skbuff: add stub to help computing crc32c on SCTP packets

sctp_compute_checksum requires crc32c symbol (provided by libcrc32c), so
it can't be used in net core. Like it has been done previously with other
symbols (e.g. ipv6_dst_lookup), introduce a stub struct skb_checksum_ops
to allow computation of crc32c checksum in net core after sctp.ko (and thus
libcrc32c) has been loaded.
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -3076,6 +3076,8 @@ struct skb_checksum_ops {
 	__wsum (*combine)(__wsum csum, __wsum csum2, int offset, int len);
 };
 
+extern const struct skb_checksum_ops *crc32c_csum_stub __read_mostly;
+
 __wsum __skb_checksum(const struct sk_buff *skb, int offset, int len,
 		      __wsum csum, const struct skb_checksum_ops *ops);
 __wsum skb_checksum(const struct sk_buff *skb, int offset, int len,

net/core/skbuff.c

@@ -2243,6 +2243,32 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 }
 EXPORT_SYMBOL(skb_copy_and_csum_bits);
 
+static __wsum warn_crc32c_csum_update(const void *buff, int len, __wsum sum)
+{
+	net_warn_ratelimited(
+		"%s: attempt to compute crc32c without libcrc32c.ko\n",
+		__func__);
+	return 0;
+}
+
+static __wsum warn_crc32c_csum_combine(__wsum csum, __wsum csum2,
+				       int offset, int len)
+{
+	net_warn_ratelimited(
+		"%s: attempt to compute crc32c without libcrc32c.ko\n",
+		__func__);
+	return 0;
+}
+
+static const struct skb_checksum_ops default_crc32c_ops = {
+	.update  = warn_crc32c_csum_update,
+	.combine = warn_crc32c_csum_combine,
+};
+
+const struct skb_checksum_ops *crc32c_csum_stub __read_mostly =
+	&default_crc32c_ops;
+EXPORT_SYMBOL(crc32c_csum_stub);
+
  /**
  *	skb_zerocopy_headlen - Calculate headroom needed for skb_zerocopy()
  *	@from: source buffer

net/sctp/offload.c

@@ -98,6 +98,11 @@ static const struct net_offload sctp6_offload = {
 	},
 };
 
+static const struct skb_checksum_ops crc32c_csum_ops = {
+	.update  = sctp_csum_update,
+	.combine = sctp_csum_combine,
+};
+
 int __init sctp_offload_init(void)
 {
 	int ret;
@@ -110,6 +115,7 @@ int __init sctp_offload_init(void)
 	if (ret)
 		goto ipv4;
 
+	crc32c_csum_stub = &crc32c_csum_ops;
 	return ret;
 
 ipv4: