Re: vlan tagged packets and libpcap breakage

On Tue, Dec 11, 2012 at 3:04 PM, Eric Dumazet [off-list ref] wrote:

On Tue, 2012-12-11 at 14:36 -0800, Ani Sinha wrote:

quoted

quoted

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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