Re: [tcpdump-workers] vlan tagged packets and libpcap breakage

[Eric Dumazet <eric.dumazet@gmail.com>]

On Tue, 2012-12-11 at 14:36 -0800, Ani Sinha wrote:
> >
> > It is possible to test for the presence of support of the new vlan bpf
> > extensions by attempting to load a filter that uses them.  As only valid
> > filters can be loaded, old kernels that do not support filtering of vlan
> > tags will fail to load the a test filter with uses them.
> 
> Unfortunately I do not see this. The sk_chk_filter() does not have a
> default in the case statement and the check will not detect an unknown
> instruction. It will fail when the filter is run and as far as I can see,
> the packet will be dropped. Something like this might help?
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index c23543c..96338aa 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -548,6 +548,8 @@ int sk_chk_filter(struct sock_filter *filter, unsigned int flen)
>  			return -EINVAL;
>  		/* Some instructions need special checks */
>  		switch (code) {
> +		/* for unknown instruction, return EINVAL */
> +		default : return -EINVAL;
>  		case BPF_S_ALU_DIV_K:
>  			/* check for division by zero */
>  			if (ftest->k == 0)

This patch is wrong.

Check lines 546, 547, 548 where we do the check for unknown instructions

code = codes[code];
if (!code)
	return -EINVAL;

If you want to test ANCILLARY possible values, its already too late, as
old kernels wont use any patch anyway.