Subject: Re: [PATCH] net: mac80211: tx.c: be sure of 'sdata->vif.type' must be NL80211_IFTYPE_AP when be in NL80211_IFTYPE_AP case

From: Chen Gang <[email-masked]>
Date: Sat, 30 Nov 2013 22:02:05 +0800

On 11/30/2013 09:50 PM, Chen Gang wrote:
> On 11/30/2013 08:53 PM, Johannes Berg wrote:
>> On Sat, 2013-11-30 at 19:59 +0800, Chen Gang wrote:
>>> On 11/29/2013 11:38 PM, Johannes Berg wrote:
>>>>
>>>>> +++ b/net/mac80211/tx.c
>>>>> @@ -1814,8 +1814,9 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
>>>>>  			break;
>>>>>  		/* fall through */
>>>>>  	case NL80211_IFTYPE_AP:
>>>>> -		if (sdata->vif.type == NL80211_IFTYPE_AP)
>>>>> -			chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
>>>>> +		if (sdata->vif.type != NL80211_IFTYPE_AP)
>>>>> +			goto fail_rcu;
>>>>> +		chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
>>>>
>>>> This change is completely wrong.
>>>>
>>>
>>> Oh, it is.
>>>
>>> Hmm... for me, this work flow still can be implemented with a little
>>> clearer way (at least it will avoid related warning):
>>>
>>> -------------------------diff begin------------------------------
>>>
>>> diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
>>> index c558b24..7076128 100644
>>> --- a/net/mac80211/tx.c
>>> +++ b/net/mac80211/tx.c
>>> @@ -1810,14 +1810,14 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
>>>  		if (!chanctx_conf)
>>>  			goto fail_rcu;
>>>  		band = chanctx_conf->def.chan->band;
>>> -		if (sta)
>>> -			break;
>>> -		/* fall through */
>>> +		if (!sta)
>>> +			goto try_next;
>>> +		break;
>>>  	case NL80211_IFTYPE_AP:
>>> -		if (sdata->vif.type == NL80211_IFTYPE_AP)
>>> -			chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
>>> +		chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
>>>  		if (!chanctx_conf)
>>>  			goto fail_rcu;
>>> +try_next:
>>
>> I don't think that's better than the (fairly obvious) fall-through, and
>> has a pretty odd goto. Also, depending on the compiler, it still knows
>> the previous case label and doesn't warn.
>>
> 
> Yeah, fall-through is obvious. But check 'A' again just near by "case A"
> seems a little strange, and some of compilers (or some of versions) are
> really not quit smart enough to know it is not a warning.
> 

Sorry, the paragraph above may lead misunderstanding, I repeated again:

 - fall-through is obvious (although I did not notice it, originally).

 - Check 'A' again just near by "case A" seems a little strange.

 - Some compilers aren't quit smart enough to know 'chanctx_conf' is OK.

Thanks.

> Hmm... for me, if the code (implementation) can express real logical
> work flow as much as directly and simply, the code (implementation) is
> clear enough (don't mind whether use 'goto' or not).
> 
> 
> And originally, at first, I am really not quite careful enough, and sent
> an incorrect patch after noticed the related compiler's warning. :-)
> 
> 
> Thanks.
> 

-- 
Chen Gang