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

From: Jarod Wilson <[email-masked]>
Date: Sat, 30 Jan 2016 15:39:01 -0500

On Sat, Jan 30, 2016 at 10:34:32AM -0800, Eric Dumazet wrote:
> 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 <[email-masked]>
> > CC: [email-masked]
> > Signed-off-by: Jarod Wilson <[email-masked]>
> > ---
> > 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 ?

Ew, no, it won't work correctly on 32-bit. The for loop is going to copy
data into dst from beyond the end of netdev_stats, and the range looks
like it won't be right either, only half of the added stats64 space will
get zeroed out. Okay, I'll fix that up correctly.

-- 
Jarod Wilson
[email-masked]