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Date:   Sat, 9 May 2020 22:18:37 -0700
From:   Yonghong Song <yhs@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Andrii Nakryiko <andriin@...com>, <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>, <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>
Subject: Re: [PATCH bpf-next v4 12/21] bpf: add PTR_TO_BTF_ID_OR_NULL support



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.

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