[PATCH][next] rpmsg: glink: Avoid -Wflex-array-member-not-at-end warnings

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

-Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
ready to enable it globally.

There is currently an object (`msg`) in multiple structures that
contains a flexible structure (`struct glink_msg`), for example:

struct glink_defer_cmd {
        ...

        struct glink_msg msg;
        u8 data[];
};

So, in order to avoid ending up with a flexible-array member in the
middle of another structure, we use the `__struct_group()` helper
to separate the flexible array from the rest of the members in the
flexible structure:

struct glink_msg {
        __struct_group(glink_msg_hdr, hdr, __packed,

        ... the rest of members

        );
        u8 data[];
} __packed;

With the change described above, we now declare objects of the type of
the tagged struct, in this case `struct glink_msg_hdr`, without
embedding flexible arrays in the middle of other structures:

struct glink_defer_cmd {
        ...

        struct glink_msg_hdr msg;
        u8 data[];
};

Also, use `container_of()` to retrieve a pointer to the flexible structure.

We also use the `DEFINE_RAW_FLEX()` helper for an in-stack definition of
a flexible structure where the size of the flexible-array member is known
at compile-time.

So, with these changes, fix the following warnings:
drivers/rpmsg/qcom_glink_native.c:51:26: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/rpmsg/qcom_glink_native.c:459:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/rpmsg/qcom_glink_native.c:846:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/rpmsg/qcom_glink_native.c:968:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
drivers/rpmsg/qcom_glink_native.c:1380:34: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 drivers/rpmsg/qcom_glink_native.c | 38 ++++++++++++++++---------------
 1 file changed, 20 insertions(+), 18 deletions(-)

diff --git a/drivers/rpmsg/qcom_glink_native.c b/drivers/rpmsg/qcom_glink_native.c
index 82d460ff4777..878e3461bce1 100644
--- a/drivers/rpmsg/qcom_glink_native.c
+++ b/drivers/rpmsg/qcom_glink_native.c
@@ -30,9 +30,12 @@
 #define RPM_GLINK_CID_MAX	65536
 
 struct glink_msg {
-	__le16 cmd;
-	__le16 param1;
-	__le32 param2;
+	/* New members must be added within the __struct_group() macro below. */
+	__struct_group(glink_msg_hdr, hdr, __packed,
+		__le16 cmd;
+		__le16 param1;
+		__le32 param2;
+	);
 	u8 data[];
 } __packed;
 
@@ -48,7 +51,7 @@ struct glink_msg {
 struct glink_defer_cmd {
 	struct list_head node;
 
-	struct glink_msg msg;
+	struct glink_msg_hdr msg;
 	u8 data[];
 };
 
@@ -455,12 +458,9 @@ static void qcom_glink_intent_req_abort(struct glink_channel *channel)
 static int qcom_glink_send_open_req(struct qcom_glink *glink,
 				    struct glink_channel *channel)
 {
-	struct {
-		struct glink_msg msg;
-		u8 name[GLINK_NAME_SIZE];
-	} __packed req;
+	DEFINE_RAW_FLEX(struct glink_msg, req, data, GLINK_NAME_SIZE);
 	int name_len = strlen(channel->name) + 1;
-	int req_len = ALIGN(sizeof(req.msg) + name_len, 8);
+	int req_len = ALIGN(sizeof(*req) + name_len, 8);
 	int ret;
 	unsigned long flags;
 
@@ -476,10 +476,10 @@ static int qcom_glink_send_open_req(struct qcom_glink *glink,
 
 	channel->lcid = ret;
 
-	req.msg.cmd = cpu_to_le16(GLINK_CMD_OPEN);
-	req.msg.param1 = cpu_to_le16(channel->lcid);
-	req.msg.param2 = cpu_to_le32(name_len);
-	strcpy(req.name, channel->name);
+	req->cmd = cpu_to_le16(GLINK_CMD_OPEN);
+	req->param1 = cpu_to_le16(channel->lcid);
+	req->param2 = cpu_to_le32(name_len);
+	strcpy(req->data, channel->name);
 
 	ret = qcom_glink_tx(glink, &req, req_len, NULL, 0, true);
 	if (ret)
@@ -826,7 +826,9 @@ static int qcom_glink_rx_defer(struct qcom_glink *glink, size_t extra)
 
 	INIT_LIST_HEAD(&dcmd->node);
 
-	qcom_glink_rx_peek(glink, &dcmd->msg, 0, sizeof(dcmd->msg) + extra);
+	qcom_glink_rx_peek(glink,
+			   container_of(&dcmd->msg, struct glink_msg, hdr), 0,
+			   sizeof(dcmd->msg) + extra);
 
 	spin_lock(&glink->rx_lock);
 	list_add_tail(&dcmd->node, &glink->rx_queue);
@@ -843,7 +845,7 @@ static int qcom_glink_rx_data(struct qcom_glink *glink, size_t avail)
 	struct glink_core_rx_intent *intent;
 	struct glink_channel *channel;
 	struct {
-		struct glink_msg msg;
+		struct glink_msg_hdr msg;
 		__le32 chunk_size;
 		__le32 left_size;
 	} __packed hdr;
@@ -965,7 +967,7 @@ static void qcom_glink_handle_intent(struct qcom_glink *glink,
 	};
 
 	struct {
-		struct glink_msg msg;
+		struct glink_msg_hdr msg;
 		struct intent_pair intents[];
 	} __packed * msg;
 
@@ -1377,7 +1379,7 @@ static int __qcom_glink_send(struct glink_channel *channel,
 	struct glink_core_rx_intent *tmp;
 	int iid = 0;
 	struct {
-		struct glink_msg msg;
+		struct glink_msg_hdr msg;
 		__le32 chunk_size;
 		__le32 left_size;
 	} __packed req;
@@ -1685,7 +1687,7 @@ static void qcom_glink_work(struct work_struct *work)
 		list_del(&dcmd->node);
 		spin_unlock_irqrestore(&glink->rx_lock, flags);
 
-		msg = &dcmd->msg;
+		msg = container_of(&dcmd->msg, struct glink_msg, hdr);
 		cmd = le16_to_cpu(msg->cmd);
 		param1 = le16_to_cpu(msg->param1);
 		param2 = le32_to_cpu(msg->param2);
-- 
2.34.1