net: Clarify meaning of CHECKSUM_PARTIAL for receive path

The current meaning of CHECKSUM_PARTIAL for validating checksums
is that _all_ checksums in the packet are considered valid.
However, in the manner that CHECKSUM_PARTIAL is set only the checksum
at csum_start+csum_offset and any preceding checksums may
be considered valid. If there are checksums in the packet after
csum_offset it is possible they have not been verfied.

This patch changes CHECKSUM_PARTIAL logic in skb_csum_unnecessary and
__skb_gro_checksum_validate_needed to only considered checksums
referring to csum_start and any preceding checksums (with starting
offset before csum_start) to be verified.
Signed-off-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -2246,7 +2246,9 @@ static inline bool __skb_gro_checksum_validate_needed(struct sk_buff *skb,
 						      bool zero_okay,
 						      __sum16 check)
 {
-	return (skb->ip_summed != CHECKSUM_PARTIAL &&
+	return ((skb->ip_summed != CHECKSUM_PARTIAL ||
+		skb_checksum_start_offset(skb) <
+		 skb_gro_offset(skb)) &&
 		NAPI_GRO_CB(skb)->csum_cnt == 0 &&
 		(!zero_okay || check));
 }

include/linux/skbuff.h

@@ -83,11 +83,15 @@
  *
  * CHECKSUM_PARTIAL:
  *
- *   This is identical to the case for output below. This may occur on a packet
+ *   A checksum is set up to be offloaded to a device as described in the
+ *   output description for CHECKSUM_PARTIAL. This may occur on a packet
  *   received directly from another Linux OS, e.g., a virtualized Linux kernel
- *   on the same host. The packet can be treated in the same way as
- *   CHECKSUM_UNNECESSARY, except that on output (i.e., forwarding) the
- *   checksum must be filled in by the OS or the hardware.
+ *   on the same host, or it may be set in the input path in GRO or remote
+ *   checksum offload. For the purposes of checksum verification, the checksum
+ *   referred to by skb->csum_start + skb->csum_offset and any preceding
+ *   checksums in the packet are considered verified. Any checksums in the
+ *   packet that are after the checksum being offloaded are not considered to
+ *   be verified.
  *
  * B. Checksumming on output.
  *
@@ -2915,7 +2919,10 @@ __sum16 __skb_checksum_complete(struct sk_buff *skb);
 
 static inline int skb_csum_unnecessary(const struct sk_buff *skb)
 {
-	return ((skb->ip_summed & CHECKSUM_UNNECESSARY) || skb->csum_valid);
+	return ((skb->ip_summed == CHECKSUM_UNNECESSARY) ||
+		skb->csum_valid ||
+		(skb->ip_summed == CHECKSUM_PARTIAL &&
+		 skb_checksum_start_offset(skb) >= 0));
 }
 
 /**