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

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Date:	Tue, 11 Dec 2012 16:46:05 -0800
From:	Ani Sinha <ani@...stanetworks.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Michael Richardson <mcr@...delman.ca>, netdev@...r.kernel.org,
	tcpdump-workers@...ts.tcpdump.org,
	Francesco Ruggeri <fruggeri@...stanetworks.com>
Subject: Re: [tcpdump-workers] vlan tagged packets and libpcap breakage

On Tue, Dec 11, 2012 at 3:04 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> 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.


yes I generated this patch wrong.

>
> Check lines 546, 547, 548 where we do the check for unknown instructions
>
> code = codes[code];
> if (!code)
>         return -EINVAL;

yepph it's OK here.

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

yepph, I was looking at possible ancilliary values. Basically this
case statement :

#define ANCILLARY(CODE) case SKF_AD_OFF + SKF_AD_##CODE:        \
                                code = BPF_S_ANC_##CODE;        \
                                break
                        switch (ftest->k) {
                        ANCILLARY(PROTOCOL);
                        ANCILLARY(PKTTYPE);
                        ANCILLARY(IFINDEX);
                        ANCILLARY(NLATTR);
                        ANCILLARY(NLATTR_NEST);
                        ANCILLARY(MARK);
                        ANCILLARY(QUEUE);
                        ANCILLARY(HATYPE);
                        ANCILLARY(RXHASH);
                        ANCILLARY(CPU);
                        ANCILLARY(ALU_XOR_X);
                        ANCILLARY(VLAN_TAG);
                        ANCILLARY(VLAN_TAG_PRESENT);
                        }
--
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