netdev - Re: [PATCH bpf-next 1/5] bpf: Handle 8-byte values in DEVMAP and DEVMAP

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Date:   Wed, 27 May 2020 07:56:25 -0600
From:   David Ahern <dsahern@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        David Ahern <dsahern@...nel.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        toke@...hat.com, daniel@...earbox.net, john.fastabend@...il.com,
        ast@...nel.org, kafai@...com, songliubraving@...com, yhs@...com,
        andriin@...com
Subject: Re: [PATCH bpf-next 1/5] bpf: Handle 8-byte values in DEVMAP and
 DEVMAP_HASH

On 5/27/20 4:26 AM, Jesper Dangaard Brouer wrote:
>> @@ -108,9 +118,13 @@ static int dev_map_init_map(struct bpf_dtab *dtab, union bpf_attr *attr)
>>  	u64 cost = 0;
>>  	int err;
>>  
>> -	/* check sanity of attributes */
>> +	/* check sanity of attributes. 2 value sizes supported:
>> +	 * 4 bytes: ifindex
>> +	 * 8 bytes: ifindex + prog fd
>> +	 */
>>  	if (attr->max_entries == 0 || attr->key_size != 4 ||
>> -	    attr->value_size != 4 || attr->map_flags & ~DEV_CREATE_FLAG_MASK)
>> +	    (attr->value_size != 4 && attr->value_size != 8) ||
> 
> IMHO we really need to leverage BTF here, as I'm sure we need to do more
> extensions, and this size matching will get more and more unmaintainable.
> 
> With BTF in place, dumping the map via bpftool, will also make the
> fields "self-documenting".
> 
> I will try to implement something that uses BTF for this case (and cpumap).
> 

as mentioned in a past response, BTF does not make any fields special
and this code should not assume it either. You need to know precisely
which 4 bytes is the program fd that needs to be looked up, and that
AFAIK is beyond the scope of BTF.