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

Am 17.10.2012 17:01, schrieb Eric Blake: > On 10/17/2012 08:02 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". > > Overnight, I realized we do have one potential safety valve: we are > guaranteed that any fd inherited by the exec() of qemu-kvm has > FD_CLOEXEC clear, and we also strive to have qemu open/dup all of its > internal fds with FD_CLOEXEC set. Therefore, it may be worth a sanity > check of fcntl(F_GETFD) to see if FD_CLOEXEC is set, and if so, the user > must have failed to pass in the fd, and we are now looking at a qemu > internal fd, and should therefore report failure. But I'm not sure if > it's worth the extra code. Hm, this sounds actually easy enough. I'll leave the decision to Corey, but I like the idea.