netdev - Re: [PATCH v9 bpf-next 06/14] bpf: Remove recursion call in btf_struct

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Date:   Tue, 4 Aug 2020 23:12:49 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andriin@...com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Martin KaFai Lau <kafai@...com>,
        David Miller <davem@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Wenbo Zhang <ethercflow@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Brendan Gregg <bgregg@...flix.com>,
        Florent Revest <revest@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v9 bpf-next 06/14] bpf: Remove recursion call in btf_struct_access

On Sat, Aug 1, 2020 at 10:04 AM Jiri Olsa <jolsa@...nel.org> wrote:
>
> Andrii suggested we can simply jump to again label
> instead of making recursion call.
>
> Suggested-by: Andrii Nakryiko <andriin@...com>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  kernel/bpf/btf.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index bc05a24f7361..0f995038b589 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3931,14 +3931,13 @@ int btf_struct_access(struct bpf_verifier_log *log,
>                 /* Only allow structure for now, can be relaxed for
>                  * other types later.
>                  */
> -               elem_type = btf_type_skip_modifiers(btf_vmlinux,
> -                                                   array_elem->type, NULL);
> -               if (!btf_type_is_struct(elem_type))
> +               t = btf_type_skip_modifiers(btf_vmlinux, array_elem->type,
> +                                           NULL);
> +               if (!btf_type_is_struct(t))
>                         goto error;
>
> -               off = (off - moff) % elem_type->size;
> -               return btf_struct_access(log, elem_type, off, size, atype,
> -                                        next_btf_id);
> +               off = (off - moff) % t->size;
> +               goto again;

Transformation looks good, thanks. So:

Acked-by: Andrii Nakryiko <andriin@...com>

But this '% t->size' makes me wonder what will happen when we have an
array of zero-sized structs or multi-dimensional arrays with
dimensions of size 0... I.e.:

struct {} arr[123];

or

int arr[0][0]0];

We should probably be more careful with division here.

>
>  error:
>                 bpf_log(log, "access beyond struct %s at off %u size %u\n",
> --
> 2.25.4
>