mac80211: Replace zero-length array with flexible-array

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200507185907.GA15102@embeddedorSigned-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/mac80211.h

@@ -230,7 +230,7 @@ struct ieee80211_chanctx_conf {
 
 	bool radar_enabled;
 
-	u8 drv_priv[0] __aligned(sizeof(void *));
+	u8 drv_priv[] __aligned(sizeof(void *));
 };
 
 /**
@@ -1670,7 +1670,7 @@ struct ieee80211_vif {
 	bool txqs_stopped[IEEE80211_NUM_ACS];
 
 	/* must be last */
-	u8 drv_priv[0] __aligned(sizeof(void *));
+	u8 drv_priv[] __aligned(sizeof(void *));
 };
 
 static inline bool ieee80211_vif_is_mesh(struct ieee80211_vif *vif)
@@ -1798,7 +1798,7 @@ struct ieee80211_key_conf {
 	s8 keyidx;
 	u16 flags;
 	u8 keylen;
-	u8 key[0];
+	u8 key[];
 };
 
 #define IEEE80211_MAX_PN_LEN	16
@@ -2053,7 +2053,7 @@ struct ieee80211_sta {
 	struct ieee80211_txq *txq[IEEE80211_NUM_TIDS + 1];
 
 	/* must be last */
-	u8 drv_priv[0] __aligned(sizeof(void *));
+	u8 drv_priv[] __aligned(sizeof(void *));
 };
 
 /**
@@ -2099,7 +2099,7 @@ struct ieee80211_txq {
 	u8 ac;
 
 	/* must be last */
-	u8 drv_priv[0] __aligned(sizeof(void *));
+	u8 drv_priv[] __aligned(sizeof(void *));
 };
 
 /**

net/mac80211/ieee80211_i.h

@@ -267,7 +267,7 @@ struct probe_resp {
 	struct rcu_head rcu_head;
 	int len;
 	u16 csa_counter_offsets[IEEE80211_MAX_CSA_COUNTERS_NUM];
-	u8 data[0];
+	u8 data[];
 };
 
 struct ps_data {