net: add skb_copy_and_crc32c_datagram_iter()

Since skb_copy_and_hash_datagram_iter() is used only with CRC32C, the
crypto_ahash abstraction provides no value.  Add
skb_copy_and_crc32c_datagram_iter() which just calls crc32c() directly.

This is faster and simpler.  It also doesn't have the weird dependency
issue where skb_copy_and_hash_datagram_iter() depends on
CONFIG_CRYPTO_HASH=y without that being expressed explicitly in the
kconfig (presumably because it was too heavyweight for NET to select).
The new function is conditional on the hidden boolean symbol NET_CRC32C,
which selects CRC32.  So it gets compiled only when something that
actually needs CRC32C packet checksums is enabled, it has no implicit
dependency, and it doesn't depend on the heavyweight crypto layer.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Link: https://patch.msgid.link/20250519175012.36581-9-ebiggers@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: ebiggers

include/linux/skbuff.h

@@ -4137,6 +4137,8 @@ int skb_copy_and_csum_datagram_msg(struct sk_buff *skb, int hlen,
 int skb_copy_and_hash_datagram_iter(const struct sk_buff *skb, int offset,
 			   struct iov_iter *to, int len,
 			   struct ahash_request *hash);
+int skb_copy_and_crc32c_datagram_iter(const struct sk_buff *skb, int offset,
+				      struct iov_iter *to, int len, u32 *crcp);
 int skb_copy_datagram_from_iter(struct sk_buff *skb, int offset,
 				 struct iov_iter *from, int len);
 int zerocopy_sg_from_iter(struct sk_buff *skb, struct iov_iter *frm);

net/core/datagram.c

@@ -52,6 +52,7 @@
 #include <linux/pagemap.h>
 #include <linux/iov_iter.h>
 #include <linux/indirect_call_wrapper.h>
+#include <linux/crc32.h>
 
 #include <net/protocol.h>
 #include <linux/skbuff.h>
@@ -519,6 +520,38 @@ int skb_copy_and_hash_datagram_iter(const struct sk_buff *skb, int offset,
 }
 EXPORT_SYMBOL(skb_copy_and_hash_datagram_iter);
 
+#ifdef CONFIG_NET_CRC32C
+static size_t crc32c_and_copy_to_iter(const void *addr, size_t bytes,
+				      void *_crcp, struct iov_iter *i)
+{
+	u32 *crcp = _crcp;
+	size_t copied;
+
+	copied = copy_to_iter(addr, bytes, i);
+	*crcp = crc32c(*crcp, addr, copied);
+	return copied;
+}
+
+/**
+ *	skb_copy_and_crc32c_datagram_iter - Copy datagram to an iovec iterator
+ *		and update a CRC32C value.
+ *	@skb: buffer to copy
+ *	@offset: offset in the buffer to start copying from
+ *	@to: iovec iterator to copy to
+ *	@len: amount of data to copy from buffer to iovec
+ *	@crcp: pointer to CRC32C value to update
+ *
+ *	Return: 0 on success, -EFAULT if there was a fault during copy.
+ */
+int skb_copy_and_crc32c_datagram_iter(const struct sk_buff *skb, int offset,
+				      struct iov_iter *to, int len, u32 *crcp)
+{
+	return __skb_datagram_iter(skb, offset, to, len, true,
+				   crc32c_and_copy_to_iter, crcp);
+}
+EXPORT_SYMBOL(skb_copy_and_crc32c_datagram_iter);
+#endif /* CONFIG_NET_CRC32C */
+
 static size_t simple_copy_to_iter(const void *addr, size_t bytes,
 		void *data __always_unused, struct iov_iter *i)
 {