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

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().