Subject: [PATCH] net: wwan: iosm: Convert single instance struct member to flexible array

From: Kees Cook <[email-masked]>
Date: Tue, 20 Jun 2023 12:42:38 -0700

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 <[email-masked]>
Closes: https://lore.kernel.org/lkml/[email-masked]/
Fixes: 1f52d7b62285 ("net: wwan: iosm: Enable M.2 7360 WWAN card support")
Cc: M Chetan Kumar <[email-masked]>
Cc: Bagas Sanjaya <[email-masked]>
Cc: Intel Corporation <[email-masked]>
Cc: Loic Poulain <[email-masked]>
Cc: Sergey Ryazanov <[email-masked]>
Cc: Johannes Berg <[email-masked]>
Cc: "David S. Miller" <[email-masked]>
Cc: Eric Dumazet <[email-masked]>
Cc: Jakub Kicinski <[email-masked]>
Cc: Paolo Abeni <[email-masked]>
Cc: "Gustavo A. R. Silva" <[email-masked]>
Cc: [email-masked]
Signed-off-by: Kees Cook <[email-masked]>
---
 drivers/net/wwan/iosm/iosm_ipc_mux_codec.c | 15 ++++++---------
 drivers/net/wwan/iosm/iosm_ipc_mux_codec.h |  2 +-
 2 files changed, 7 insertions(+), 10 deletions(-)

diff --git a/drivers/net/wwan/iosm/iosm_ipc_mux_codec.c b/drivers/net/wwan/iosm/iosm_ipc_mux_codec.c
index d6b166fc5c0e..bff46f7ca59f 100644
--- a/drivers/net/wwan/iosm/iosm_ipc_mux_codec.c
+++ b/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) <
diff --git a/drivers/net/wwan/iosm/iosm_ipc_mux_codec.h b/drivers/net/wwan/iosm/iosm_ipc_mux_codec.h
index 5d4e3b89542c..f8df88f816c4 100644
--- a/drivers/net/wwan/iosm/iosm_ipc_mux_codec.h
+++ b/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[];
 };
 
 /**
-- 
2.34.1