mac80211: avoid excessive stack usage in sta_info

commit 0ef049dc upstream.

When CONFIG_OPTIMIZE_INLINING is set, the sta_info_insert_finish
function consumes more stack than normally, exceeding the
1024 byte limit on ARM:

net/mac80211/sta_info.c: In function 'sta_info_insert_finish':
net/mac80211/sta_info.c:561:1: error: the frame size of 1080 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

It turns out that there are two functions that put a 'struct station_info'
on the stack: __sta_info_destroy_part2 and sta_info_insert_finish, and
this structure alone requires up to 792 bytes.

Hoping that both are called rarely enough, this replaces the
on-stack structure with a dynamic allocation, which unfortunately
requires some suboptimal error handling for out-of-memory.

The __sta_info_destroy_part2 function is actually affected by the
stack usage twice because it calls cfg80211_del_sta_sinfo(), which
has another instance of struct station_info on its stack.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 98b62183 ("mac80211/cfg80211: add station events")
Fixes: 6f7a8d26 ("mac80211: send statistics with delete station event")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[bwh: Backported to 3.16: there's only one instance to fix]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

net/mac80211/sta_info.c

@@ -487,11 +487,17 @@ static int sta_info_insert_finish(struct sta_info *sta) __acquires(RCU)
 {
 	struct ieee80211_local *local = sta->local;
 	struct ieee80211_sub_if_data *sdata = sta->sdata;
-	struct station_info sinfo;
+	struct station_info *sinfo;
 	int err = 0;
 
 	lockdep_assert_held(&local->sta_mtx);
 
+	sinfo = kzalloc(sizeof(struct station_info), GFP_KERNEL);
+	if (!sinfo) {
+		err = -ENOMEM;
+		goto out_err;
+	}
+
 	/* check if STA exists already */
 	if (sta_info_get_bss(sdata, sta->sta.addr)) {
 		err = -EEXIST;
@@ -523,10 +529,9 @@ static int sta_info_insert_finish(struct sta_info *sta) __acquires(RCU)
 	ieee80211_sta_debugfs_add(sta);
 	rate_control_add_sta_debugfs(sta);
 
-	memset(&sinfo, 0, sizeof(sinfo));
-	sinfo.filled = 0;
-	sinfo.generation = local->sta_generation;
-	cfg80211_new_sta(sdata->dev, sta->sta.addr, &sinfo, GFP_KERNEL);
+	sinfo->generation = local->sta_generation;
+	cfg80211_new_sta(sdata->dev, sta->sta.addr, sinfo, GFP_KERNEL);
+	kfree(sinfo);
 
 	sta_dbg(sdata, "Inserted STA %pM\n", sta->sta.addr);