iwlwifi: 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 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200507185538.GA14674@embeddedor
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/dvm/commands.h

@@ -1023,7 +1023,7 @@ struct iwl_wep_cmd {
 	u8 global_key_type;
 	u8 flags;
 	u8 reserved;
-	struct iwl_wep_key key[0];
+	struct iwl_wep_key key[];
 } __packed;
 
 #define WEP_KEY_WEP_TYPE 1
@@ -1305,7 +1305,7 @@ struct iwl_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;
 
 /*
@@ -2380,7 +2380,7 @@ struct iwl_scan_cmd {
 	 * for one scan to complete (i.e. receive SCAN_COMPLETE_NOTIFICATION)
 	 * before requesting another scan.
 	 */
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 /* Can abort will notify by complete notification with abort status. */
@@ -2475,7 +2475,7 @@ struct iwl_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;
 
 /******************************************************************************
@@ -3188,7 +3188,7 @@ struct iwl_calib_hdr {
 
 struct iwl_calib_cmd {
 	struct iwl_calib_hdr hdr;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 struct iwl_calib_xtal_freq_cmd {
@@ -3216,7 +3216,7 @@ struct iwl_calib_temperature_offset_v2_cmd {
 /* IWL_PHY_CALIBRATE_CHAIN_NOISE_RESET_CMD */
 struct iwl_calib_chain_noise_reset_cmd {
 	struct iwl_calib_hdr hdr;
-	u8 data[0];
+	u8 data[];
 };
 
 /* IWL_PHY_CALIBRATE_CHAIN_NOISE_GAIN_CMD */

drivers/net/wireless/intel/iwlwifi/fw/api/nvm-reg.h

@@ -361,7 +361,7 @@ struct iwl_mcc_update_resp_v3 {
 	__le16 time;
 	__le16 geo_info;
 	__le32 n_channels;
-	__le32 channels[0];
+	__le32 channels[];
 } __packed; /* LAR_UPDATE_MCC_CMD_RESP_S_VER_3 */
 
 /**
@@ -390,7 +390,7 @@ struct iwl_mcc_update_resp {
 	u8 source_id;
 	u8 reserved[3];
 	__le32 n_channels;
-	__le32 channels[0];
+	__le32 channels[];
 } __packed; /* LAR_UPDATE_MCC_CMD_RESP_S_VER_4 */
 
 /**

drivers/net/wireless/intel/iwlwifi/fw/debugfs.c

@@ -260,7 +260,7 @@ struct hcmd_write_data {
 	__be32 cmd_id;
 	__be32 flags;
 	__be16 length;
-	u8 data[0];
+	u8 data[];
 } __packed;
 
 static ssize_t iwl_dbgfs_send_hcmd_write(struct iwl_fw_runtime *fwrt, char *buf,

drivers/net/wireless/intel/iwlwifi/iwl-op-mode.h

@@ -175,7 +175,7 @@ void iwl_opmode_deregister(const char *name);
 struct iwl_op_mode {
 	const struct iwl_op_mode_ops *ops;
 
-	char op_mode_specific[0] __aligned(sizeof(void *));
+	char op_mode_specific[] __aligned(sizeof(void *));
 };
 
 static inline void iwl_op_mode_stop(struct iwl_op_mode *op_mode)

drivers/net/wireless/intel/iwlwifi/iwl-trans.h

@@ -1006,7 +1006,7 @@ struct iwl_trans {
 
 	/* pointer to trans specific struct */
 	/*Ensure that this pointer will always be aligned to sizeof pointer */
-	char trans_specific[0] __aligned(sizeof(void *));
+	char trans_specific[] __aligned(sizeof(void *));
 };
 
 const char *iwl_get_cmd_string(struct iwl_trans *trans, u32 id);