Re: [dpdk-dev] [PATCH] net: fix unneeded replacement of 0 by ffff for TCP checksum

[Olivier Matz <olivier.matz@6wind.com>]

On Fri, Jul 10, 2020 at 03:10:34PM +0200, Morten Brørup wrote:
> > From: Olivier Matz [mailto:olivier.matz@6wind.com]
> > Sent: Friday, July 10, 2020 2:41 PM
> > 
> > On Fri, Jul 10, 2020 at 02:55:51PM +0800, Hongzhi Guo wrote:
> > > Per RFC768:
> > > If the computed checksum is zero, it is transmitted as all ones.
> > > An all zero transmitted checksum value means that the transmitter
> > > generated no checksum.
> > >
> > > RFC793 for TCP has no such special treatment for the checksum of
> > zero.
> > >
> > > Fixes: 6006818cfb26 ("net: new checksum functions")
> > > Cc: stable@dpdk.org
> > >
> > > Signed-off-by: Hongzhi Guo <guohongzhi1@huawei.com>
> > > ---
> > > v2:
> > > * Fixed commit tile
> > > * Fixed the API comment
> > > ---
> > > ---
> > >  lib/librte_net/rte_ip.h | 18 +++++++++++++++---
> > >  1 file changed, 15 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/lib/librte_net/rte_ip.h b/lib/librte_net/rte_ip.h
> > > index 292f63fd7..d03c77120 100644
> > > --- a/lib/librte_net/rte_ip.h
> > > +++ b/lib/librte_net/rte_ip.h
> > > @@ -325,7 +325,7 @@ rte_ipv4_phdr_cksum(const struct rte_ipv4_hdr
> > *ipv4_hdr, uint64_t ol_flags)
> > >   *   The pointer to the beginning of the L4 header.
> > >   * @return
> > >   *   The complemented checksum to set in the IP packet
> > > - *   or 0 on error
> > > + *   or 0 if the IP length is invalid in the header.
> > >   */
> > >  static inline uint16_t
> > >  rte_ipv4_udptcp_cksum(const struct rte_ipv4_hdr *ipv4_hdr, const
> > void *l4_hdr)
> 
> 0 is a valid return value, so I suggest omitting it from the return value description:
> 
>   * @return
> - *   The complemented checksum to set in the IP packet
> - *   or 0 on error
> + *   The complemented checksum to set in the IP packet.
> 
> The comparison "if (l3_len < sizeof(struct rte_ipv4_hdr))" is only there to protect against invalid input; it prevents l4_len from becoming negative.

I don't get why "0 if the IP length is invalid in the header" should
be removed from the comment: 0 is both a valid return value and
the value returned on invalid packet.

> For the same reason, unlikely() should be added to this comparison.

Maybe yes, but that's another story I think.

> Otherwise,
> 
> Acked-by: Morten Brørup <mb@smartsharesystems.com>
>