         __u16 kind;
> @@ -2275,6 +2275,10 @@ static int btf_dedup_struct_type(struct btf_dedup *d, __u32 type_id)
>         for_each_dedup_cand(d, h, cand_node) {
>                 int eq;
>
> +               cand_type = d->btf->types[cand_node->type_id];
> +               if (!btf_equal_struct(t, cand_type))
> +                       continue;
> +
>                 btf_dedup_clear_hypot_map(d);
>                 eq = btf_dedup_is_equiv(d, type_id, cand_node->type_id);
>                 if (eq < 0)
> --
> 2.17.1
>