[PATCH net-next 02/10] net: add skb_crc32c()

[Eric Biggers <ebiggers@kernel.org>]

From: Eric Biggers <ebiggers@google.com>

Add skb_crc32c(), which calculates the CRC32C of a sk_buff.  It will
replace __skb_checksum(), which unnecessarily supports arbitrary
checksums.  Compared to __skb_checksum(), skb_crc32c():

   - Uses the correct type for CRC32C values (u32, not __wsum).

   - Does not require the caller to provide a skb_checksum_ops struct.

   - Is faster because it does not use indirect calls and does not use
     the very slow crc32c_combine().

According to commit 2817a336d4d5 ("net: skb_checksum: allow custom
update/combine for walking skb") which added __skb_checksum(), the
original motivation for the abstraction layer was to avoid code
duplication for CRC32C and other checksums in the future.  However:

   - No additional checksums showed up after CRC32C.  __skb_checksum()
     is only used with the "regular" net checksum and CRC32C.

   - Indirect calls are expensive.  Commit 2544af0344ba ("net: avoid
     indirect calls in L4 checksum calculation") worked around this
     using the INDIRECT_CALL_1 macro. But that only avoided the indirect
     call for the net checksum, and at the cost of an extra branch.

   - The checksums use different types (__wsum and u32), causing casts
     to be needed.

   - It made the checksums of fragments be combined (rather than
     chained) for both checksums, despite this being highly
     counterproductive for CRC32C due to how slow crc32c_combine() is.
     This can clearly be seen in commit 4c2f24549644 ("sctp: linearize
     early if it's not GSO") which tried to work around this performance
     bug.  With a dedicated function for each checksum, we can instead
     just use the proper strategy for each checksum.

Signed-off-by: Eric Biggers <ebiggers@google.com>
---
 include/linux/skbuff.h |  1 +
 net/core/skbuff.c      | 73 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index f3e72be6f634..33b33bb18aa6 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -4192,10 +4192,11 @@ 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,
 		    __wsum csum);
+u32 skb_crc32c(const struct sk_buff *skb, int offset, int len, u32 crc);
 
 static inline void * __must_check
 __skb_header_pointer(const struct sk_buff *skb, int offset, int len,
 		     const void *data, int hlen, void *buffer)
 {
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index d73ad79fe739..b9900cc16a24 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -62,10 +62,11 @@
 #include <linux/bitfield.h>
 #include <linux/if_vlan.h>
 #include <linux/mpls.h>
 #include <linux/kcov.h>
 #include <linux/iov_iter.h>
+#include <linux/crc32.h>
 
 #include <net/protocol.h>
 #include <net/dst.h>
 #include <net/sock.h>
 #include <net/checksum.h>
@@ -3626,10 +3627,82 @@ __wsum skb_copy_and_csum_bits(const struct sk_buff *skb, int offset,
 	BUG_ON(len);
 	return csum;
 }
 EXPORT_SYMBOL(skb_copy_and_csum_bits);
 
+#ifdef CONFIG_NET_CRC32C
+u32 skb_crc32c(const struct sk_buff *skb, int offset, int len, u32 crc)
+{
+	int start = skb_headlen(skb);
+	int i, copy = start - offset;
+	struct sk_buff *frag_iter;
+
+	if (copy > 0) {
+		copy = min(copy, len);
+		crc = crc32c(crc, skb->data + offset, copy);
+		len -= copy;
+		if (len == 0)
+			return crc;
+		offset += copy;
+	}
+
+	if (WARN_ON_ONCE(!skb_frags_readable(skb)))
+		return 0;
+
+	for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
+		int end;
+		skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
+
+		WARN_ON(start > offset + len);
+
+		end = start + skb_frag_size(frag);
+		copy = end - offset;
+		if (copy > 0) {
+			u32 p_off, p_len, copied;
+			struct page *p;
+			u8 *vaddr;
+
+			copy = min(copy, len);
+			skb_frag_foreach_page(frag,
+					      skb_frag_off(frag) + offset - start,
+					      copy, p, p_off, p_len, copied) {
+				vaddr = kmap_atomic(p);
+				crc = crc32c(crc, vaddr + p_off, p_len);
+				kunmap_atomic(vaddr);
+			}
+			len -= copy;
+			if (len == 0)
+				return crc;
+			offset += copy;
+		}
+		start = end;
+	}
+
+	skb_walk_frags(skb, frag_iter) {
+		int end;
+
+		WARN_ON(start > offset + len);
+
+		end = start + frag_iter->len;
+		copy = end - offset;
+		if (copy > 0) {
+			copy = min(copy, len);
+			crc = skb_crc32c(frag_iter, offset - start, copy, crc);
+			len -= copy;
+			if (len == 0)
+				return crc;
+			offset += copy;
+		}
+		start = end;
+	}
+	BUG_ON(len);
+
+	return crc;
+}
+EXPORT_SYMBOL(skb_crc32c);
+#endif /* CONFIG_NET_CRC32C */
+
 __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len)
 {
 	__sum16 sum;
 
 	sum = csum_fold(skb_checksum(skb, 0, len, skb->csum));
-- 
2.49.0