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

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

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.

-- 
Regards,
Corey Bryant