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

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On 01/04/2017 07:45 PM, Sowmini Varadhan wrote:
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 <sowmini.varadhan@xxxxxxxxxx>
Acked-by: Willem de Bruijn <willemb@xxxxxxxxxx>
---
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'
+	 */
  	struct sock_filter bpf_filter[] = {
-		{ 0x80, 0, 0, 0x00000000 },  /* LD  pktlen		      */
-		{ 0x35, 0, 4, DATA_LEN   },  /* JGE DATA_LEN  [f goto nomatch]*/
-		{ 0x30, 0, 0, 0x00000050 },  /* LD  ip[80]		      */
-		{ 0x15, 1, 0, DATA_CHAR  },  /* JEQ DATA_CHAR   [t goto match]*/
-		{ 0x15, 0, 1, DATA_CHAR_1},  /* JEQ DATA_CHAR_1 [t goto match]*/
-		{ 0x06, 0, 0, 0x00000060 },  /* RET match	              */
-		{ 0x06, 0, 0, 0x00000000 },  /* RET no match		      */
+		BPF_STMT(BPF_LD | BPF_H | BPF_ABS, 12),	/* LD ethertype */
+		BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, ETH_P_IP, 0, 8),
+		BPF_STMT(BPF_LD|BPF_B|BPF_ABS, 23),	/* LD ip_proto */
+		BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, IPPROTO_UDP, 0, 6),
+		BPF_STMT(BPF_LD|BPF_H|BPF_ABS, 16),	/* LD ip_len */
+		BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, ip_len, 0, 4),
+		BPF_STMT(BPF_LD|BPF_B|BPF_ABS, 80),	/* LD udp[38] */
+		BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, DATA_CHAR, 1, 0),
+		BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, DATA_CHAR_1, 0, 1),
+		BPF_STMT(BPF_RET | BPF_K, ~0),		/* match */
+		BPF_STMT(BPF_RET | BPF_K, 0)		/* no match */

Just reading up on the thread, sorry to jump in late. Can't you just
use the generated code from bpf_asm (tools/net/) and add the asm program
as a comment above? Something like we do in net/core/ptp_classifier.c +13.
As it stands it makes it a bit harder to parse / less readable with macros
actually. Rest seems fine, thanks.

  	};
  	struct sock_fprog bpf_prog;



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