Re: [PATCHv3 bpf-next 1/8] bpf: Honor const on generic kfunc memory arguments

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Nov 05, 2026 at 04:34:44PM -0800, Andrii Nakryiko wrote:

SNIP

> > +/* A kfunc only reads through 'const T *arg'. The compiler enforces it. */
> > +static enum bpf_type_flag kfunc_arg_mem_flag(const struct btf *btf,
> > +                                            const struct btf_param *arg)
> > +{
> > +       const struct btf_type *t = btf_type_skip_modifiers(btf, arg->type, NULL);
> > +       u32 id, ref_id;
> > +
> > +       btf_type_skip_modifiers(btf, t->type, &ref_id);
> > +       for (id = t->type; id != ref_id; id = t->type) {
> > +               t = btf_type_by_id(btf, id);
> > +               if (btf_kind(t) == BTF_KIND_CONST)
> > +                       return MEM_RDONLY;
> > +       }
> 
> quite weird implementation, skip modifier, and then re-walk
> modifiers... while not have a loop from t->type while
> btf_type_is_modifier() and if you find btf_kind(t) == BTF_KIND_CONST
> -> return true?

the intention was not to introduce another helper with modifier
classification, the tradeoff is 2 loops [1]

jirka


[1] https://lore.kernel.org/bpf/DMLRV3E28SC8.3FJZXLBLZI4DD@gmail.com/

> 
> 
> > +       return MEM_WRITE;
> > +}
> > +
> >  static bool is_kfunc_arg_constant(const struct btf *btf, const struct btf_param *arg)
> >  {
> >         return btf_param_match_suffix(btf, arg, "__k");
> > @@ -13618,7 +13634,8 @@ static int resolve_func_arg_type(struct bpf_verifier_env *env,
> >                         PTR_ERR(resolve_ret));
> >                 return -EINVAL;
> >         }
> > -       *arg_type = ARG_PTR_TO_MEM | MEM_FIXED_SIZE | MEM_WRITE |
> > +       *arg_type = ARG_PTR_TO_MEM | MEM_FIXED_SIZE |
> > +                   kfunc_arg_mem_flag(meta->btf, &args[arg]) |
> >                     (*arg_type & (PTR_MAYBE_NULL | MEM_UNINIT));
> >
> >         return 0;
> > @@ -14170,7 +14187,7 @@ get_kfunc_arg_type(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
> >                                 reg_arg_name(env, argno), btf_type_str(ref_t), ref_tname);
> >                         return -EINVAL;
> >                 }
> > -               arg_type = ARG_PTR_TO_MEM | MEM_WRITE;
> > +               arg_type = ARG_PTR_TO_MEM | kfunc_arg_mem_flag(meta->btf, &args[arg]);
> >         } else if (btf_type_is_struct(ref_t))
> >                 /* A pointer to a struct without a size argument is classified as ARG_PTR_TO_BTF_ID */
> >                 arg_type = ARG_PTR_TO_BTF_ID;
> > @@ -14195,7 +14212,8 @@ get_kfunc_arg_type(struct bpf_verifier_env *env, struct bpf_call_arg_meta *meta,
> >                         return -EINVAL;
> >                 }
> >                 proto->arg_size[arg] = type_size;
> > -               arg_type = ARG_PTR_TO_MEM | MEM_FIXED_SIZE | MEM_WRITE;
> > +               arg_type = ARG_PTR_TO_MEM | MEM_FIXED_SIZE |
> > +                          kfunc_arg_mem_flag(meta->btf, &args[arg]);
> >         }
> >
> >         if (is_kfunc_arg_uninit(meta->btf, &args[arg]))
> > --
> > 2.43.0
> >