net: explain __skb_checksum_complete() with comments

Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/core/dev.c b/net/core/dev.c
index f69b2fcdee4091d38278f367aced4a0dee85ad46..abe50c424b298575a8aa714fa4ddfa17b6af3a65 100644 (file)
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5791,6 +5791,7 @@ __sum16 __skb_gro_checksum_complete(struct sk_buff *skb)
 
        /* NAPI_GRO_CB(skb)->csum holds pseudo checksum */
        sum = csum_fold(csum_add(NAPI_GRO_CB(skb)->csum, wsum));
+       /* See comments in __skb_checksum_complete(). */
        if (likely(!sum)) {
                if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) &&
                    !skb->csum_complete_sw)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 02cd7ae3d0fb26ef0a8b006390154fdefd0d292f..3c814565ed7cd89af02b7169fc92b7224a699c0a 100644 (file)
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -2635,6 +2635,7 @@ __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len)
        __sum16 sum;
 
        sum = csum_fold(skb_checksum(skb, 0, len, skb->csum));
+       /* See comments in __skb_checksum_complete(). */
        if (likely(!sum)) {
                if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) &&
                    !skb->csum_complete_sw)
@@ -2646,6 +2647,15 @@ __sum16 __skb_checksum_complete_head(struct sk_buff *skb, int len)
 }
 EXPORT_SYMBOL(__skb_checksum_complete_head);
 
+/* This function assumes skb->csum already holds pseudo header's checksum,
+ * which has been changed from the hardware checksum, for example, by
+ * __skb_checksum_validate_complete(). And, the original skb->csum must
+ * have been validated unsuccessfully for CHECKSUM_COMPLETE case.
+ *
+ * It returns non-zero if the recomputed checksum is still invalid, otherwise
+ * zero. The new checksum is stored back into skb->csum unless the skb is
+ * shared.
+ */
 __sum16 __skb_checksum_complete(struct sk_buff *skb)
 {
        __wsum csum;
@@ -2653,8 +2663,14 @@ __sum16 __skb_checksum_complete(struct sk_buff *skb)
 
        csum = skb_checksum(skb, 0, skb->len, 0);
 
-       /* skb->csum holds pseudo checksum */
        sum = csum_fold(csum_add(skb->csum, csum));
+       /* This check is inverted, because we already knew the hardware
+        * checksum is invalid before calling this function. So, if the
+        * re-computed checksum is valid instead, then we have a mismatch
+        * between the original skb->csum and skb_checksum(). This means either
+        * the original hardware checksum is incorrect or we screw up skb->csum
+        * when moving skb->data around.
+        */
        if (likely(!sum)) {
                if (unlikely(skb->ip_summed == CHECKSUM_COMPLETE) &&
                    !skb->csum_complete_sw)