Don't timeout poll() with TPACKET_V3 in 3.19 and later.

We don't need to do the workaround for not getting wakeups when buffers
are handed to us, nor do we have to do a timeout in poll() for the
benefit of programs that expect the timeout to expire even if no packets
have arrived, so, with TPACKET_V3 in 3.19 and later, we always use a
timeout of -1 for poll().

This eliminates some spurious wakeups from poll(), due to the poll()
call *itself* timing out, that turn into spurious timeouts; see GitHub
issue #383.

1 files changed, 62 insertions, 18 deletions

diff --git a/pcap-linux.c b/pcap-linux.c
index b19e4a1a..e08f0b12 100644
--- a/pcap-linux.c
+++ b/pcap-linux.c
@@ -1300,33 +1300,77 @@ static void	pcap_cleanup_linux( pcap_t *handle )
 static void
 set_poll_timeout(struct pcap_linux *handlep)
 {
+#ifdef HAVE_TPACKET3
+	struct utsname utsname;
+	char *version_component, *endp;
+	int major, minor;
+	int broken_tpacket_v3 = 1;
+
+	/*
+	 * Some versions of TPACKET_V3 have annoying bugs/misfeatures
+	 * around which we have to work.  Determine if we have those
+	 * problems or not.
+	 */
+	if (uname(&utsname) == 0) {
+		/*
+		 * 3.19 is the first release with a fixed version of
+		 * TPACKET_V3.  We treat anything before that as
+		 * not haveing a fixed version; that may really mean
+		 * it has *no* version.
+		 */
+		version_component = utsname.release;
+		major = strtol(version_component, &endp, 10);
+		if (endp != version_component && *endp == '.') {
+			/*
+			 * OK, that was a valid major version.
+			 * Get the minor version.
+			 */
+			version_component = endp + 1;
+			minor = strtol(version_component, &endp, 10);
+			if (endp != version_component &&
+			    (*endp == '.' || *endp == '\0')) {
+				/*
+				 * OK, that was a valid minor version.
+				 * Is this 3.19 or newer?
+				 */
+				if (major >= 4 || (major == 3 && minor >= 19)) {
+					/* Yes. TPACKET_V3 works correctly. */
+					broken_tpacket_v3 = 0;
+				}
+			}
+		}
+	}
+#endif
 	if (handlep->timeout == 0) {
 #ifdef HAVE_TPACKET3
 		/*
-		 * XXX - due to a set of (mis)features in the
-		 * TPACKET_V3 kernel code, blocking forever with
-		 * a TPACKET_V3 socket can, if few packets
-		 * are arriving and passing the socket filter,
-		 * cause most packets to be dropped.  See
-		 * libpcap issue #335 for the full painful
-		 * story.  The workaround is to have poll()
-		 * time out very quickly, so we grab the
-		 * frames handed to us, and return them to
-		 * the kernel, ASAP.
+		 * XXX - due to a set of (mis)features in the TPACKET_V3
+		 * kernel code prior to the 3.19 kernel, blocking forever
+		 * with a TPACKET_V3 socket can, if few packets are
+		 * arriving and passing the socket filter, cause most
+		 * packets to be dropped.  See libpcap issue #335 for the
+		 * full painful story.
 		 *
-		 * If those issues are ever fixed, we might
-		 * want to check the kernel version and block
-		 * forever with TPACKET_V3 if we're running
-		 * with a kernel that has the fix.
+		 * The workaround is to have poll() time out very quickly,
+		 * so we grab the frames handed to us, and return them to
+		 * the kernel, ASAP.
 		 */
-		if (handlep->tp_version == TPACKET_V3)
+		if (handlep->tp_version == TPACKET_V3 && broken_tpacket_v3)
 			handlep->poll_timeout = 1;	/* don't block for very long */
 		else
 #endif
 			handlep->poll_timeout = -1;	/* block forever */
-	} else if (handlep->timeout > 0)
-		handlep->poll_timeout = handlep->timeout;	/* block for that amount of time */
-	else
+	} else if (handlep->timeout > 0) {
+		/*
+		 * For TPACKET_V3, the timeout is handled by the kernel,
+		 * so block forever; that way, we don't get extra timeouts.
+		 * Don't do that if we have a broken TPACKET_V3, though.
+		 */
+		if (handlep->tp_version == TPACKET_V3 && !broken_tpacket_v3)
+			handlep->poll_timeout = -1;	/* block forever, let TPACKET_V3 wake us up */
+		else
+			handlep->poll_timeout = handlep->timeout;	/* block for that amount of time */
+	} else
 		handlep->poll_timeout = 0;	/* non-blocking mode - poll to pick up errors */
 }