netdev - Re: [PATCH bpf-next v4 12/21] bpf: add PTR_TO_BTF_ID_OR

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Date:   Sun, 10 May 2020 10:05:17 -0700
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Network Development <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next v4 12/21] bpf: add PTR_TO_BTF_ID_OR_NULL support



On 5/10/20 9:11 AM, Alexei Starovoitov wrote:
> On Sat, May 9, 2020 at 10:19 PM Yonghong Song <yhs@...com> wrote:
>>
>>
>>
>> On 5/9/20 5:50 PM, Alexei Starovoitov wrote:
>>> On Sat, May 09, 2020 at 10:59:12AM -0700, Yonghong Song wrote:
>>>> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
>>>> index a2cfba89a8e1..c490fbde22d4 100644
>>>> --- a/kernel/bpf/btf.c
>>>> +++ b/kernel/bpf/btf.c
>>>> @@ -3790,7 +3790,10 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>>>>               return true;
>>>>
>>>>       /* this is a pointer to another type */
>>>> -    info->reg_type = PTR_TO_BTF_ID;
>>>> +    if (off != 0 && prog->aux->btf_id_or_null_non0_off)
>>>> +            info->reg_type = PTR_TO_BTF_ID_OR_NULL;
>>>> +    else
>>>> +            info->reg_type = PTR_TO_BTF_ID;
>>>
>>> I think the verifier should be smarter than this.
>>> It's too specific and inflexible. All ctx fields of bpf_iter execpt first
>>> will be such ? let's figure out a different way to tell verifier about this.
>>> How about using typedef with specific suffix? Like:
>>> typedef struct bpf_map *bpf_map_or_null;
>>>    struct bpf_iter__bpf_map {
>>>      struct bpf_iter_meta *meta;
>>>      bpf_map_or_null map;
>>>    };
>>> or use a union with specific second member? Like:
>>>    struct bpf_iter__bpf_map {
>>>      struct bpf_iter_meta *meta;
>>>      union {
>>>        struct bpf_map *map;
>>>        long null;
>>>      };
>>>    };
>>
>> I have an alternative approach to refactor this for future
>> support for map elements as well.
>>
>> For example, for bpf_map_elements iterator the prog context type
>> can be
>>       struct bpf_iter_bpf_map_elem {
>>          struct bpf_iter_meta *meta;
>>          strruct bpf_map *map;
>>          <key type>  *key;
>>          <value type> *val;
>>      };
>>
>> target will pass the following information to bpf_iter registration:
>>      arg 1: PTR_TO_BTF_ID
>>      arg 2: PTR_TO_BTF_ID_OR_NULL
>>      arg 3: PTR_TO_BUFFER
>>      arg 4: PTR_TO_BUFFER
>>
>> verifier will retrieve the reg_type from target.
> 
> you mean to introduce something like 'struct bpf_func_proto'
> that describes types of helpers, but instead something similar
> to clarify the types in ctx ? That should work. Thanks

Yes, this is what I think will be extensible.