netdev - Re: [PATCH bpf-next 1/2] bpf: Fix context type resolving for extension programs

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Message-ID: <CAADnVQ+3sBR7dTQhX+eHvzJajtnm0QctjrWFyc+LMkHJOoOabA@mail.gmail.com>
Date:   Thu, 10 Sep 2020 11:00:45 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eelco Chaudron <echaudro@...hat.com>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH bpf-next 1/2] bpf: Fix context type resolving for
 extension programs

On Wed, Sep 9, 2020 at 8:11 AM Jiri Olsa <jolsa@...nel.org> wrote:
>
> Eelco reported we can't properly access arguments if the tracing
> program is attached to extension program.
>
> Having following program:
>
>   SEC("classifier/test_pkt_md_access")
>   int test_pkt_md_access(struct __sk_buff *skb)
>
> with its extension:
>
>   SEC("freplace/test_pkt_md_access")
>   int test_pkt_md_access_new(struct __sk_buff *skb)
>
> and tracing that extension with:
>
>   SEC("fentry/test_pkt_md_access_new")
>   int BPF_PROG(fentry, struct sk_buff *skb)
>
> It's not possible to access skb argument in the fentry program,
> with following error from verifier:
>
>   ; int BPF_PROG(fentry, struct sk_buff *skb)
>   0: (79) r1 = *(u64 *)(r1 +0)
>   invalid bpf_context access off=0 size=8
>
> The problem is that btf_ctx_access gets the context type for the
> traced program, which is in this case the extension.
>
> But when we trace extension program, we want to get the context
> type of the program that the extension is attached to, so we can
> access the argument properly in the trace program.
>
> Reported-by: Eelco Chaudron <echaudro@...hat.com>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  kernel/bpf/btf.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index f9ac6935ab3c..37ad01c32e5a 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -3859,6 +3859,14 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>         }
>
>         info->reg_type = PTR_TO_BTF_ID;
> +
> +       /* When we trace extension program, we want to get the context
> +        * type of the program that the extension is attached to, so
> +        * we can access the argument properly in the trace program.
> +        */
> +       if (tgt_prog && tgt_prog->type == BPF_PROG_TYPE_EXT)
> +               tgt_prog = tgt_prog->aux->linked_prog;
> +
>         if (tgt_prog) {
>                 ret = btf_translate_to_vmlinux(log, btf, t, tgt_prog->type, arg);

I think it would be cleaner to move resolve_prog_type() from verifier.c
and use that helper function here.