netdev - Re: [PATCH] net: mac80211: tx.c: be sure of 'sdata->vif.type' must be NL80211_IFTYPE_AP when be in NL80211_IFTYPE

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Date:	Sat, 30 Nov 2013 12:39:00 -0800
From:	Joe Perches <joe@...ches.com>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Chen Gang <gang.chen.5i5j@...il.com>,
	"John W. Linville" <linville@...driver.com>,
	rkuo <rkuo@...eaurora.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	linux-wireless@...r.kernel.org, netdev <netdev@...r.kernel.org>
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

On Sat, 2013-11-30 at 21:08 +0100, Johannes Berg wrote:
> On Sat, 2013-11-30 at 22:02 +0800, Chen Gang wrote:
> 
> > >>>  	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.
> 
> I know. If you have any good ideas of how to make it more obvious to the
> compiler, I'm all ears, I just don't like any of the solutions offered
> so far (and you aren't the first to do so either) :-)
> 
> FWIW, I find the label to be odd because if you're familiar with the
> code then AP/AP_VLAN *should* be identical except for two special things
> that are now linearly & neatly handled in the code (the first being the
> 4-addr station, the second the chanctx assignment which always has to be
> done regardless of 4-addr). IMHO the == check after case should be
> enough to make a human reader take a closer look. I understand that you
> didn't and that's OK since you were just trying to squelch compile
> warnings, but I don't see that this one warrants much attention.

The label/test could be moved to save a couple of lines
of duplicated code.

Maybe:

 net/mac80211/tx.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
index 70b5a05..b2160f4 100644
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c
@@ -1777,18 +1777,16 @@ netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 		}
 		ap_sdata = container_of(sdata->bss, struct ieee80211_sub_if_data,
 					u.ap);
-		chanctx_conf = rcu_dereference(ap_sdata->vif.chanctx_conf);
-		if (!chanctx_conf)
-			goto fail_rcu;
-		band = chanctx_conf->def.chan->band;
-		if (sta)
-			break;
 		/* fall through */
 	case NL80211_IFTYPE_AP:
-		if (sdata->vif.type == NL80211_IFTYPE_AP)
-			chanctx_conf = rcu_dereference(sdata->vif.chanctx_conf);
+		chanctx_conf = rcu_dereference(ap_sdata->vif.chanctx_conf);
 		if (!chanctx_conf)
 			goto fail_rcu;
+		if (sdata->vif.type == NL80211_IFTYPE_AP_VLAN) {
+			band = chanctx_conf->def.chan->band;
+			if (sta)
+				break;
+		}
 		fc |= cpu_to_le16(IEEE80211_FCTL_FROMDS);
 		/* DA BSSID SA */
 		memcpy(hdr.addr1, skb->data, ETH_ALEN);


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