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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
 drivers/net/wireless/intel/iwlwifi/dvm/commands.h   |   12 ++++++------
 drivers/net/wireless/intel/iwlwifi/fw/api/nvm-reg.h |    4 ++--
 drivers/net/wireless/intel/iwlwifi/fw/debugfs.c     |    2 +-
 drivers/net/wireless/intel/iwlwifi/iwl-op-mode.h    |    2 +-
 drivers/net/wireless/intel/iwlwifi/iwl-trans.h      |    2 +-
 5 files changed, 11 insertions(+), 11 deletions(-)

drivers/net/wireless/intel/iwlwifi/dvm/commands.h +6 -6

index 0f4be4be181c..fdcc1292a92b 100644
@@ -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 +2 -2

index 97b49843e318..397ac89a04c2 100644
@@ -351,7 +351,7 @@ 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 */
 
 /**

@@ -380,7 +380,7 @@ 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 +1 -1

index 89f74116569d..cc1d93606d9b 100644
@@ -261,7 +261,7 @@ 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 +1 -1

index 3008a5246be8..b35b8920941b 100644
@@ -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 +1 -1

index bba527b339b5..3fbbbac0953d 100644
@@ -878,7 +878,7 @@ 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);

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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by Gustavo A. R. Silva <[email protected]>