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





On 10/18/2012 10:29 AM, Corey Bryant wrote:


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.


After digging into this some more it appears to be a non issue. Only qemu_open() and qemu_close() deal with fdsets. The QMP fd is created with qemu_socket(), not qemu_open(), so it doesn't deal with fdsets. The ensuing bind() call that specifies the path ends up failing with ENOENT because the actual path "/dev/fdset/1" doesn't exist:

bind(unix:/dev/fdset/1): No such file or directory

--
Regards,
Corey Bryant


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