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

From: Stephen Hemminger <[email-masked]>
Date: Tue, 11 Dec 2012 15:12:31 -0800

On Tue, 11 Dec 2012 14:36:33 -0800 (PST)
Ani Sinha <[email-masked]> 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)

Did you test this? I think it will blow up for some existing instructions
like BPF_S_ALU_XOR_X or any of the other non-special instructions.

Also it is not formatted correctly for the kernel programming style.

ERROR: space prohibited before that ':' (ctx:WxW)
#86: FILE: net/core/filter.c:552:
+		default : return -EINVAL;
 		        ^