On Sun, 2015-09-27 at 22:37 -0700, Tom Herbert wrote: > > Which drivers are doing this? It is up to the driver to determine > whether a particular packet being sent can have checksum offloaded to > the device. If it cannot offload the checksum it must call > skb_checksum_help.
Not so. A driver sets the NETIF_F_IP_CSUM feature to indicate that it can do the checksum on Legacy IP TCP or UDP frames and *nothing* else. It most certainly does not expect to be handed any other kind of packet for checksumming, and bad things will often happen if it is. If drivers *do* spot that they've been given something they don't handle, I see BUG() calls and warnings, but I don't see any of them calling skb_checksum_help() to silently cope. Many of them just feed it to the hardware and don't even notice at all because it's the *hardware* which decides whether to do a TCP or a UDP checksum. So who knows what'll happen. The check is supposed to be done in can_checksum_protocol(), called from harmonize_features(). But as noted, that check has false positives and lets some inappropriate packets through — for NETIF_F_IP_CSUM it lets through *all* skbuffs with ->protocol == ETH_P_IP instead of only TCP and UDP. I originally couldn't see how to deal with this except by looking at the contents of the packet, which sucked. But I think I've found a somewhat more acceptable approach now: http://lists.openwall.net/netdev/2015/09/25/85 -- David Woodhouse Open Source Technology Centre david.woodho...@intel.com Intel Corporation
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