udp: fix skb_copy_and_csum_datagram with odd segment sizes

When iteratively computing a checksum with csum_block_add, track the
offset "pos" to correctly rotate in csum_block_add when offset is odd.

The open coded implementation of skb_copy_and_csum_datagram did this.
With the switch to __skb_datagram_iter calling csum_and_copy_to_iter,
pos was reinitialized to 0 on each call.

Bring back the pos by passing it along with the csum to the callback.

Changes v1->v2
- pass csum value, instead of csump pointer (Alexander Duyck)

Link: https://lore.kernel.org/netdev/20210128152353.GB27281@optiplex/
Fixes: 950fcaecd5cc ("datagram: consolidate datagram copy to iter helpers")
Reported-by: Oliver Graute <[e-mail]>
Signed-off-by: Willem de Bruijn <[e-mail]>
Reviewed-by: Alexander Duyck <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/uio.h

···
 {
 	i->count = count;
 }
-size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *csump, struct iov_iter *i);
+
+struct csum_state {
+	__wsum csum;
+	size_t off;
+};
+
+size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *csstate, struct iov_iter *i);
 size_t csum_and_copy_from_iter(void *addr, size_t bytes, __wsum *csum, struct iov_iter *i);
 bool csum_and_copy_from_iter_full(void *addr, size_t bytes, __wsum *csum, struct iov_iter *i);
 size_t hash_and_copy_to_iter(const void *addr, size_t bytes, void *hashp,

lib/iov_iter.c

···
 }
 
 static size_t csum_and_copy_to_pipe_iter(const void *addr, size_t bytes,
-				__wsum *csum, struct iov_iter *i)
+					 struct csum_state *csstate,
+					 struct iov_iter *i)
 {
 	struct pipe_inode_info *pipe = i->pipe;
 	unsigned int p_mask = pipe->ring_size - 1;
+	__wsum sum = csstate->csum;
+	size_t off = csstate->off;
 	unsigned int i_head;
 	size_t n, r;
-	size_t off = 0;
-	__wsum sum = *csum;
 
 	if (!sanity(i))
 		return 0;
···
 		i_head++;
 	} while (n);
 	i->count -= bytes;
-	*csum = sum;
+	csstate->csum = sum;
+	csstate->off = off;
 	return bytes;
 }
 
···
 }
 EXPORT_SYMBOL(csum_and_copy_from_iter_full);
 
-size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *csump,
+size_t csum_and_copy_to_iter(const void *addr, size_t bytes, void *_csstate,
 			     struct iov_iter *i)
 {
+	struct csum_state *csstate = _csstate;
 	const char *from = addr;
-	__wsum *csum = csump;
 	__wsum sum, next;
-	size_t off = 0;
+	size_t off;
 
 	if (unlikely(iov_iter_is_pipe(i)))
-		return csum_and_copy_to_pipe_iter(addr, bytes, csum, i);
+		return csum_and_copy_to_pipe_iter(addr, bytes, _csstate, i);
 
-	sum = *csum;
+	sum = csstate->csum;
+	off = csstate->off;
 	if (unlikely(iov_iter_is_discard(i))) {
 		WARN_ON(1);	/* for now */
 		return 0;
···
 		off += v.iov_len;
 	})
 	)
-	*csum = sum;
+	csstate->csum = sum;
+	csstate->off = off;
 	return bytes;
 }
 EXPORT_SYMBOL(csum_and_copy_to_iter);

net/core/datagram.c

···
 				      struct iov_iter *to, int len,
 				      __wsum *csump)
 {
-	return __skb_datagram_iter(skb, offset, to, len, true,
-			csum_and_copy_to_iter, csump);
+	struct csum_state csdata = { .csum = *csump };
+	int ret;
+
+	ret = __skb_datagram_iter(skb, offset, to, len, true,
+				  csum_and_copy_to_iter, &csdata);
+	if (ret)
+		return ret;
+
+	*csump = csdata.csum;
+	return 0;
 }
 
 /**