Subject: Re: net: Zeroing the structure ethtool_wolinfo in ethtool_get_wol()

From: Joe Perches <[email-masked]>
Date: Tue, 23 Aug 2016 08:05:16 -0700

On Tue, 2016-08-23 at 07:21 -0700, Eric Dumazet wrote:
> On Tue, 2016-08-23 at 14:41 +0100, Luis Henriques wrote:
> > From: Avijit Kanti Das <[email-masked]>
> > 
> > memset() the structure ethtool_wolinfo that has padded bytes
> > but the padded bytes have not been zeroed out.
[]
> > diff --git a/net/core/ethtool.c b/net/core/ethtool.c
[]
> > @@ -1435,11 +1435,13 @@ static int ethtool_reset(struct net_device *dev, char __user *useraddr)
> >  
> >  static int ethtool_get_wol(struct net_device *dev, char __user *useraddr)
> >  {
> > -	struct ethtool_wolinfo wol = { .cmd = ETHTOOL_GWOL };
> > +	struct ethtool_wolinfo wol;
> >  
> >  	if (!dev->ethtool_ops->get_wol)
> >  		return -EOPNOTSUPP;
> >  
> > +	memset(&wol, 0, sizeof(struct ethtool_wolinfo));
> > +	wol.cmd = ETHTOOL_GWOL;
> >  	dev->ethtool_ops->get_wol(dev, &wol);
> >  
> >  	if (copy_to_user(useraddr, &wol, sizeof(wol)))
> This would suggest a compiler bug to me.

A compiler does not have a standards based requirement to
initialize arbitrary padding bytes.

I believe gcc always does zero all padding anyway.

> I checked that my compiler does properly put zeros there, even in the
> padding area.
> 
> If we can not rely on such constructs, we have hundreds of similar
> patches to submit.

True.

>>From a practical point of view, does any compiler used for
kernel compilation (gcc/icc/llvm/any others?) not always
perform zero padding of alignment bytes?