[Qemu-devel] [PATCH] net: provide a friendly message when a user passes a bad -net tap, fd=X (v2)

[Anthony Liguori]

A lot of people copy libvirt's command line from ps -ef and then wonder why the
VM isn't working correctly.  Let's be kind and tell them what they should do
instead.

Without this patch, if you run with an invalid -net tap,fd=X, the guest still
runs and we poll 100% on a bad file descriptor.  With this patch, you get:

 qemu: -net tap,fd=42: invalid fd= for tap network device.  If you're copying
 from libvirt, use `virsh dom2xml-to-native' instead
 qemu: -net tap,fd=42: Device 'tap' could not be initialized

Signed-off-by: Anthony Liguori <address@hidden>
---
v1 -> v2
 - Remove garbage from unclean commit (spotted by Alex Graf)

diff --git a/net/tap.c b/net/tap.c
index 0147dab..45b1fb6 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -405,6 +405,8 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char 
*name, VLANState *vlan
     int fd, vnet_hdr = 0;
 
     if (qemu_opt_get(opts, "fd")) {
+        int flags;
+
         if (qemu_opt_get(opts, "ifname") ||
             qemu_opt_get(opts, "script") ||
             qemu_opt_get(opts, "downscript") ||
@@ -418,6 +420,11 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char 
*name, VLANState *vlan
             return -1;
         }
 
+        if (fcntl(fd, F_GETFL, &flags) == -1) {
+            error_report("invalid fd= for tap network device.  If you're 
copying from libvirt, use `virsh dom2xml-to-native' instead");
+            return -1;
+        }
+
         fcntl(fd, F_SETFL, O_NONBLOCK);
 
         vnet_hdr = tap_probe_vnet_hdr(fd);
-- 
1.7.0.4