Launching kvm with -net nic -net tap triggers the /etc/qemu-ifup /etc/qemu-ifdown scripts. They are (in this version) hardcoding brctl as /usr/sbin/brctl but actually installed as /sbin/brctl.
/etc/qemu-ifup /etc/qmeu-ifdown are provided by qemu-common with source package of qemu-kvm
ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: qemu-common 0.14.1+noroms-0ubuntu2
ProcVersionSignature: Ubuntu 3.0.0-9.14-server 3.0.3
Uname: Linux 3.0.0-9-server x86_64
Architecture: amd64
CurrentDmesg:
[ 18.522844] nf_conntrack version 0.5.0 (16384 buckets, 65536 max)
[ 18.550476] ADDRCONF(NETDEV_UP): virbr0: link is not ready
[ 18.872333] ip6_tables: (C) 2000-2006 Netfilter Core Team
[ 25.430014] br0: no IPv6 routers present
Date: Wed Aug 24 19:32:00 2011
Dependencies:
seabios 0.6.2-0ubuntu1
vgabios 0.6c-2ubuntu3
InstallationMedia: Ubuntu-Server 11.10 "Oneiric Ocelot" - Alpha amd64 (20110802.1)
KvmCmdLine: Error: command ['ps', '-C', 'kvm', '-F'] failed with exit code 1: UID PID PPID C SZ RSS PSR STIME TTY TIME CMD
MachineType: ECS IC780M-A
PackageArchitecture: all
ProcEnviron:
LANGUAGE=en_US:
LANG=en_US.UTF-8
SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-3.0.0-9-server root=UUID=f2e68c5d-89b6-4c8f-b83a-e59f307cbf47 ro
SourcePackage: qemu-kvm
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 04/19/2010
dmi.bios.vendor: American Megatrends Inc.
dmi.bios.version: 080015
dmi.board.name: IC780M-A
dmi.board.vendor: ECS
dmi.board.version: 1.0
dmi.chassis.type: 3
dmi.chassis.vendor: ECS
dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr080015:bd04/19/2010:svnECS:pnIC780M-A:pvr1.0:rvnECS:rnIC780M-A:rvr1.0:cvnECS:ct3:cvr:
dmi.product.name: IC780M-A
dmi.product.version: 1.0
dmi.sys.vendor: ECS
I suspect adding a dependency on bridge-utils may help in preventing more bugs like this.
I also see that /etc/qemu-ifup (and down) are symlinks to /usr/bin/qemu-ifup (and down).