Subject: Re: [PATCH v4 bpf-next 05/14] bpf: Remove btf_id helpers resolving

From: Jiri Olsa <[email-masked]>
Date: Sun, 28 Jun 2020 22:16:08 +0200

On Fri, Jun 26, 2020 at 02:36:37PM -0700, Yonghong Song wrote:

SNIP

> > -	}
> > -
> > -	t = btf_type_by_id(btf_vmlinux, t->type);
> > -	if (!btf_type_is_ptr(t))
> > -		return -EFAULT;
> > -	t = btf_type_by_id(btf_vmlinux, t->type);
> > -	if (!btf_type_is_func_proto(t))
> > -		return -EFAULT;
> > -
> > -	args = (const struct btf_param *)(t + 1);
> > -	if (arg >= btf_type_vlen(t)) {
> > -		bpf_log(log, "bpf helper %s doesn't have %d-th argument\n",
> > -			fnname, arg);
> > +	if (WARN_ON_ONCE(!fn->btf_id))
> 
> The original code does not have this warning. It directly did
> "ret = READ_ONCE(*btf_id);" after testing reg arg type ARG_PTR_TO_BTF_ID.

not sure why I put it in there, it's probably enough guarded
by arg_type having ARG_PTR_TO_BTF_ID, will remove

> 
> >   		return -EINVAL;
> > -	}
> > -	t = btf_type_by_id(btf_vmlinux, args[arg].type);
> > -	if (!btf_type_is_ptr(t) || !t->type) {
> > -		/* anything but the pointer to struct is a helper config bug */
> > -		bpf_log(log, "ARG_PTR_TO_BTF is misconfigured\n");
> > -		return -EFAULT;
> > -	}
> > -	btf_id = t->type;
> > -	t = btf_type_by_id(btf_vmlinux, t->type);
> > -	/* skip modifiers */
> > -	while (btf_type_is_modifier(t)) {
> > -		btf_id = t->type;
> > -		t = btf_type_by_id(btf_vmlinux, t->type);
> > -	}
> > -	if (!btf_type_is_struct(t)) {
> > -		bpf_log(log, "ARG_PTR_TO_BTF is not a struct\n");
> > -		return -EFAULT;
> > -	}
> > -	bpf_log(log, "helper %s arg%d has btf_id %d struct %s\n", fnname + 4,
> > -		arg, btf_id, __btf_name_by_offset(btf_vmlinux, t->name_off));
> > -	return btf_id;
> > -}
> > +	id = fn->btf_id[arg];
> 
> The corresponding BTF_ID definition here is:
>   BTF_ID_LIST(bpf_skb_output_btf_ids)
>   BTF_ID(struct, sk_buff)
> 
> The bpf helper writer needs to ensure proper declarations
> of BTF_IDs like the above matching helpers definition.
> Support we have arg1 and arg3 as BTF_ID. then the list
> definition may be
> 
>   BTF_ID_LIST(bpf_skb_output_btf_ids)
>   BTF_ID(struct, sk_buff)
>   BTF_ID(struct, __unused)
>   BTF_ID(struct, task_struct)
> 
> This probably okay, I guess.

right, AFAIK we don't have such case yet, but would be good
to be ready and have something like

  BTF_ID(struct, __unused)

maybe adding new type for that will be better:

  BTF_ID(none, unused)

jirka