netdev - Re: [PATCH 1/6] bpf: Allow ctx access for pointers to scalar

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Message-ID: <CAADnVQKeR1VFEaRGY7Zy=P7KF8=TKshEy2inhFfi9qis9osS3A@mail.gmail.com>
Date:   Tue, 21 Jan 2020 17:51:12 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Andrii Nakryiko <andriin@...com>,
        Yonghong Song <yhs@...com>, Martin KaFai Lau <kafai@...com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        David Miller <davem@...hat.com>,
        Björn Töpel <bjorn.topel@...el.com>
Subject: Re: [PATCH 1/6] bpf: Allow ctx access for pointers to scalar

On Tue, Jan 21, 2020 at 4:05 AM Jiri Olsa <jolsa@...nel.org> wrote:
>
> When accessing the context we allow access to arguments with
> scalar type and pointer to struct. But we omit pointer to scalar
> type, which is the case for many functions and same case as
> when accessing scalar.
>
> Adding the check if the pointer is to scalar type and allow it.
>
> Acked-by: John Fastabend <john.fastabend@...il.com>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  kernel/bpf/btf.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 832b5d7fd892..207ae554e0ce 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3668,7 +3668,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>                     const struct bpf_prog *prog,
>                     struct bpf_insn_access_aux *info)
>  {
> -       const struct btf_type *t = prog->aux->attach_func_proto;
> +       const struct btf_type *tp, *t = prog->aux->attach_func_proto;
>         struct bpf_prog *tgt_prog = prog->aux->linked_prog;
>         struct btf *btf = bpf_prog_get_target_btf(prog);
>         const char *tname = prog->aux->attach_func_name;
> @@ -3730,6 +3730,17 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>                  */
>                 return true;
>
> +       tp = btf_type_by_id(btf, t->type);
> +       /* skip modifiers */
> +       while (btf_type_is_modifier(tp))
> +               tp = btf_type_by_id(btf, tp->type);
> +
> +       if (btf_type_is_int(tp) || btf_type_is_enum(tp))
> +               /* This is a pointer scalar.
> +                * It is the same as scalar from the verifier safety pov.
> +                */
> +               return true;

The reason I didn't do it earlier is I was thinking to represent it
as PTR_TO_BTF_ID as well, so that corresponding u8..u64
access into this memory would still be possible.
I'm trying to analyze the situation that returning a scalar now
and converting to PTR_TO_BTF_ID in the future will keep progs
passing the verifier. Is it really the case?
Could you give a specific example that needs this support?
It will help me understand this backward compatibility concern.
What prog is doing with that 'u32 *' that is seen as scalar ?
It cannot dereference it. Use it as what?