Re: [PATCH net v2 1/4] net/core: relax BUILD_BUG_ON in netdev_stats_to_stats64

[Eric Dumazet]

On Sat, 2016-01-30 at 13:19 -0500, Jarod Wilson wrote:
> The netdev_stats_to_stats64 function copies the deprecated
> net_device_stats format stats into rtnl_link_stats64 for legacy support
> purposes, but with the BUILD_BUG_ON as it was, it wasn't possible to
> extend rtnl_link_stats64 without also extending net_device_stats. Relax
> the BUILD_BUG_ON to only require that rtnl_link_stats64 is larger, and
> zero out all the stat counters that aren't present in net_device_stats.
> 
> CC: Eric Dumazet <edumazet@xxxxxxxxxx>
> CC: netdev@xxxxxxxxxxxxxxx
> Signed-off-by: Jarod Wilson <jarod@xxxxxxxxxx>
> ---
> Re-re-sending, hopefully getting the patch to the right list this time.
> 
>  net/core/dev.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 8cba3d8..575a7df 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -7253,25 +7253,30 @@ void netdev_run_todo(void)
>  	}
>  }
>  
> -/* Convert net_device_stats to rtnl_link_stats64.  They have the same
> - * fields in the same order, with only the type differing.
> +/* Convert net_device_stats to rtnl_link_stats64. rtnl_link_stats64 has
> + * all the same fields in the same order as net_device_stats, with only
> + * the type differing, but rtnl_link_stats64 may have additional fields
> + * at the end for newer counters.
>   */
>  void netdev_stats_to_stats64(struct rtnl_link_stats64 *stats64,
>  			     const struct net_device_stats *netdev_stats)
>  {
>  #if BITS_PER_LONG == 64
> -	BUILD_BUG_ON(sizeof(*stats64) != sizeof(*netdev_stats));
> +	BUILD_BUG_ON(sizeof(*stats64) < sizeof(*netdev_stats));
>  	memcpy(stats64, netdev_stats, sizeof(*stats64));
>  #else
>  	size_t i, n = sizeof(*stats64) / sizeof(u64);
>  	const unsigned long *src = (const unsigned long *)netdev_stats;
>  	u64 *dst = (u64 *)stats64;
>  
> -	BUILD_BUG_ON(sizeof(*netdev_stats) / sizeof(unsigned long) !=
> +	BUILD_BUG_ON(sizeof(*netdev_stats) / sizeof(unsigned long) >
>  		     sizeof(*stats64) / sizeof(u64));
>  	for (i = 0; i < n; i++)
>  		dst[i] = src[i];
>  #endif
> +	/* zero out counters that only exist in rtnl_link_stats64 */
> +	memset((char *)stats64 + sizeof(*netdev_stats), 0,
> +	       sizeof(*stats64) - sizeof(*netdev_stats));

Are you sure it works on 32bit arches ?