Re: [PATCH v2 06/20] randstruct: Whitelist UNIXCB cast

[Christoph Hellwig]

On Fri, May 26, 2017 at 01:17:10PM -0700, Kees Cook wrote:
> This is another false positive in bad cast detection:
> 
> net/unix/af_unix.c: In function âunix_skb_scm_eqâ:
> net/unix/af_unix.c:1621:31: note: found mismatched rhs struct pointer types: âstruct unix_skb_parmsâ and âcharâ
> 
>   const struct unix_skb_parms *u = &UNIXCB(skb);
>                                ^
> 
> UNIXCB is:
> 
> 	#define UNIXCB(skb)     (*(struct unix_skb_parms *)&((skb)->cb))
> 
> And ->cb is:
> 
> 	char                    cb[48] __aligned(8);
> 
> This is a rather crazy cast, but appears to be safe in the face of
> randomization, so whitelist it in the plugin.

We have a lot of network protocol that use the ->cb area, which makes me
wonder why this one would be so special.

It seems like everyone is just using a plain cast to a pointer without
doing the address taking trick that doesn't make sense for arrays
anyway.

Maybe we just need to fix up the af_unix code to work the same as all
other protocols?