iov: Add iov_truncate() helper and use it in vu handlers

Add a generic iov_truncate() function that truncates an IO vector to a
given number of bytes, returning the number of iov entries that contain
data after truncation.

Use it in udp_vu_sock_recv() and tcp_vu_sock_recv() to replace the
open-coded truncation logic that adjusted iov entries after recvmsg().
Also convert the direct iov_len assignment in tcp_vu_send_flag() to use
iov_truncate() for consistency.

Add an ASSERT() in tcp_vu_data_from_sock() to quiet the Coverity error:

passt/tcp_vu.c:457:3:
  19. overflow_const: Expression "dlen + hdrlen", where "dlen" is known to
      be equal to -86,  and "hdrlen" is known to be equal to 86, underflows
      the type of "dlen + hdrlen", which is type "unsigned long".

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

4 files changed, 34 insertions, 21 deletions

diff --git a/iov.c b/iov.c
index ad726da..31a3f5b 100644
--- a/iov.c
+++ b/iov.c
@@ -148,6 +148,28 @@ size_t iov_size(const struct iovec *iov, size_t iov_cnt)
 }
 
 /**
+ * iov_truncate() - Truncate an IO vector to a given number of bytes
+ * @iov:	IO vector (modified)
+ * @iov_cnt:	Number of entries in @iov
+ * @size:	Total number of bytes to keep
+ *
+ * Return: number of iov entries that contain data after truncation
+ */
+size_t iov_truncate(struct iovec *iov, size_t iov_cnt, size_t size)
+{
+	size_t i, offset;
+
+	i = iov_skip_bytes(iov, iov_cnt, size, &offset);
+
+	if (i < iov_cnt) {
+		iov[i].iov_len = offset;
+		i += !!offset;
+	}
+
+	return i;
+}
+
+/**
  * iov_tail_prune() - Remove any unneeded buffers from an IOV tail
  * @tail:	IO vector tail (modified)
  *
diff --git a/iov.h b/iov.h
index d1ab91a..b4e50b0 100644
--- a/iov.h
+++ b/iov.h
@@ -29,6 +29,7 @@ size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
 size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
 		  size_t offset, void *buf, size_t bytes);
 size_t iov_size(const struct iovec *iov, size_t iov_cnt);
+size_t iov_truncate(struct iovec *iov, size_t iov_cnt, size_t size);
 
 /*
  * DOC: Theory of Operation, struct iov_tail
diff --git a/tcp_vu.c b/tcp_vu.c
index 88be232..fd734e8 100644
--- a/tcp_vu.c
+++ b/tcp_vu.c
@@ -131,7 +131,7 @@ int tcp_vu_send_flag(const struct ctx *c, struct tcp_tap_conn *conn, int flags)
 		return ret;
 	}
 
-	flags_elem[0].in_sg[0].iov_len = hdrlen + optlen;
+	iov_truncate(&flags_iov[0], 1, hdrlen + optlen);
 	payload = IOV_TAIL(flags_elem[0].in_sg, 1, hdrlen);
 
 	if (flags & KEEPALIVE)
@@ -192,9 +192,9 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
 	struct msghdr mh_sock = { 0 };
 	uint16_t mss = MSS_GET(conn);
 	int s = conn->sock;
-	ssize_t ret, len;
 	size_t hdrlen;
 	int elem_cnt;
+	ssize_t ret;
 	int i;
 
 	*iov_cnt = 0;
@@ -247,15 +247,7 @@ static ssize_t tcp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq,
 		ret -= already_sent;
 
 	/* adjust iov number and length of the last iov */
-	len = ret;
-	for (i = 0; len && i < elem_cnt; i++) {
-		struct iovec *iov = &elem[i].in_sg[0];
-
-		if (iov->iov_len > (size_t)len)
-			iov->iov_len = len;
-
-		len -= iov->iov_len;
-	}
+	i = iov_truncate(&iov_vu[DISCARD_IOV_NUM], elem_cnt, ret);
 
 	/* adjust head count */
 	while (*head_cnt > 0 && head[*head_cnt - 1] >= i)
@@ -448,10 +440,14 @@ int tcp_vu_data_from_sock(const struct ctx *c, struct tcp_tap_conn *conn)
 	for (i = 0, previous_dlen = -1, check = NULL; i < head_cnt; i++) {
 		struct iovec *iov = &elem[head[i]].in_sg[0];
 		int buf_cnt = head[i + 1] - head[i];
-		ssize_t dlen = iov_size(iov, buf_cnt) - hdrlen;
+		size_t frame_size = iov_size(iov, buf_cnt);
 		bool push = i == head_cnt - 1;
+		ssize_t dlen;
 		size_t l2len;
 
+		ASSERT(frame_size >= hdrlen);
+
+		dlen = frame_size - hdrlen;
 		vu_set_vnethdr(iov->iov_base, buf_cnt);
 
 		/* The IPv4 header checksum varies only with dlen */
diff --git a/udp_vu.c b/udp_vu.c
index 3520f89..5effca7 100644
--- a/udp_vu.c
+++ b/udp_vu.c
@@ -71,9 +71,9 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
 			    bool v6, ssize_t *dlen)
 {
 	const struct vu_dev *vdev = c->vdev;
-	int iov_cnt, idx, iov_used;
-	size_t off, hdrlen, l2len;
 	struct msghdr msg  = { 0 };
+	int iov_cnt, iov_used;
+	size_t hdrlen, l2len;
 
 	ASSERT(!c->no_udp);
 
@@ -115,13 +115,7 @@ static int udp_vu_sock_recv(const struct ctx *c, struct vu_virtq *vq, int s,
 	iov_vu[0].iov_base = (char *)iov_vu[0].iov_base - hdrlen;
 	iov_vu[0].iov_len += hdrlen;
 
-	/* count the numbers of buffer filled by recvmsg() */
-	idx = iov_skip_bytes(iov_vu, iov_cnt, *dlen + hdrlen, &off);
-
-	/* adjust last iov length */
-	if (idx < iov_cnt)
-		iov_vu[idx].iov_len = off;
-	iov_used = idx + !!off;
+	iov_used = iov_truncate(iov_vu, iov_cnt, *dlen + hdrlen);
 
 	/* pad frame to 60 bytes: first buffer is at least ETH_ZLEN long */
 	l2len = *dlen + hdrlen - VNET_HLEN;