Subject: Re: [PATCH bpf-next v1 01/13] bpf: Support getting referenced kptr from struct_ops argument

From: Martin KaFai Lau <[email-masked]>
Date: Thu, 19 Dec 2024 15:06:28 -0800

On 12/18/24 8:57 AM, Amery Hung wrote:
>> At the first glance, the ref_obj_id assignment looks racy because ctx_arg_info
>> is shared by different bpf progs that may be verified in parallel. After another
>> thought, this should be fine because it should always end up having the same
>> ref_obj_id for the same arg-no, right? Not sure if UBSAN can understand this
>> without using the READ/WRITE_ONCE. but adding READ/WRITE_ONCE when using
>> ref_obj_id will be quite puzzling when reading the verifier code. Any better idea?
>>
> It looks like ref_obj_id cannot be reused (id always comes from
> ++env->id_gen), and these will be the earliest references to acquire.
> So, maybe we can assume the ref_obj_id without needing to store it in
> ctx_arg_info? E.g., the first __ref argument's ref_obj_id is always 1.

That seems reasonable to me. Then ctx_arg_info can stay read-only after the very 
first initialization during bpf_struct_ops_desc_init().