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

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Date:	Tue, 11 Dec 2012 14:36:33 -0800 (PST)
From:	Ani Sinha <ani@...stanetworks.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
cc:	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

>
> 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)
--
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