iwlegacy: 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>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200507185529.GA14639@embeddedor

drivers/net/wireless/intel/iwlegacy/commands.h

@@ -203,7 +203,7 @@ struct il_cmd_header {
 	__le16 sequence;
 
 	/* command or response/notification data follows immediately */
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 /**
@@ -1112,7 +1112,7 @@ struct il_wep_cmd {
 	u8 global_key_type;
 	u8 flags;
 	u8 reserved;
-	struct il_wep_key key[0];
+	struct il_wep_key key[];
 } __packed;
 
 #define WEP_KEY_WEP_TYPE 1
@@ -1166,7 +1166,7 @@ struct il3945_rx_frame_stats {
 	u8 agc;
 	__le16 sig_avg;
 	__le16 noise_diff;
-	u8 payload[0];
+	u8 payload[];
 } __packed;
 
 struct il3945_rx_frame_hdr {
@@ -1175,7 +1175,7 @@ struct il3945_rx_frame_hdr {
 	u8 reserved1;
 	u8 rate;
 	__le16 len;
-	u8 payload[0];
+	u8 payload[];
 } __packed;
 
 struct il3945_rx_frame_end {
@@ -1211,7 +1211,7 @@ struct il4965_rx_non_cfg_phy {
 	__le16 ant_selection;	/* ant A bit 4, ant B bit 5, ant C bit 6 */
 	__le16 agc_info;	/* agc code 0:6, agc dB 7:13, reserved 14:15 */
 	u8 rssi_info[6];	/* we use even entries, 0/2/4 for A/B/C rssi */
-	u8 pad[0];
+	u8 pad[];
 } __packed;
 
 /*
@@ -1409,7 +1409,7 @@ struct il3945_tx_cmd {
 	 * length is 26 or 30 bytes, followed by payload data
 	 */
 	u8 payload[0];
-	struct ieee80211_hdr hdr[0];
+	struct ieee80211_hdr hdr[];
 } __packed;
 
 /*
@@ -1511,7 +1511,7 @@ struct il_tx_cmd {
 	 * length is 26 or 30 bytes, followed by payload data
 	 */
 	u8 payload[0];
-	struct ieee80211_hdr hdr[0];
+	struct ieee80211_hdr hdr[];
 } __packed;
 
 /* TX command response is sent after *3945* transmission attempts.
@@ -2520,7 +2520,7 @@ struct il3945_scan_cmd {
 	 * for one scan to complete (i.e. receive N_SCAN_COMPLETE)
 	 * before requesting another scan.
 	 */
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct il_scan_cmd {
@@ -2564,7 +2564,7 @@ struct il_scan_cmd {
 	 * for one scan to complete (i.e. receive N_SCAN_COMPLETE)
 	 * before requesting another scan.
 	 */
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 /* Can abort will notify by complete notification with abort status. */
@@ -2664,7 +2664,7 @@ struct il3945_tx_beacon_cmd {
 	__le16 tim_idx;
 	u8 tim_size;
 	u8 reserved1;
-	struct ieee80211_hdr frame[0];	/* beacon frame */
+	struct ieee80211_hdr frame[];	/* beacon frame */
 } __packed;
 
 struct il_tx_beacon_cmd {
@@ -2672,7 +2672,7 @@ struct il_tx_beacon_cmd {
 	__le16 tim_idx;
 	u8 tim_size;
 	u8 reserved1;
-	struct ieee80211_hdr frame[0];	/* beacon frame */
+	struct ieee80211_hdr frame[];	/* beacon frame */
 } __packed;
 
 /******************************************************************************

drivers/net/wireless/intel/iwlegacy/iwl-spectrum.h

@@ -53,7 +53,7 @@ struct ieee80211_measurement_params {
 struct ieee80211_info_element {
 	u8 id;
 	u8 len;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct ieee80211_measurement_request {
@@ -61,7 +61,7 @@ struct ieee80211_measurement_request {
 	u8 token;
 	u8 mode;
 	u8 type;
-	struct ieee80211_measurement_params params[0];
+	struct ieee80211_measurement_params params[];
 } __packed;
 
 struct ieee80211_measurement_report {