linux-hardening - Re: [netfilter-nf-next:testing 2/9] ./usr/include/linux/netfilter_bridge/ebtables.h:163:26: warning: field 'target' with variable sized type 'struct ebt_entry_target' not at the end of a struct or class is a GNU extension

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Date:   Thu, 17 Aug 2023 10:02:35 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Florian Westphal <fw@...len.de>
Cc:     "GONG, Ruiqi" <gongruiqi1@...wei.com>,
        kernel test robot <lkp@...el.com>, llvm@...ts.linux.dev,
        oe-kbuild-all@...ts.linux.dev, linux-hardening@...r.kernel.org
Subject: Re: [netfilter-nf-next:testing 2/9]
 ./usr/include/linux/netfilter_bridge/ebtables.h:163:26: warning: field
 'target' with variable sized type 'struct ebt_entry_target' not at the end
 of a struct or class is a GNU extension

On Thu, Aug 17, 2023 at 10:26:18AM +0200, Florian Westphal wrote:
> Kees Cook <keescook@...omium.org> wrote:
> > On Thu, Aug 17, 2023 at 01:03:20PM +0800, kernel test robot wrote:
> > > tree:   git://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf-next.git testing
> > > head:   015e2d9101d3713c7bee16dccad171df04a3bbd5
> > > commit: 61b9e6bd48a6317c0a44ee4f3fecdec9de5baa9e [2/9] netfilter: ebtables: replace zero-length array members
> > > config: i386-buildonly-randconfig-r004-20230817 (https://download.01.org/0day-ci/archive/20230817/202308171249.g1ywxhII-lkp@intel.com/config)
> > > compiler: clang version 16.0.4 (https://github.com/llvm/llvm-project.git ae42196bc493ffe877a7e3dff8be32035dea4d07)
> > > reproduce: (https://download.01.org/0day-ci/archive/20230817/202308171249.g1ywxhII-lkp@intel.com/reproduce)
> > > 
> > > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > > the same patch/commit), kindly add following tags
> > > | Reported-by: kernel test robot <lkp@...el.com>
> > > | Closes: https://lore.kernel.org/oe-kbuild-all/202308171249.g1ywxhII-lkp@intel.com/
> > > 
> > > All warnings (new ones prefixed by >>):
> > > 
> > >    In file included from <built-in>:1:
> > > >> ./usr/include/linux/netfilter_bridge/ebtables.h:163:26: warning: field 'target' with variable sized type 'struct ebt_entry_target' not at the end of a struct or class is a GNU extension [-Wgnu-variable-sized-type-not-at-end]
> > >            struct ebt_entry_target target;
> > >                                    ^
> > >    1 warning generated.
> > 
> > Eww, it looks like "struct ebt_entry_target" is used _within_ another
> > struct:
> > 
> > struct ebt_standard_target {
> >         struct ebt_entry_target target;
> >         int verdict;
> > };
> 
> Yes, same as xt_standard_target.
> 
> > These have been fixed in the past in a variety of ways -- it all depends
> > on how userspace is using them. In looking at Debian Code Search:
> > https://codesearch.debian.net/search?q=struct+ebt_standard_target&literal=1
> > 
> > It is exclusively doing casts and looking at the "verdict" member. So
> > the easiest conversion might be this:
> > 
> >  struct ebt_standard_target {
> > -       struct ebt_entry_target target;
> > +       unsigned char hdr[sizeof(struct ebt_entry_target)];
> >         int verdict;
> 
> I don't think its worth doing all of this.
> 
> Can't we just keep it as-is and drop the relevant hunk from the patch?

For now, yeah. But we'll need to find a solution as flex-array
structures overlapping variables is considered deprecated (it can lead
to ambiguous sizing results). But as long as the kernel itself doesn't
use struct ebt_standard_target, we can kick the can down the road a bit
more. :)

-- 
Kees Cook