net: fix recent csum changes

Vladimir reported csum issues after my recent change in skb_postpull_rcsum()

Issue here is the following:

initial skb->csum is the csum of

[part to be pulled][rest of packet]

Old code:
 skb->csum = csum_sub(skb->csum, csum_partial(pull, pull_length, 0));

New code:
 skb->csum = ~csum_partial(pull, pull_length, ~skb->csum);

This is broken if the csum of [pulled part]
happens to be equal to skb->csum, because end
result of skb->csum is 0 in new code, instead
of being 0xffffffff

David Laight suggested to use

skb->csum = -csum_partial(pull, pull_length, -skb->csum);

I based my patches on existing code present in include/net/seg6.h,
update_csum_diff4() and update_csum_diff16() which might need
a similar fix.

I guess that my tests, mostly pulling 40 bytes of IPv6 header
were not providing enough entropy to hit this bug.

v2: added wsum_negate() to make sparse happy.

Fixes: 29c3002644 ("net: optimize skb_postpull_rcsum()")
Fixes: 0bd28476f6 ("gro: optimize skb_gro_postpull_rcsum()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Suggested-by: David Laight <David.Laight@ACULAB.COM>
Cc: David Lebrun <dlebrun@google.com>
Tested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20211204045356.3659278-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/linux/skbuff.h

@@ -3486,7 +3486,8 @@ static inline void skb_postpull_rcsum(struct sk_buff *skb,
 				      const void *start, unsigned int len)
 {
 	if (skb->ip_summed == CHECKSUM_COMPLETE)
-		skb->csum = ~csum_partial(start, len, ~skb->csum);
+		skb->csum = wsum_negate(csum_partial(start, len,
+						     wsum_negate(skb->csum)));
 	else if (skb->ip_summed == CHECKSUM_PARTIAL &&
 		 skb_checksum_start_offset(skb) < 0)
 		skb->ip_summed = CHECKSUM_NONE;

include/net/checksum.h

@@ -180,4 +180,8 @@ static inline void remcsum_unadjust(__sum16 *psum, __wsum delta)
 	*psum = csum_fold(csum_sub(delta, (__force __wsum)*psum));
 }
 
+static inline __wsum wsum_negate(__wsum val)
+{
+	return (__force __wsum)-((__force u32)val);
+}
 #endif

include/net/gro.h

@@ -173,8 +173,8 @@ static inline void skb_gro_postpull_rcsum(struct sk_buff *skb,
 					const void *start, unsigned int len)
 {
 	if (NAPI_GRO_CB(skb)->csum_valid)
-		NAPI_GRO_CB(skb)->csum = ~csum_partial(start, len,
-						       ~NAPI_GRO_CB(skb)->csum);
+		NAPI_GRO_CB(skb)->csum = wsum_negate(csum_partial(start, len,
+						wsum_negate(NAPI_GRO_CB(skb)->csum)));
 }
 
 /* GRO checksum functions. These are logical equivalents of the normal