net: wwan: iosm: Convert single instance struct member to flexible array

struct mux_adth actually ends with multiple struct mux_adth_dg members.
This is seen both in the comments about the member:

/**
 * struct mux_adth - Structure of the Aggregated Datagram Table Header.
 ...
 * @dg:		datagramm table with variable length
 */

and in the preparation for populating it:

                        adth_dg_size = offsetof(struct mux_adth, dg) +
                                        ul_adb->dg_count[i] * sizeof(*dg);
			...
                        adth_dg_size -= offsetof(struct mux_adth, dg);
                        memcpy(&adth->dg, ul_adb->dg[i], adth_dg_size);

This was reported as a run-time false positive warning:

memcpy: detected field-spanning write (size 16) of single field "&adth->dg" at drivers/net/wwan/iosm/iosm_ipc_mux_codec.c:852 (size 8)

Adjust the struct mux_adth definition and associated sizeof() math; no binary
output differences are observed in the resulting object file.

Reported-by: Florian Klink <flokli@flokli.de>
Closes: https://lore.kernel.org/lkml/dbfa25f5-64c8-5574-4f5d-0151ba95d232@gmail.com/
Fixes: 1f52d7b ("net: wwan: iosm: Enable M.2 7360 WWAN card support")
Cc: M Chetan Kumar <m.chetan.kumar@intel.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Intel Corporation <linuxwwan@intel.com>
Cc: Loic Poulain <loic.poulain@linaro.org>
Cc: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230620194234.never.023-kees@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: kees

drivers/net/wwan/iosm/iosm_ipc_mux_codec.c

@@ -626,14 +626,12 @@ static void mux_dl_adb_decode(struct iosm_mux *ipc_mux,
 		if (adth->signature != cpu_to_le32(IOSM_AGGR_MUX_SIG_ADTH))
 			goto adb_decode_err;
 
-		if (le16_to_cpu(adth->table_length) < (sizeof(struct mux_adth) -
-				sizeof(struct mux_adth_dg)))
+		if (le16_to_cpu(adth->table_length) < sizeof(struct mux_adth))
 			goto adb_decode_err;
 
 		/* Calculate the number of datagrams. */
 		nr_of_dg = (le16_to_cpu(adth->table_length) -
-					sizeof(struct mux_adth) +
-					sizeof(struct mux_adth_dg)) /
+					sizeof(struct mux_adth)) /
 					sizeof(struct mux_adth_dg);
 
 		/* Is the datagram table empty ? */
@@ -649,7 +647,7 @@ static void mux_dl_adb_decode(struct iosm_mux *ipc_mux,
 		}
 
 		/* New aggregated datagram table. */
-		dg = &adth->dg;
+		dg = adth->dg;
 		if (mux_dl_process_dg(ipc_mux, adbh, dg, skb, if_id,
 				      nr_of_dg) < 0)
 			goto adb_decode_err;
@@ -849,7 +847,7 @@ static void ipc_mux_ul_encode_adth(struct iosm_mux *ipc_mux,
 			adth->if_id = i;
 			adth->table_length = cpu_to_le16(adth_dg_size);
 			adth_dg_size -= offsetof(struct mux_adth, dg);
-			memcpy(&adth->dg, ul_adb->dg[i], adth_dg_size);
+			memcpy(adth->dg, ul_adb->dg[i], adth_dg_size);
 			ul_adb->if_cnt++;
 		}
 
@@ -1426,14 +1424,13 @@ static int ipc_mux_get_payload_from_adb(struct iosm_mux *ipc_mux,
 
 		if (adth->signature == cpu_to_le32(IOSM_AGGR_MUX_SIG_ADTH)) {
 			nr_of_dg = (le16_to_cpu(adth->table_length) -
-					sizeof(struct mux_adth) +
-					sizeof(struct mux_adth_dg)) /
+					sizeof(struct mux_adth)) /
 					sizeof(struct mux_adth_dg);
 
 			if (nr_of_dg <= 0)
 				return payload_size;
 
-			dg = &adth->dg;
+			dg = adth->dg;
 
 			for (i = 0; i < nr_of_dg; i++, dg++) {
 				if (le32_to_cpu(dg->datagram_index) <

drivers/net/wwan/iosm/iosm_ipc_mux_codec.h

@@ -161,7 +161,7 @@ struct mux_adth {
 	u8 opt_ipv4v6;
 	__le32 next_table_index;
 	__le32 reserved2;
-	struct mux_adth_dg dg;
+	struct mux_adth_dg dg[];
 };
 
 /**