cfg80211: 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/20200507183909.GA12993@embeddedorSigned-off-by: Johannes Berg <johannes.berg@intel.com>

include/linux/ieee80211.h

@@ -800,7 +800,7 @@ struct ieee80211_msrment_ie {
 	u8 token;
 	u8 mode;
 	u8 type;
-	u8 request[0];
+	u8 request[];
 } __packed;
 
 /**
@@ -1781,7 +1781,7 @@ struct ieee80211_he_operation {
 	__le32 he_oper_params;
 	__le16 he_mcs_nss_set;
 	/* Optional 0,1,3,4,5,7 or 8 bytes: depends on @he_oper_params */
-	u8 optional[0];
+	u8 optional[];
 } __packed;
 
 /**
@@ -1793,7 +1793,7 @@ struct ieee80211_he_operation {
 struct ieee80211_he_spr {
 	u8 he_sr_control;
 	/* Optional 0 to 19 bytes: depends on @he_sr_control */
-	u8 optional[0];
+	u8 optional[];
 } __packed;
 
 /**

include/net/cfg80211.h

@@ -2037,7 +2037,7 @@ struct cfg80211_scan_request {
 	bool no_cck;
 
 	/* keep last */
-	struct ieee80211_channel *channels[0];
+	struct ieee80211_channel *channels[];
 };
 
 static inline void get_random_mask_addr(u8 *buf, const u8 *addr, const u8 *mask)
@@ -2183,7 +2183,7 @@ struct cfg80211_sched_scan_request {
 	struct list_head list;
 
 	/* keep last */
-	struct ieee80211_channel *channels[0];
+	struct ieee80211_channel *channels[];
 };
 
 /**
@@ -2305,7 +2305,7 @@ struct cfg80211_bss {
 	u8 bssid_index;
 	u8 max_bssid_indicator;
 
-	u8 priv[0] __aligned(sizeof(void *));
+	u8 priv[] __aligned(sizeof(void *));
 };
 
 /**
@@ -4852,7 +4852,7 @@ struct wiphy {
 
 	u8 max_data_retry_count;
 
-	char priv[0] __aligned(NETDEV_ALIGN);
+	char priv[] __aligned(NETDEV_ALIGN);
 };
 
 static inline struct net *wiphy_net(struct wiphy *wiphy)

net/wireless/core.h

@@ -286,7 +286,7 @@ struct cfg80211_cqm_config {
 	u32 rssi_hyst;
 	s32 last_rssi_event_value;
 	int n_rssi_thresholds;
-	s32 rssi_thresholds[0];
+	s32 rssi_thresholds[];
 };
 
 void cfg80211_destroy_ifaces(struct cfg80211_registered_device *rdev);