Re: [PATCH bpf-next v4 08/12] bpf: verifier: Relax caller requirements for kfunc projection type args

[Alexei Starovoitov]

On Sat, Jun 8, 2024 at 2:16 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
>
> Currently, if a kfunc accepts a projection type as an argument (eg
> struct __sk_buff *), the caller must exactly provide exactly the same
> type with provable provenance.
>
> However in practice, kfuncs that accept projection types _must_ cast to
> the underlying type before use b/c projection type layouts are
> completely made up. Thus, it is ok to relax the verifier rules around
> implicit conversions.
>
> We will use this functionality in the next commit when we align kfuncs
> to user-facing types.
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
>  kernel/bpf/verifier.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 81a3d2ced78d..0808beca3837 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -11257,6 +11257,8 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
>         bool strict_type_match = false;
>         const struct btf *reg_btf;
>         const char *reg_ref_tname;
> +       bool taking_projection;
> +       bool struct_same;
>         u32 reg_ref_id;
>
>         if (base_type(reg->type) == PTR_TO_BTF_ID) {
> @@ -11300,7 +11302,13 @@ static int process_kf_arg_ptr_to_btf_id(struct bpf_verifier_env *env,
>
>         reg_ref_t = btf_type_skip_modifiers(reg_btf, reg_ref_id, &reg_ref_id);
>         reg_ref_tname = btf_name_by_offset(reg_btf, reg_ref_t->name_off);
> -       if (!btf_struct_ids_match(&env->log, reg_btf, reg_ref_id, reg->off, meta->btf, ref_id, strict_type_match)) {
> +       struct_same = btf_struct_ids_match(&env->log, reg_btf, reg_ref_id, reg->off, meta->btf, ref_id, strict_type_match);
> +       /* If kfunc is accepting a projection type (ie. __sk_buff), it cannot
> +        * actually use it -- it must cast to the underlying type. So we allow
> +        * caller to pass in the underlying type.
> +        */
> +       taking_projection = !strcmp(ref_tname, "__sk_buff") && !strcmp(reg_ref_tname, "sk_buff");

xdp_md/buff probably as well?

And with that share the code with btf_is_prog_ctx_type() ?

> +       if (!taking_projection && !struct_same) {
>                 verbose(env, "kernel function %s args#%d expected pointer to %s %s but R%d has a pointer to %s %s\n",
>                         meta->func_name, argno, btf_type_str(ref_t), ref_tname, argno + 1,
>                         btf_type_str(reg_ref_t), reg_ref_tname);
> --
> 2.44.0
>