net: introduce skb_crc32c_csum_help

skb_crc32c_csum_help is like skb_checksum_help, but it is designed for
checksumming SCTP packets using crc32c (see RFC3309), provided that
libcrc32c.ko has been loaded before. In case libcrc32c is not loaded,
invoking skb_crc32c_csum_help on a skb results in one the following
printouts:

warn_crc32c_csum_update: attempt to compute crc32c without libcrc32c.ko
warn_crc32c_csum_combine: attempt to compute crc32c without libcrc32c.ko
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -3931,6 +3931,7 @@ void netdev_rss_key_fill(void *buffer, size_t len);
 
 int dev_get_nest_level(struct net_device *dev);
 int skb_checksum_help(struct sk_buff *skb);
+int skb_crc32c_csum_help(struct sk_buff *skb);
 struct sk_buff *__skb_gso_segment(struct sk_buff *skb,
 				  netdev_features_t features, bool tx_path);
 struct sk_buff *skb_mac_gso_segment(struct sk_buff *skb,

include/linux/skbuff.h

@@ -193,7 +193,8 @@
  *     accordingly. Note the there is no indication in the skbuff that the
  *     CHECKSUM_PARTIAL refers to an SCTP checksum, a driver that supports
  *     both IP checksum offload and SCTP CRC offload must verify which offload
- *     is configured for a packet presumably by inspecting packet headers.
+ *     is configured for a packet presumably by inspecting packet headers; in
+ *     case, skb_crc32c_csum_help is provided to compute CRC on SCTP packets.
  *
  *   NETIF_F_FCOE_CRC - This feature indicates that a device is capable of
  *     offloading the FCOE CRC in a packet. To perform this offload the stack

net/core/dev.c

@@ -143,6 +143,7 @@
 #include <linux/hrtimer.h>
 #include <linux/netfilter_ingress.h>
 #include <linux/crash_dump.h>
+#include <linux/sctp.h>
 
 #include "net-sysfs.h"
 
@@ -2612,6 +2613,46 @@ int skb_checksum_help(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(skb_checksum_help);
 
+int skb_crc32c_csum_help(struct sk_buff *skb)
+{
+	__le32 crc32c_csum;
+	int ret = 0, offset, start;
+
+	if (skb->ip_summed != CHECKSUM_PARTIAL)
+		goto out;
+
+	if (unlikely(skb_is_gso(skb)))
+		goto out;
+
+	/* Before computing a checksum, we should make sure no frag could
+	 * be modified by an external entity : checksum could be wrong.
+	 */
+	if (unlikely(skb_has_shared_frag(skb))) {
+		ret = __skb_linearize(skb);
+		if (ret)
+			goto out;
+	}
+	start = skb_checksum_start_offset(skb);
+	offset = start + offsetof(struct sctphdr, checksum);
+	if (WARN_ON_ONCE(offset >= skb_headlen(skb))) {
+		ret = -EINVAL;
+		goto out;
+	}
+	if (skb_cloned(skb) &&
+	    !skb_clone_writable(skb, offset + sizeof(__le32))) {
+		ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
+		if (ret)
+			goto out;
+	}
+	crc32c_csum = cpu_to_le32(~__skb_checksum(skb, start,
+						  skb->len - start, ~(__u32)0,
+						  crc32c_csum_stub));
+	*(__le32 *)(skb->data + offset) = crc32c_csum;
+	skb->ip_summed = CHECKSUM_NONE;
+out:
+	return ret;
+}
+
 __be16 skb_network_protocol(struct sk_buff *skb, int *depth)
 {
 	__be16 type = skb->protocol;