tls: rx: replace 'back' with 'offset'

The padding length TLS 1.3 logic is searching for content_type from
the end of text. IMHO the code is easier to parse if we calculate
offset and decrement it rather than try to maintain positive offset
from the end of the record called "back".
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tls/tls_sw.c

@@ -136,22 +136,21 @@ static int padding_length(struct tls_prot_info *prot, struct sk_buff *skb)
 
 	/* Determine zero-padding length */
 	if (prot->version == TLS_1_3_VERSION) {
-		int back = TLS_TAG_SIZE + 1;
+		int offset = rxm->full_len - TLS_TAG_SIZE - 1;
 		char content_type = 0;
 		int err;
 
 		while (content_type == 0) {
-			if (back > rxm->full_len - prot->prepend_size)
+			if (offset < prot->prepend_size)
 				return -EBADMSG;
-			err = skb_copy_bits(skb,
-					    rxm->offset + rxm->full_len - back,
+			err = skb_copy_bits(skb, rxm->offset + offset,
 					    &content_type, 1);
 			if (err)
 				return err;
 			if (content_type)
 				break;
 			sub++;
-			back++;
+			offset--;
 		}
 		tlm->control = content_type;
 	}