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Re: [libvirt] [Qemu-devel] [PATCH v2 3/3] qemu-config: Add new -add-fd command line option

On 10/11/2012 08:45 AM, Corey Bryant wrote:

>> Another missing validation check is for duplicate use.  With the monitor
>> command, you ALWAYS have a unique fd (thanks to SCM_RIGHTS).  But with
>> the command line, I can type 'qemu -add-fd fd=4,set=1 -add-fd
>> fd=4,set=2'.  Oops - I've now corrupted your set layout, unless you
>> validate that every fd requested in -add-fd does not already reside in
>> any existing set.
> I don't see this validation check for duplicate use of fd's being
> necessary.  Like you say below, in the QMP add-fd case we can add the
> same fd multiple times.  So we should be able to add the same fd
> multiple times via the command line.  The only difference between QMP
> and command line in this case is that the QMP fd is a dup and therefore
> a different number and the command line fd will be the same fd.  I'd
> prefer to leave this alone unless there's a compelling reason to block
> adding of the same fd.

There is a compelling reason to prevent duplicates among your sets:

Suppose I add fd 4 into set 1 and 2, and then discard set 2 via monitor
commands.  Then, when qemu_close() drops the last reference to set 2, it
steps through and calls close() on all fds in that set, including fd 4.
 Oops - now set 1 is invalid, because it is tracking a closed fd.  And
worse, if qemu then does something else to open a new fd, it will get fd
4 again, and now set 1 will be tracking the WRONG fd.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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