3 files changed, 50 insertions, 2 deletions

diff --git a/iov.c b/iov.c
index 6fd684a..968a365 100644
--- a/iov.c
+++ b/iov.c
@@ -450,3 +450,38 @@ ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
 
 	return j;
 }
+
+/**
+ * iov_tail_trim() - Limit a tail to @len bytes via a scratch iovec array
+ * @tail:	 Pointer to the iov_tail to trim; rebuilt on success to
+ *		 reference @scratch
+ * @len:	 Number of bytes to keep
+ * @scratch:	 Scratch iovec array backing the trimmed tail; must stay
+ *		 valid as long as the trimmed tail is in use
+ * @scratch_cnt: Number of elements in @scratch
+ *
+ * Return: true on success, false if @tail is shorter than @len or does
+ *	   not fit in @scratch (@tail is unchanged on failure)
+ */
+bool iov_tail_trim(struct iov_tail *tail, size_t len,
+		   struct iovec *scratch, size_t scratch_cnt)
+{
+	ssize_t cnt = iov_tail_clone(scratch, scratch_cnt, tail);
+	size_t left = len;
+	unsigned int i;
+
+	if (cnt < 0)
+		return false;
+
+	for (i = 0; i < (size_t)cnt && left; i++) {
+		if (scratch[i].iov_len > left)
+			scratch[i].iov_len = left;
+		left -= scratch[i].iov_len;
+	}
+
+	if (left)
+		return false;
+
+	*tail = IOV_TAIL(scratch, i, 0);
+	return true;
+}
diff --git a/iov.h b/iov.h
index 4fdf14a..3af467e 100644
--- a/iov.h
+++ b/iov.h
@@ -97,6 +97,8 @@ size_t iov_push_header_(struct iov_tail *tail, const void *v, size_t len);
 void *iov_remove_header_(struct iov_tail *tail, void *v, size_t len, size_t align);
 ssize_t iov_tail_clone(struct iovec *dst_iov, size_t dst_iov_cnt,
 		       struct iov_tail *tail);
+bool iov_tail_trim(struct iov_tail *tail, size_t len,
+		   struct iovec *scratch, size_t scratch_cnt);
 
 /**
  * IOV_PEEK_HEADER() - Get typed pointer to a header from an IOV tail
diff --git a/tap.c b/tap.c
index 4cba4c7..6ed7f8d 100644
--- a/tap.c
+++ b/tap.c
@@ -716,7 +716,8 @@ static int tap4_handler(struct ctx *c, const struct pool *in,
 	i = 0;
 resume:
 	for (seq_count = 0, seq = NULL; i < in->count; i++) {
-		size_t l3len, hlen, l4len;
+		struct iovec trim_iov[UIO_MAXIOV];
+		size_t l3len, hlen, l4len, check;
 		struct ethhdr eh_storage;
 		struct iphdr iph_storage;
 		struct udphdr uh_storage;
@@ -775,7 +776,17 @@ resume:
 
 		if (!iov_drop_header(&data, hlen))
 			continue;
-		if (iov_tail_size(&data) != l4len)
+
+		check = iov_tail_size(&data);
+		if (check < l4len)
+			continue;
+
+		/* Drivers modelled on real hardware (Plan 9's virtio, for
+		 * one) pad short frames to the 60 byte Ethernet minimum:
+		 * trim trailing padding instead of dropping the packet.
+		 */
+		if (check > l4len &&
+		    !iov_tail_trim(&data, l4len, trim_iov, ARRAY_SIZE(trim_iov)))
 			continue;
 
 		if (iph->protocol == IPPROTO_ICMP) {