ath11k: 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 <gustavo@embeddedor.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200504201224.GA32282@embeddedor

drivers/net/wireless/ath/ath11k/debug.h

@@ -67,7 +67,7 @@ struct debug_htt_stats_req {
 	u8 peer_addr[ETH_ALEN];
 	struct completion cmpln;
 	u32 buf_len;
-	u8 buf[0];
+	u8 buf[];
 };
 
 struct ath_pktlog_hdr {
@@ -77,7 +77,7 @@ struct ath_pktlog_hdr {
 	u16 size;
 	u32 timestamp;
 	u32 type_specific_data;
-	u8 payload[0];
+	u8 payload[];
 };
 
 #define ATH11K_HTT_PEER_STATS_RESET BIT(16)

drivers/net/wireless/ath/ath11k/debug_htt_stats.h

@@ -239,7 +239,7 @@ struct htt_tx_pdev_stats_tx_ppdu_stats_tlv_v {
  */
 struct htt_tx_pdev_stats_tried_mpdu_cnt_hist_tlv_v {
 	u32 hist_bin_size;
-	u32 tried_mpdu_cnt_hist[0]; /* HTT_TX_PDEV_TRIED_MPDU_CNT_HIST */
+	u32 tried_mpdu_cnt_hist[]; /* HTT_TX_PDEV_TRIED_MPDU_CNT_HIST */
 };
 
 /* == SOC ERROR STATS == */
@@ -550,7 +550,7 @@ struct htt_tx_hwq_stats_cmn_tlv {
 struct htt_tx_hwq_difs_latency_stats_tlv_v {
 	u32 hist_intvl;
 	/* histogram of ppdu post to hwsch - > cmd status received */
-	u32 difs_latency_hist[0]; /* HTT_TX_HWQ_MAX_DIFS_LATENCY_BINS */
+	u32 difs_latency_hist[]; /* HTT_TX_HWQ_MAX_DIFS_LATENCY_BINS */
 };
 
 /* NOTE: Variable length TLV, use length spec to infer array size */
@@ -586,7 +586,7 @@ struct htt_tx_hwq_fes_result_stats_tlv_v {
 struct htt_tx_hwq_tried_mpdu_cnt_hist_tlv_v {
 	u32 hist_bin_size;
 	/* Histogram of number of mpdus on tried mpdu */
-	u32 tried_mpdu_cnt_hist[0]; /* HTT_TX_HWQ_TRIED_MPDU_CNT_HIST */
+	u32 tried_mpdu_cnt_hist[]; /* HTT_TX_HWQ_TRIED_MPDU_CNT_HIST */
 };
 
 /* NOTE: Variable length TLV, use length spec to infer array size
@@ -1584,7 +1584,7 @@ struct htt_pdev_stats_twt_session_tlv {
 struct htt_pdev_stats_twt_sessions_tlv {
 	u32 pdev_id;
 	u32 num_sessions;
-	struct htt_pdev_stats_twt_session_tlv twt_session[0];
+	struct htt_pdev_stats_twt_session_tlv twt_session[];
 };
 
 enum htt_rx_reo_resource_sample_id_enum {

drivers/net/wireless/ath/ath11k/hal_desc.h

@@ -477,7 +477,7 @@ enum hal_tlv_tag {
 
 struct hal_tlv_hdr {
 	u32 tl;
-	u8 value[0];
+	u8 value[];
 } __packed;
 
 #define RX_MPDU_DESC_INFO0_MSDU_COUNT		GENMASK(7, 0)
@@ -1972,7 +1972,7 @@ struct hal_rx_reo_queue {
 	u32 processed_total_bytes;
 	u32 info5;
 	u32 rsvd[3];
-	struct hal_rx_reo_queue_ext ext_desc[0];
+	struct hal_rx_reo_queue_ext ext_desc[];
 } __packed;
 
 /* hal_rx_reo_queue

drivers/net/wireless/ath/ath11k/hal_rx.h

@@ -23,7 +23,7 @@ struct hal_rx_wbm_rel_info {
 
 struct hal_rx_mon_status_tlv_hdr {
 	u32 hdr;
-	u8 value[0];
+	u8 value[];
 };
 
 enum hal_rx_su_mu_coding {

drivers/net/wireless/ath/ath11k/hw.h

@@ -111,7 +111,7 @@ struct ath11k_hw_params {
 struct ath11k_fw_ie {
 	__le32 id;
 	__le32 len;
-	u8 data[0];
+	u8 data[];
 };
 
 enum ath11k_bd_ie_board_type {

drivers/net/wireless/ath/ath11k/wmi.h

@@ -39,7 +39,7 @@ struct wmi_cmd_hdr {
 
 struct wmi_tlv {
 	u32 header;
-	u8 value[0];
+	u8 value[];
 } __packed;
 
 #define WMI_TLV_LEN	GENMASK(15, 0)