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authorCong Wang <xiyou.wangcong@gmail.com>2018-11-26 09:31:26 -0800
committerDavid S. Miller <davem@davemloft.net>2018-11-29 13:47:16 -0800
commit1464193107da8041e05341388964733bbba3be27 (patch)
tree4cb7f9a9342d0a532d10965098faee255632efe4 /net/core/skbuff.c
parent19bf62613a800ef4ffa26cbae6b723d9f46740ed (diff)
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>
Diffstat (limited to 'net/core/skbuff.c')
-rw-r--r--net/core/skbuff.c18
1 files changed, 17 insertions, 1 deletions
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 02cd7ae3d0fb..3c814565ed7c 100644
--- 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)