ixgbe: fix ipsec trailer length

Fix up the Tx trailer length calculation.  We can't believe the
trailer len from the xstate information because it was calculated
before the packet was put together and padding added.  This bit
of code finds the padding value in the trailer, adds it to the
authentication length, and saves it so later we can put it into
the Tx descriptor to tell the device where to stop the checksum
calculation.

Fixes: 592594704761 ("ixgbe: process the Tx ipsec offload")
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/ixgbe/ixgbe_ipsec.c

@@ -789,11 +789,33 @@ int ixgbe_ipsec_tx(struct ixgbe_ring *tx_ring,
 
 	itd->flags = 0;
 	if (xs->id.proto == IPPROTO_ESP) {
+		struct sk_buff *skb = first->skb;
+		int ret, authlen, trailerlen;
+		u8 padlen;
+
 		itd->flags |= IXGBE_ADVTXD_TUCMD_IPSEC_TYPE_ESP |
 			      IXGBE_ADVTXD_TUCMD_L4T_TCP;
 		if (first->protocol == htons(ETH_P_IP))
 			itd->flags |= IXGBE_ADVTXD_TUCMD_IPV4;
-		itd->trailer_len = xs->props.trailer_len;
+
+		/* The actual trailer length is authlen (16 bytes) plus
+		 * 2 bytes for the proto and the padlen values, plus
+		 * padlen bytes of padding.  This ends up not the same
+		 * as the static value found in xs->props.trailer_len (21).
+		 *
+		 * The "correct" way to get the auth length would be to use
+		 *    authlen = crypto_aead_authsize(xs->data);
+		 * but since we know we only have one size to worry about
+		 * we can let the compiler use the constant and save us a
+		 * few CPU cycles.
+		 */
+		authlen = IXGBE_IPSEC_AUTH_BITS / 8;
+
+		ret = skb_copy_bits(skb, skb->len - (authlen + 2), &padlen, 1);
+		if (unlikely(ret))
+			return 0;
+		trailerlen = authlen + 2 + padlen;
+		itd->trailer_len = trailerlen;
 	}
 	if (tsa->encrypt)
 		itd->flags |= IXGBE_ADVTXD_TUCMD_IPSEC_ENCRYPT_EN;