Re: [PATCH bpf-next v2 2/6] resolve_btfids: Process KF_ARENA_* flags in resolve_btfids

On Wed, 2026-08-05 at 16:06 -0700, Ihor Solodrai wrote:

...

> +static s32 add_arena_tagged_proto(struct btf *btf, struct kfunc *kfunc)
> +{
> +	const struct btf_type *func = btf__type_by_id(btf, kfunc->btf_id);
> +	u32 proto_id = func->type;
> +	const struct btf_type *proto = btf__type_by_id(btf, proto_id);
> +	const struct btf_param *params = btf_params(proto);
> +	u32 nr_params = btf_vlen(proto);
> +	s32 arg0_type_id = nr_params > 0 ? (s32)params[0].type : -1;
> +	s32 arg1_type_id = nr_params > 1 ? (s32)params[1].type : -1;
> +	s32 new_proto_id, id, param_type_id;
> +	s32 ret_type_id = proto->type;
> +	const char *name;
> +	int err;
> +
> +	if (kfunc->flags & KF_ARENA_RET) {
> +		id = arena_tag_ptr(btf, ret_type_id);
> +		if (id < 0) {
> +			pr_err("ERROR: resolve_btfids: kfunc %s: KF_ARENA_RET but return type is not a pointer\n",
> +			       kfunc->name);
> +			return id;
> +		}
> +		ret_type_id = id;
> +	}
> +
> +	if (kfunc->flags & KF_ARENA_ARG1) {

Nit: let's avoid the copy paste and make this code prepared for the
     __arena suffixes by moving the logic inside the parameter
     processing loop below:

     for (i in params) {
       bool add_tag = false;

       param_type_id = params[i].type;
       switch(i) { 0: add_tag = kfunc->flags & KF_ARENA_ARG1; break; ... }
       if (add_tag)
          param_type_id = arena_tag_ptr(btf, param_type_id);
       if (param_type_id < 0)
         ...
     }

> +		if (nr_params < 1) {
> +			pr_err("ERROR: resolve_btfids: kfunc %s: KF_ARENA_ARG1 but it has no argument 1\n",
> +			       kfunc->name);
> +			return -EINVAL;
> +		}
> +		id = arena_tag_ptr(btf, arg0_type_id);
> +		if (id < 0) {
> +			pr_err("ERROR: resolve_btfids: kfunc %s: KF_ARENA_ARG1 but argument 1 is not a pointer\n",
> +			       kfunc->name);

Nit: not a pointer is not the only error condition, btf__add_*()
     functions might fail as well, maybe just push pr_err() down
     to the arena_tag_ptr()?

> +			return id;
> +		}
> +		arg0_type_id = id;
> +	}
> +
> +	if (kfunc->flags & KF_ARENA_ARG2) {
> +		if (nr_params < 2) {
> +			pr_err("ERROR: resolve_btfids: kfunc %s: KF_ARENA_ARG2 but it has no argument 2\n",
> +			       kfunc->name);
> +			return -EINVAL;
> +		}
> +		id = arena_tag_ptr(btf, arg1_type_id);
> +		if (id < 0) {
> +			pr_err("ERROR: resolve_btfids: kfunc %s: KF_ARENA_ARG2 but argument 2 is not a pointer\n",
> +			       kfunc->name);
> +			return id;
> +		}
> +		arg1_type_id = id;
> +	}
> +
> +	new_proto_id = btf__add_func_proto(btf, ret_type_id);
> +	if (new_proto_id < 0) {
> +		pr_err("ERROR: resolve_btfids: kfunc %s: failed to add a func proto to BTF\n",
> +		       kfunc->name);
> +		return new_proto_id;
> +	}
> +
> +	for (u32 i = 0; i < nr_params; i++) {
> +		proto = btf__type_by_id(btf, proto_id);
> +		params = btf_params(proto);

Nit: these two do not need to be in the loop body.

> +		name = btf__name_by_offset(btf, params[i].name_off);
> +
> +		switch (i) {
> +		case 0:
> +			param_type_id = arg0_type_id;
> +			break;
> +		case 1:
> +			param_type_id = arg1_type_id;
> +			break;
> +		default:
> +			param_type_id = params[i].type;
> +			break;
> +		}
> +
> +		err = btf__add_func_param(btf, name ?: "", param_type_id);
> +		if (err < 0) {
> +			pr_err("ERROR: resolve_btfids: kfunc %s: failed to add a proto param to BTF\n",
> +			       kfunc->name);
> +			return err;
> +		}
> +	}
> +
> +	pr_debug("added arena-tagged proto for kfunc %s: %d\n", kfunc->name, new_proto_id);
> +
> +	return new_proto_id;
> +}

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