Re: [libvirt] [PATCH v3 4/4] qemu-config: Add new -add-fd command line option

On 10/17/2012 10:02 AM, Kevin Wolf wrote: > Am 17.10.2012 06:16, schrieb Eric Blake: >> I'm still seeing the corner case of: >> >> qemu-kvm -add-fd fd=3,set=1 -add-fd fd=4,set=2 4<&- >> >> where the dup(3) will populate fd 4 prior to the point where we get to >> process the -add-fd fd=4 command to notice that the user started >> qemu-kvm with fd 4 closed, and thus qemu will silently proceed to use >> the wrong fd. >> >> On the other hand, I'm not sure if that corner case is worth worrying >> about, or if we just chalk it up to user stupidity (aka libvirt >> programmer stupidity) if they did something like that (most likely, >> because the management app forgot to clear FD_CLOEXEC before exec()ing >> qemu-kvm). > > If you specify an FD number that isn't actually open when qemu is > stared, you can get any FD that qemu opens internally. I think the > correct answer to this problem is "then don't do that". > I'd also say "then don't do that". Or maybe "why are you doing that?". But I'm not opposed to closing a corner case if it's not cluttering the code base. >> Hmm, this makes me wonder if I can do something crazy like: >> >> qemu-kvm -add-fd fd=4,set=1 -qmp /dev/fdset/1 >> >> to open a monitor on the fd I just passed in? > > I think so. At least on my side it was intended to allow this. > >> And what if so, what then >> happens on that monitor if I request that fdset 1 be removed? > > The same as with block devices: The fd stays open until the monitor > connection is closed. A closed monitor also triggers fd garbage > collection, so at this point the original fd would be closed (well, > assuming that you had only one monitor). > > Kevin > True, but I think in this case we care more about the dup'd fd staying open than the fd in the fdset. Remember that qemu_open() dups the fd from the fd set. So assuming the open/close of the QMP fd occurs in qemu_open()/qemu_close(), the QMP fd would be a dup of the fd that was added to the fd set. So if remove-fd removed the fd from the fdset, or it removed the entire fdset, the QMP fd would remain open until qemu_close() was called. I'll try this out today to make sure but I don't think this is an issue.