tap: Don't pcap frames that didn't get sent

In tap_send_frames() we send a number of frames to the tap device, then
also write them to the pcap capture file (if configured).  However the tap
send can partially fail (short write()s or similar), meaning that some
of the requested frames weren't actually sent, but we still write those
frames to the capture file.

We do give a debug message in this case, but it's misleading to add frames
that we know weren't sent to the capture file.  Rework to avoid this.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

1 files changed, 20 insertions, 10 deletions

diff --git a/tap.c b/tap.c
index e4c5b5b..4857599 100644
--- a/tap.c
+++ b/tap.c
@@ -309,10 +309,12 @@ void tap_icmp6_send(const struct ctx *c,
  * @iov:	Array of buffers, each containing one frame
  * @n:		Number of buffers/frames in @iov
  *
+ * Return: number of frames successfully sent
+ *
  * #syscalls:pasta write
  */
-static void tap_send_frames_pasta(struct ctx *c,
-				  const struct iovec *iov, size_t n)
+static size_t tap_send_frames_pasta(struct ctx *c,
+				    const struct iovec *iov, size_t n)
 {
 	size_t i;
 
@@ -325,6 +327,8 @@ static void tap_send_frames_pasta(struct ctx *c,
 			i--;
 		}
 	}
+
+	return n;
 }
 
 /**
@@ -357,10 +361,12 @@ static void tap_send_remainder(const struct ctx *c, const struct iovec *iov,
  * @iov:	Array of buffers, each containing one frame
  * @n:		Number of buffers/frames in @iov
  *
+ * Return: number of frames successfully sent
+ *
  * #syscalls:passt sendmsg
  */
-static void tap_send_frames_passt(const struct ctx *c,
-				  const struct iovec *iov, size_t n)
+static size_t tap_send_frames_passt(const struct ctx *c,
+				    const struct iovec *iov, size_t n)
 {
 	struct msghdr mh = {
 		.msg_iov = (void *)iov,
@@ -371,7 +377,7 @@ static void tap_send_frames_passt(const struct ctx *c,
 
 	sent = sendmsg(c->fd_tap, &mh, MSG_NOSIGNAL | MSG_DONTWAIT);
 	if (sent < 0)
-		return;
+		return 0;
 
 	/* Check for any partial frames due to short send */
 	for (i = 0; i < n; i++) {
@@ -386,8 +392,7 @@ static void tap_send_frames_passt(const struct ctx *c,
 		i++;
 	}
 
-	if (i < n)
-		debug("tap: dropped %lu frames due to short send", n - i);
+	return i;
 }
 
 /**
@@ -398,15 +403,20 @@ static void tap_send_frames_passt(const struct ctx *c,
  */
 void tap_send_frames(struct ctx *c, const struct iovec *iov, size_t n)
 {
+	size_t m;
+
 	if (!n)
 		return;
 
 	if (c->mode == MODE_PASST)
-		tap_send_frames_passt(c, iov, n);
+		m = tap_send_frames_passt(c, iov, n);
 	else
-		tap_send_frames_pasta(c, iov, n);
+		m = tap_send_frames_pasta(c, iov, n);
+
+	if (m < n)
+		debug("tap: dropped %lu frames of %lu due to short send", n - m, n);
 
-	pcap_multiple(iov, n, c->mode == MODE_PASST ? sizeof(uint32_t) : 0);
+	pcap_multiple(iov, m, c->mode == MODE_PASST ? sizeof(uint32_t) : 0);
 }
 
 /**