Merge branch 'net-fix-CRC32c-in-the-forwarding-path'

Davide Caratti says:

====================
net: fix CRC32c in the forwarding path

Current kernel allows offloading CRC32c computation when SCTP packets
are generated, setting skb->ip_summed to CHECKSUM_PARTIAL, if the
underlying device features have NETIF_F_SCTP_CRC set. However, after these
packets are forwarded, they may land on a device where CRC32c offloading is
not available: as a consequence, transmission is done with wrong CRC32c.
It's not possible to use sctp_compte_cksum() in the forwarding path
and in most drivers, because it needs symbols exported by libcrc32c module.

Patch 1 and 2 of this series try to solve this problem, introducing a new
helper function, namely skb_crc32c_csum_help(), that can be used to resolve
CHECKSUM_PARTIAL when crc32c is needed instead of Internet Checksum.

Currently, we need to parse the packet headers to understand what algorithm
is needed to resolve CHECKSUM_PARTIAL. We can speedup things by storing
this information in the skb metadata, and use it to call an appropriate
helper (skb_checksum_help or skb_crc32c_csum_help), or leave the packet
unmodified when the NIC is able to offload the checksum computation.

Patch 3 deprecates skb->csum_bad to free one bit in skb metadata; patch 4
introduces skb->csum_not_inet, providing skb with an indication on the
algorithm needed to resolve CHECKSUM_PARTIAL.
Patch 5 and 6 fix the kernel forwarding path and openvswitch datapath,
where skb_checksum_help was unconditionally called to resolve CHECKSUM_PARTIAL,
thus generating wrong CRC32c in forwarded SCTP packets.
Finally, patch 7 updates documentation to provide a better description of
possible values of skb->ip_summed.

Some further work is still possible:
* drivers that parse the packet header to correctly resolve CHECKSUM_PARTIAL
(e.g. ixgbe_tx_csum()) can benefit from testing skb->csum_not_inet to avoid
calling ip_hdr(skb)->protocol or ixgbe_ipv6_csum_is_sctp(skb).

* drivers that call skb_checksum_help() to resolve CHECKSUM_PARTIAL can
call skb_csum_hwoffload_help to avoid corrupting SCTP packets.

Changes v2->v3:
- patch 1/7: more standard declaration of stub variables

Changes v1->v2:
- none

Changes RFCv4->v1:
- patch 2/7: use WARN_ON_ONCE() instead of BUG_ON(), and avoid computing
CRC32c on the error path.
- patch 3/7: don't invert tests on the values of same_flow and
NAPI_GRO_CB(skb)->flush in dev_gro_receive(), it's useless and it breaks
GRO functionality as reported by kernel test robot.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>