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

[Eric Blake <eblake@redhat.com>]

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On 10/10/2012 04:31 PM, Eric Blake 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.
> 
> Thinking aloud:
> On the other hand, being able to pass in one fd to multiple sets MIGHT
> be useful; in the SCM_RIGHTS monitor command case, I can pass the same
> fd from the management perspective into multiple sets, even though in
> qemu's perspective, there will be multiple fds created (one per call).
> Perhaps instead of directly adding the inherited fd to a set, and having
> to then sweep all sets to check for duplicates, it might make sense to
> add dup(fd) to a set, so that if I call:
> 
> qemu -add-fd fd=4,set=1 -add-fd fd=4,set=2 -add-fd fd=5,set=2
> 
> what REALLY happens is that qemu adds dup(4)==6 to set 1, dup(4)==7 to
> set 2, and dup(5)==8 to set 3.  Then, after all ALL -add-fd have been
> processed, qemu then does another pass through them calling close(4) and
> close(5) (to avoid holding the original fds open indefinitely if the
> corresponding sets are discarded).

Another idea: a hybrid approach - the _first_ -add-fd 4 directly adds 4
to the set, all other -add-fd 4 end up adding dup(4) instead (well,
fcntl(F_DUPFD_CLOEXEC), but you get the picture).  That is, do the
duplicate scanning, and if there is no duplicate, use the fd directly;
if there IS a duplicate, then put a unique fd number as a copy into the
remaining sets.  That way, you don't have to do a final close() sweep
across the -add-fd arguments passed on the command line, and you still
don't have to worry about duplicated fds across multiple sets causing
mayhem in qemu_close().

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


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