[PATCH 4/6] tap: Don't risk truncating frames on full buffer in tap_pasta_input()

[David Gibson <david@gibson.dropbear.id.au>]

tap_pasta_input() keeps reading frames from the tap device until the
buffer is full.  However, this has an ugly edge case, when we get close
to buffer full, we will provide just the remaining space as a read()
buffer.  If this is shorter than the next frame to read, the tap device
will truncate the frame and discard the remainder.

Adjust the code to make sure we always have room for a maximum size frame.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tap.c b/tap.c
index 71742748..2fbcef04 100644
--- a/tap.c
+++ b/tap.c
@@ -1076,8 +1076,8 @@ static void tap_pasta_input(struct ctx *c, const struct timespec *now)
 
 	tap_flush_pools();
 
-	for (n = 0; n < (ssize_t)TAP_BUF_BYTES; n += len) {
-		len = read(c->fd_tap, pkt_buf + n, TAP_BUF_BYTES - n);
+	for (n = 0; n < (ssize_t)TAP_BUF_BYTES - ETH_MAX_MTU; n += len) {
+		len = read(c->fd_tap, pkt_buf + n, ETH_MAX_MTU);
 
 		if (len <= 0) {
 			if (errno == EINTR) {
-- 
@@ -1076,8 +1076,8 @@ static void tap_pasta_input(struct ctx *c, const struct timespec *now)
 
 	tap_flush_pools();
 
-	for (n = 0; n < (ssize_t)TAP_BUF_BYTES; n += len) {
-		len = read(c->fd_tap, pkt_buf + n, TAP_BUF_BYTES - n);
+	for (n = 0; n < (ssize_t)TAP_BUF_BYTES - ETH_MAX_MTU; n += len) {
+		len = read(c->fd_tap, pkt_buf + n, ETH_MAX_MTU);
 
 		if (len <= 0) {
 			if (errno == EINTR) {
-- 
2.46.0