netdev - Re: [PATCH v4 bpf-next 07/14] bpf: Allow nested BTF object to be refferenced by BTF object + offset

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Date:   Mon, 29 Jun 2020 18:52:21 -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>,
        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>,
        Andrii Nakryiko <andriin@...com>,
        Brendan Gregg <bgregg@...flix.com>,
        Florent Revest <revest@...omium.org>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v4 bpf-next 07/14] bpf: Allow nested BTF object to be
 refferenced by BTF object + offset

On Thu, Jun 25, 2020 at 4:49 PM Jiri Olsa <jolsa@...nel.org> wrote:
>
> Adding btf_struct_address function that takes 2 BTF objects
> and offset as arguments and checks whether object A is nested
> in object B on given offset.
>
> This function will be used when checking the helper function
> PTR_TO_BTF_ID arguments. If the argument has an offset value,
> the btf_struct_address will check if the final address is
> the expected BTF ID.
>
> This way we can access nested BTF objects under PTR_TO_BTF_ID
> pointer type and pass them to helpers, while they still point
> to valid kernel BTF objects.
>
> Using btf_struct_access to implement new btf_struct_address
> function, because it already walks down the given BTF object.
>
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---

This logic is very hard to follow. Each type I try to review it, I get
lost very fast. TBH, this access_data struct is not just not helpful,
but actually just complicates everything.

I'll get to this tomorrow morning with fresh brains and will try to do
another pass.

[...]

>  int btf_resolve_helper_id(struct bpf_verifier_log *log,
>                           const struct bpf_func_proto *fn, int arg)
>  {
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 7de98906ddf4..da7351184295 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3808,6 +3808,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>         struct bpf_reg_state *regs = cur_regs(env), *reg = &regs[regno];
>         enum bpf_reg_type expected_type, type = reg->type;
>         enum bpf_arg_type arg_type = fn->arg_type[arg];
> +       const struct btf_type *btf_type;
>         int err = 0;
>
>         if (arg_type == ARG_DONTCARE)
> @@ -3887,24 +3888,34 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 arg,
>                 expected_type = PTR_TO_BTF_ID;
>                 if (type != expected_type)
>                         goto err_type;
> -               if (!fn->check_btf_id) {
> -                       if (reg->btf_id != meta->btf_id) {
> -                               verbose(env, "Helper has type %s got %s in R%d\n",
> +               if (reg->off) {


This non-zero offset only logic looks fishy, tbh. What if the struct
you are trying to access is at offset zero? E.g., bpf_link is pretty
much always a first field of whatever specific link struct it is
contained within. The fact that we allow only non-zero offsets for
such use case looks like an arbitrary limitation.

> +                       btf_type = btf_type_by_id(btf_vmlinux, reg->btf_id);
> +                       if (btf_struct_address(&env->log, btf_type, reg->off, meta->btf_id)) {
> +                               verbose(env, "Helper has type %s got %s in R%d, off %d\n",
>                                         kernel_type_name(meta->btf_id),
> +                                       kernel_type_name(reg->btf_id), regno, reg->off);
> +                               return -EACCES;
> +                       }

[...]