Re: [PATCH v3 net-next 1/2] tools: psock_lib: tighten conditions checked in sock_setfilter

The bpf_prog used in sock_setfilter() only attempts to check for
ip pktlen, and verifies that the contents of the 80'th packet in
the ethernet frame is 'a' or 'b'.  Thus many non-udp packets
could incorrectly pass through this filter and cause incorrect
test results.

This commit hardens the conditions checked by the filter so
that only UDP/IPv4 packets with the matching length and test-character
will be permitted by the filter. The filter has been cleaned up
to explicitly use the BPF macros to make it more readable.

Signed-off-by: Sowmini Varadhan <redacted>
Acked-by: Willem de Bruijn <willemb@google.com>
---
v2: commit comment edited based on Willem de Bruijn review
v3: Shuah Khan nit.

 tools/testing/selftests/net/psock_lib.h |   29 ++++++++++++++++++++++-------
 1 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/net/psock_lib.h b/tools/testing/selftests/net/psock_lib.h
index 24bc7ec..9e5553a 100644
--- a/tools/testing/selftests/net/psock_lib.h
+++ b/tools/testing/selftests/net/psock_lib.h

@@ -27,6 +27,7 @@
 #include <string.h>
 #include <arpa/inet.h>
 #include <unistd.h>
+#include <netinet/udp.h>
 
 #define DATA_LEN			100
 #define DATA_CHAR			'a'

@@ -40,14 +41,28 @@
 
 static __maybe_unused void sock_setfilter(int fd, int lvl, int optnum)
 {
+	uint16_t ip_len = DATA_LEN +
+			  sizeof(struct iphdr) +
+			  sizeof(struct udphdr);
+	/* the filter below checks for all of the following conditions that
+	 * are based on the contents of create_payload()
+	 *  ether type 0x800 and
+	 *  ip proto udp     and
+	 *  ip len == ip_len and
+	 *  udp[38] == 'a' or udp[38] == 'b'
+	 */