Two problems exist in brcmf_netif_mon_rx():

1. skb_push() is called without first verifying that the skb has
   sufficient headroom. If the skb arrives with zero headroom the
   kernel will panic. Use skb_cow_head() before each skb_push() and
   free the skb on allocation failure.

2. When the firmware provides a hardware RX header
   (BRCMF_FEAT_MONITOR_FMT_HW_RX_HDR), the driver strips it and
   inserts a blank ieee80211_radiotap_header with no signal data.
   Monitor-mode packet captures therefore show no signal strength,
   making tools like Wireshark and iw unable to report RSSI.

Define struct brcmf_radiotap_info that embeds the standard radiotap
header followed by a dbm_antsignal field. Extract the rssi value from
wlc_d11rxhdr and populate it_present with
IEEE80211_RADIOTAP_DBM_ANTSIGNAL so userspace tools can read signal
strength from monitor-mode frames.

Also replace the open-coded "skb->len -= 4" with skb_trim(), which
is the correct API for shortening a linear skb.

Fixes: e665988be29c ("brcmfmac: support monitor frames with the hardware/ucode header")
Signed-off-by: Shivesh <chanelshivesh@gmail.com>
---
 .../broadcom/brcm80211/brcmfmac/core.c        | 44 ++++++++++++-------
 1 file changed, 29 insertions(+), 15 deletions(-)

diff --git a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c
index ec170647800d..eefc437dd055 100644
--- a/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c
+++ b/drivers/net/wireless/broadcom/brcm80211/brcmfmac/core.c
@@ -431,45 +431,55 @@ void brcmf_netif_rx(struct brcmf_if *ifp, struct sk_buff *skb)
 	netif_rx(skb);
 }
 
+struct brcmf_radiotap_info {
+	struct ieee80211_radiotap_header hdr;
+	s8 dbm_antsignal;
+} __packed;
+
 void brcmf_netif_mon_rx(struct brcmf_if *ifp, struct sk_buff *skb)
 {
 	if (brcmf_feat_is_enabled(ifp, BRCMF_FEAT_MONITOR_FMT_RADIOTAP)) {
-		/* Do nothing */
+		/* Firmware already provided a full radiotap header; do nothing */
 	} else if (brcmf_feat_is_enabled(ifp, BRCMF_FEAT_MONITOR_FMT_HW_RX_HDR)) {
 		struct wlc_d11rxhdr *wlc_rxhdr = (struct wlc_d11rxhdr *)skb->data;
-		struct ieee80211_radiotap_header *radiotap;
+		struct brcmf_radiotap_info *rtap;
 		unsigned int offset;
 		u16 RxStatus1;
+		s8 rssi;
 
 		RxStatus1 = le16_to_cpu(wlc_rxhdr->rxhdr.RxStatus1);
+		rssi = wlc_rxhdr->rssi;
 
 		offset = sizeof(struct wlc_d11rxhdr);
-		/* MAC inserts 2 pad bytes for a4 headers or QoS or A-MSDU
-		 * subframes
-		 */
+		/* MAC inserts 2 pad bytes for a4 headers or QoS or A-MSDU subframes */
 		if (RxStatus1 & RXS_PBPRES)
 			offset += 2;
 		offset += D11_PHY_HDR_LEN;
 
 		skb_pull(skb, offset);
 
-		/* TODO: use RX header to fill some radiotap data */
-		radiotap = skb_push(skb, sizeof(*radiotap));
-		memset(radiotap, 0, sizeof(*radiotap));
-		radiotap->it_len = cpu_to_le16(sizeof(*radiotap));
-
-		/* TODO: 4 bytes with receive status? */
-		skb->len -= 4;
+		/* Insert our radiotap header with RSSI data */
+		if (skb_cow_head(skb, sizeof(*rtap)))
+			goto drop;
+		rtap = skb_push(skb, sizeof(*rtap));
+		memset(rtap, 0, sizeof(*rtap));
+		rtap->hdr.it_len = cpu_to_le16(sizeof(*rtap));
+		rtap->hdr.it_present = cpu_to_le32(1 << IEEE80211_RADIOTAP_DBM_ANTSIGNAL);
+		rtap->dbm_antsignal = rssi;
+
+		/* Strip the 4-byte receive status / FCS tail */
+		skb_trim(skb, skb->len - 4);
 	} else {
 		struct ieee80211_radiotap_header *radiotap;
 
-		/* TODO: use RX status to fill some radiotap data */
+		if (skb_cow_head(skb, sizeof(*radiotap)))
+			goto drop;
 		radiotap = skb_push(skb, sizeof(*radiotap));
 		memset(radiotap, 0, sizeof(*radiotap));
 		radiotap->it_len = cpu_to_le16(sizeof(*radiotap));
 
-		/* TODO: 4 bytes with receive status? */
-		skb->len -= 4;
+		/* Strip the 4-byte receive status / FCS tail */
+		skb_trim(skb, skb->len - 4);
 	}
 
 	skb->dev = ifp->ndev;
@@ -478,6 +488,10 @@ void brcmf_netif_mon_rx(struct brcmf_if *ifp, struct sk_buff *skb)
 	skb->protocol = htons(ETH_P_802_2);
 
 	brcmf_netif_rx(ifp, skb);
+	return;
+
+drop:
+	brcmu_pkt_buf_free_skb(skb);
 }
 
 static int brcmf_rx_hdrpull(struct brcmf_pub *drvr, struct sk_buff *skb,
-- 
2.53.0