[Qemu-devel] File descriptor "leak" through /etc/qemu-ifup script

["Alain Knaff (qemu)" <qemu@misc.lka.org.lu>]

Qemu does not close its filedescriptors (or setting the FD_CLOEXEC) when 
invoking the /etc/qemu-ifup script.

Hence any background process spawned from there (such as a dhcpd) will 
also inherit the open filedescriptor, preventing the relevant decide 
(/dev/net/tup) to be reused later on:

If for example you chose a fixed network name using the -net ...ifname= 
  option and have an /etc/init.d/dhcp start in your if-up qemu (in order 
to instruct dhcpd to handle the new interface), the newly spawned dhcp 
will inherit the filedescriptor controlling the tun device, a continue 
hogging it even after the qemu is stopped, and is started again.

Workaround:

Putting the following into the script:
exec 3>/dev/null
exec 4>/dev/null
exec 5>/dev/null
exec 6>/dev/null


But a cleaner solution would be if qemu itself was careful not to leak 
those descriptors in the first place.

Thanks,

Alain