Re: [dpdk-dev] [PATCH] net: fix the way how L4 checksum choice is tested

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: Ananyev, Konstantin
> Sent: Monday, June 24, 2019 12:52 PM
> To: 'Ivan Malov' <ivan.malov@oktetlabs.ru>; Olivier Matz <olivier.matz@6wind.com>
> Cc: dev@dpdk.org; Kulasek, TomaszX <tomaszx.kulasek@intel.com>; stable@dpdk.org
> Subject: RE: [dpdk-dev] [PATCH] net: fix the way how L4 checksum choice is tested
> 
> 
> > The API to prepare checksum offloads mistreats L4
> > checksum type enum values as self-contained flags.
> >
> > Turning these flag checks into enum checks causes
> > warnings by GCC about possibly uninitialised IPv4
> > header pointer. The issue was found to show up in
> > the case of GCC versions 4.8.5 and 5.4.0, however,
> > it might be the case for a wider variety of other
> > versions. As GCC version 7.4.0 is not susceptible
> > to the said false positive assessment, this patch
> > maintains a compiler barrier for earlier versions.
> >
> > Fixes: 4fb7e803eb1a ("ethdev: add Tx preparation")
> > Cc: Tomasz Kulasek <tomaszx.kulasek@intel.com>
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
> > ---
> >  lib/librte_net/rte_net.h | 16 ++++++++++++++--
> >  1 file changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/lib/librte_net/rte_net.h b/lib/librte_net/rte_net.h
> > index 7088584..fb09431 100644
> > --- a/lib/librte_net/rte_net.h
> > +++ b/lib/librte_net/rte_net.h
> > @@ -151,7 +151,19 @@ uint32_t rte_net_get_ptype(const struct rte_mbuf *m,
> >  			ipv4_hdr->hdr_checksum = 0;
> >  	}
> >
> 
> As I remember, saw something similar before...
> Probably the eaiser way to overcome it, is just to always initialize ipv4_hdr above,
> something like:
> 
> +ipv4_hdr = NULL;
> if (ol_flags & PKT_TX_IPV4) {
>                 ipv4_hdr = rte_pktmbuf_mtod_offset(m, struct rte_ipv4_hdr *,
>                                 inner_l3_offset);
> 
>                 if (ol_flags & PKT_TX_IP_CKSUM)
>                         ipv4_hdr->hdr_checksum = 0;
>  }

As another possible option  always initialisze both, and then use either one or another,
depending on flags:

ipv4_hdr = rte_pktmbuf_mtod_offset(m, struct rte_ipv4_hdr *, inner_l3_offset);
ipv6_hdr = rte_pktmbuf_mtod_offset(m, struct rte_ipv6_hdr *, inner_l3_offset);
....

> 
> 
> > -	if ((ol_flags & PKT_TX_UDP_CKSUM) == PKT_TX_UDP_CKSUM) {
> > +#ifdef GCC_VERSION
> > +#if GCC_VERSION < 70400
> > +	/*
> > +	 * Earlier versions of GCC suspect access to possibly
> > +	 * uninitialised ipv4_hdr in the code below, although
> > +	 * the said variable is properly initialised above.
> > +	 * Use compiler barrier to cope with the problem.
> > +	 */
> > +	rte_compiler_barrier();
> > +#endif
> > +#endif
> > +
> > +	if ((ol_flags & PKT_TX_L4_MASK) == PKT_TX_UDP_CKSUM) {
> >  		if (ol_flags & PKT_TX_IPV4) {
> >  			udp_hdr = (struct rte_udp_hdr *)((char *)ipv4_hdr +
> >  					m->l3_len);
> > @@ -167,7 +179,7 @@ uint32_t rte_net_get_ptype(const struct rte_mbuf *m,
> >  			udp_hdr->dgram_cksum = rte_ipv6_phdr_cksum(ipv6_hdr,
> >  					ol_flags);
> >  		}
> > -	} else if ((ol_flags & PKT_TX_TCP_CKSUM) ||
> > +	} else if ((ol_flags & PKT_TX_L4_MASK) == PKT_TX_TCP_CKSUM ||
> >  			(ol_flags & PKT_TX_TCP_SEG)) {
> >  		if (ol_flags & PKT_TX_IPV4) {
> >  			/* non-TSO tcp or TSO */
> > --
> > 1.8.3.1