[PATCH] net/vmnet: Pad short Ethernet frames

[William Hooper]

At least on macOS 12.7.2, vmnet doesn't pad Ethernet frames, such as the
host's ARP replies, to the minimum size (60 bytes before the frame check
sequence) defined in IEEE Std 802.3-2022, so guests' Ethernet device
drivers may drop them with "frame too short" errors.

This patch calls eth_pad_short_frame() to add padding, as in net/tap.c
and net/slirp.c.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2058
Signed-off-by: William Hooper <wsh@wshooper.org>
---
 net/vmnet-common.m | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/net/vmnet-common.m b/net/vmnet-common.m
index 2958283485..f8f7163226 100644
--- a/net/vmnet-common.m
+++ b/net/vmnet-common.m
@@ -18,6 +18,7 @@
 #include "qemu/error-report.h"
 #include "qapi/error.h"
 #include "sysemu/runstate.h"
+#include "net/eth.h"
 
 #include <vmnet/vmnet.h>
 #include <dispatch/dispatch.h>
@@ -150,10 +151,23 @@ static int vmnet_read_packets(VmnetState *s)
  */
 static void vmnet_write_packets_to_qemu(VmnetState *s)
 {
+    uint8_t *pkt;
+    size_t pktsz;
+    uint8_t min_pkt[ETH_ZLEN];
+    size_t min_pktsz = sizeof(min_pkt);
+
     while (s->packets_send_current_pos < s->packets_send_end_pos) {
-        ssize_t size = qemu_send_packet_async(&s->nc,
-                                      
s->iov_buf[s->packets_send_current_pos].iov_base,
-                                      
s->packets_buf[s->packets_send_current_pos].vm_pkt_size,
+        pkt = s->iov_buf[s->packets_send_current_pos].iov_base;
+        pktsz = s->packets_buf[s->packets_send_current_pos].vm_pkt_size;
+
+        if (net_peer_needs_padding(&s->nc)) {
+            if (eth_pad_short_frame(min_pkt, &min_pktsz, pkt, pktsz)) {
+                pkt = min_pkt;
+                pktsz = min_pktsz;
+            }
+        }
+
+        ssize_t size = qemu_send_packet_async(&s->nc, pkt, pktsz,
                                       vmnet_send_completed);
 
         if (size == 0) {
-- 
2.37.1