net/core/skbuff.c

2076 	/* Only adjust this if it actually is csum_start rather than csum */
2078 		skb->csum_start += off;
4806 				SKB_GSO_CB(nskb)->csum_start =
4898 			SKB_GSO_CB(nskb)->csum_start =
5551  * for skb->csum_start and skb->csum_offset are valid so we don't oops.
5559 	u32 csum_start = skb_headroom(skb) + (u32)start;
5561 	if (unlikely(csum_start >= U16_MAX || csum_end > skb_headlen(skb))) {
5567 	skb->csum_start = csum_start;
5569 	skb->transport_header = csum_start;