hw/usb/network: Remove hardcoded 0x40 prefix in STRING_ETHADDR response

USB NICs have a "40:" prefix hardcoded for all MAC addresses when we
return the guest the MAC address if it queries the STRING_ETHADDR USB
string property.  This doesn't match what we use for the
OID_802_3_PERMANENT_ADDRESS or OID_802_3_CURRENT_ADDRESS OIDs for
NDIS, or the MAC address we actually use in the QEMU networking code
to send/receive packets for this device, or the NIC info string we
print for users.  In all those other places we directly use
s->conf.macaddr.a, which is the full thing the user asks for.

This overrides user-provided configuration and leads to an inconsistent
experience.

I couldn't find any documented reason (comment or git commits) for
this behavior.  It seems like everyone is just expecting the MAC
address to be fully passed through to the guest, but it isn't.

This may have been a debugging hack that accidentally made it through
to the accepted patch: it has been in the code since it was originally
added back in 2008.

This is also particularly problematic as the "40:" prefix isn't a
reserved prefix for MAC addresses (IEEE OUI).  There are a number of
valid allocations out there which use this prefix, meaning that QEMU
may be causing MAC address conflicts.

Cc: qemu-stable@nongnu.org
Fixes: 6c9f886ceae5b ("Add CDC-Ethernet usb NIC (original patch from Thomas Sailer)"
Signed-off-by: Stéphane Graber <stgraber@stgraber.org>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2951
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
[PMM: beef up commit message based on mailing list discussion]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

diff --git a/hw/usb/dev-network.c b/hw/usb/dev-network.c
index 81cc09dcac90ff896698ff07ea64cb481a15bce9..1df24541814c2278d046e1c4d3ee8b38cc4123c5 100644 (file)
--- a/hw/usb/dev-network.c
+++ b/hw/usb/dev-network.c
@@ -1383,7 +1383,7 @@ static void usb_net_realize(USBDevice *dev, Error **errp)
     qemu_format_nic_info_str(qemu_get_queue(s->nic), s->conf.macaddr.a);
     snprintf(s->usbstring_mac, sizeof(s->usbstring_mac),
              "%02x%02x%02x%02x%02x%02x",
-             0x40,
+             s->conf.macaddr.a[0],
              s->conf.macaddr.a[1],
              s->conf.macaddr.a[2],
              s->conf.macaddr.a[3],