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

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

> 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;
 }


> -	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