[PATCH v2][next] net: wwan: mhi_wwan_mbim: Avoid -Wflex-array-member-not-at-end warning

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

Use DEFINE_RAW_FLEX() to avoid a -Wflex-array-member-not-at-end warning.

Remove fixed-size array struct usb_cdc_ncm_dpe16 dpe16[2]; from struct
mbim_tx_hdr, so that flex-array member struct mbim_tx_hdr::ndp16.dpe16[]
ends last in this structure.

Compensate for this by using the DEFINE_RAW_FLEX() helper to declare the
on-stack struct instance that contains struct usb_cdc_ncm_ndp16 as a
member. Adjust the rest of the code, accordingly.

So, with these changes fix the following warning:

drivers/net/wwan/mhi_wwan_mbim.c:81:34: 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>
---
Changes in v2:
 - Add code comment to prevent people from adding new members after
   flex struct member `struct usb_cdc_ncm_ndp16 ndp16;`

v1:
 - Link: https://lore.kernel.org/linux-hardening/aSUubvYfGJ-BIeDq@kspp/

 drivers/net/wwan/mhi_wwan_mbim.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/drivers/net/wwan/mhi_wwan_mbim.c b/drivers/net/wwan/mhi_wwan_mbim.c
index c814fbd756a1..313dc5207c93 100644
--- a/drivers/net/wwan/mhi_wwan_mbim.c
+++ b/drivers/net/wwan/mhi_wwan_mbim.c
@@ -78,8 +78,9 @@ struct mhi_mbim_context {
 
 struct mbim_tx_hdr {
 	struct usb_cdc_ncm_nth16 nth16;
+
+	/* Must be last as it ends in a flexible-array member. */
 	struct usb_cdc_ncm_ndp16 ndp16;
-	struct usb_cdc_ncm_dpe16 dpe16[2];
 } __packed;
 
 static struct mhi_mbim_link *mhi_mbim_get_link_rcu(struct mhi_mbim_context *mbim,
@@ -107,20 +108,20 @@ static int mhi_mbim_get_link_mux_id(struct mhi_controller *cntrl)
 static struct sk_buff *mbim_tx_fixup(struct sk_buff *skb, unsigned int session,
 				     u16 tx_seq)
 {
+	DEFINE_RAW_FLEX(struct mbim_tx_hdr, mbim_hdr, ndp16.dpe16, 2);
 	unsigned int dgram_size = skb->len;
 	struct usb_cdc_ncm_nth16 *nth16;
 	struct usb_cdc_ncm_ndp16 *ndp16;
-	struct mbim_tx_hdr *mbim_hdr;
 
 	/* Only one NDP is sent, containing the IP packet (no aggregation) */
 
 	/* Ensure we have enough headroom for crafting MBIM header */
-	if (skb_cow_head(skb, sizeof(struct mbim_tx_hdr))) {
+	if (skb_cow_head(skb, __struct_size(mbim_hdr))) {
 		dev_kfree_skb_any(skb);
 		return NULL;
 	}
 
-	mbim_hdr = skb_push(skb, sizeof(struct mbim_tx_hdr));
+	mbim_hdr = skb_push(skb, __struct_size(mbim_hdr));
 
 	/* Fill NTB header */
 	nth16 = &mbim_hdr->nth16;
@@ -133,12 +134,11 @@ static struct sk_buff *mbim_tx_fixup(struct sk_buff *skb, unsigned int session,
 	/* Fill the unique NDP */
 	ndp16 = &mbim_hdr->ndp16;
 	ndp16->dwSignature = cpu_to_le32(USB_CDC_MBIM_NDP16_IPS_SIGN | (session << 24));
-	ndp16->wLength = cpu_to_le16(sizeof(struct usb_cdc_ncm_ndp16)
-					+ sizeof(struct usb_cdc_ncm_dpe16) * 2);
+	ndp16->wLength = cpu_to_le16(struct_size(ndp16, dpe16, 2));
 	ndp16->wNextNdpIndex = 0;
 
 	/* Datagram follows the mbim header */
-	ndp16->dpe16[0].wDatagramIndex = cpu_to_le16(sizeof(struct mbim_tx_hdr));
+	ndp16->dpe16[0].wDatagramIndex = cpu_to_le16(__struct_size(mbim_hdr));
 	ndp16->dpe16[0].wDatagramLength = cpu_to_le16(dgram_size);
 
 	/* null termination */
@@ -584,7 +584,7 @@ static void mhi_mbim_setup(struct net_device *ndev)
 {
 	ndev->header_ops = NULL;  /* No header */
 	ndev->type = ARPHRD_RAWIP;
-	ndev->needed_headroom = sizeof(struct mbim_tx_hdr);
+	ndev->needed_headroom = struct_size_t(struct mbim_tx_hdr, ndp16.dpe16, 2);
 	ndev->hard_header_len = 0;
 	ndev->addr_len = 0;
 	ndev->flags = IFF_POINTOPOINT | IFF_NOARP;
-- 
2.43.0