Re: [PATCH] Make padding in struct sockaddr_storage explicit [BZ #20111]

[Florian Weimer <fweimer at redhat dot com>]

On 05/19/2016 03:40 PM, Andreas Schwab wrote:
> fweimer@redhat.com (Florian Weimer) writes:
>
> > diff --git a/bits/socket.h b/bits/socket.h
> > index ab9f242..b3f2753 100644
> > --- a/bits/socket.h
> > +++ b/bits/socket.h
> > @@ -164,6 +164,7 @@ struct sockaddr
> >  struct sockaddr_storage
> >    {
> >      __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
> > +    char __ss_padding_init[sizeof (__ss_aligntype) - __SOCKADDR_COMMON_SIZE];
>
> That needs to be __alignof, otherwise you change the layout, and you
> need to watch out for zero padding.

Hmm.  This would have to be conditional on __GNUC__.

What if we put the padding in the middle, like this?

#define _SS_PADSIZE \
  _SS_SIZE - __SOCKADDR_COMMON_SIZE - sizeof (__ss_aligntype)

struct sockaddr_storage
  {
    __SOCKADDR_COMMON (ss_);	/* Address family, etc.  */
    char __ss_padding[_SS_PADSIZE];
    __ss_aligntype __ss_align;	/* Force desired alignment.  */
  };

Then there should be no undeclared padding, and the __ss_align member 
should increase the alignment of the entire struct.

Thanks,
Florian