Re: [libvirt] [PATCH v3 0/5] file descriptor passing using pass-fd

On 06/19/2012 09:57 AM, Kevin Wolf wrote: Agreed, as that's a lot of work for a lot of commands. Indeed, having one additional up-front command in the pass-fd/closefd family might make this easier. But how would it work reliably? Remember, the current proposal is: libvirt opens backing file O_RDONLY, and calls 'pass-fd name' qemu returns 21 libvirt tells qemu to hotplug a drive with /dev/fd/21 as backing file qemu dup()s 21, and proceeds to use fd 22 for all its real work libvirt calls 'closefd name', to avoid the leak on fd 21 sometime later... qemu has opened something else that now occupies fd 21 libvirt wants to call 'block-commit', and knows qemu now needs O_RDWR access to the file under your idea, that would mean libvirt would call something like 'pass-reopen-fd name /dev/fd/21', and qemu would get a new fd (let's assume 23), so that qemu would now know that fd 23 should be used the next time any qemu interface wants to reopen '/dev/fd/21' But that's not safe - there could easily have been more than one 'pass-fd' all resulting in 21, as long as each was separated by 'closefd' in the meantime; and since libvirt already called 'closefd' to avoid an indefinite fd leak, qemu is tracking neither /dev/fd/21 nor 'name' in its passfd list. That is, the only place tracking '/dev/fd/21' is the block driver where we used /dev/fd to pass in the O_RDONLY fd in the first place. Qemu can't dup2() to move 23 into 21 at the time of the 'pass-reopen-fd', as that might not be safe. Unless you have any bright ideas that I'm overlooking, I don't see how an additional 'pass-reopen-fd' can be made to do what we want. I'm afraid that the only way to make a reopen operation reliable is to be able to specify a new file name to pass in a new /dev/fd/nnn to use as part of any reopen operation, which means touching every command like 'block-commit' that needs to do a reopen. -- Eric Blake eblake(a)redhat.com +1-919-301-3266 Libvirt virtualization library http://libvirt.org