Re: [PATCH V7 1/2] nl80211: add feature for BSS selection support

[Arend Van Spriel <arend.vanspriel@broadcom.com>]

On 23-2-2016 21:52, Johannes Berg wrote:
> On Tue, 2016-02-23 at 21:46 +0100, Arend Van Spriel wrote:
>>  
>>> Maybe we should be less specific here regarding the NLA_FLAG and
>>> say
>>> that it will contain attributes for each, indicating which
>>> behaviours
>>> are supported, and say there's behaviour-dependent data inside each
>>> of
>>> those attributes?
>>
>> For the GET_WIPHY case there will be not behaviour dependent data.
>>
>>> It seems entirely plausible that future behaviours would require
>>> their
>>> own sub-capabilities; perhaps that's even the case today for having
>>> an
>>> adjustment range for example, not that I think it's really
>>> necessary
>>> now.
>>
>> At least it does not harm to take that into account.
> 
> Right; there isn't today, but there might be later, so for the
> GET_WIPHY case it might be worth just specifying that there will be an
> attribute, not that it's NLA_FLAG?

Will try and be vague :-)

> Not that it'll matter much - only when we actually add such could
> applications possibly want to parse them :)

That goes without saying :-p

>>>> +	/* only process one nested attribute */
>>>> +	nest = nla_data(nla);
>>>> +	if (!nla_ok(nest, nla_len(nest)))
>>>> +		return -EBADF;
>>>
>>> This isn't quite clear to me. Shouldn't you do this by declaring it
>>> NLA_TESTED in the nl80211_policy?
>>
>> Guess you mean NLA_NESTED.
> 
> Heh, yeah.
> 
>>> Actually, not really? you use nla_ok() on the inner (without having
>>> checked at all that it's even long enough btw, since you didn't do
>>> the
>>> policy change), but that would already be done by nla_parse()
>>> below?
>>
>> Because ATTR_BSS_SELECT is used in CMD_GET_WIPHY and CMD_CONNECT the
>> length of the attribute is not a constant. Or do I only need to put
>> length in nl80211_policy for userspace-2-kernel direction, ie. for
>> ATTR_BSS_SELECT in CMD_CONNECT.
> 
> Ah. kernel->userspace messages don't use the policy in any way, it's
> just for the userspace->kernel message validation, so you can put there
> what's needed for CMD_CONNECT and ignore GET_WIPHY.

I actually do not see any NLA_NESTED attributes with an explicit length.
As you mentioned the nla_parse() of the nested attribute will validate
the length of the stream so no need to put that in the policy.

Regards,
Arend