Re: [libvirt] [PATCH v4 4/4] qemu-config: Add new -add-fd command line option

On 10/18/2012 04:43 PM, Eric Blake wrote: > On 10/18/2012 01:19 PM, Corey Bryant wrote: >> This option can be used for passing file descriptors on the >> command line. It mirrors the existing add-fd QMP command which >> allows an fd to be passed to QEMU via SCM_RIGHTS and added to an >> fd set. >> >> This can be combined with commands such as -drive to link file >> descriptors in an fd set to a drive: >> >> qemu-kvm -add-fd fd=3,set=2,opaque="rdwr:/path/to/file" >> -add-fd fd=4,set=2,opaque="rdonly:/path/to/file" >> -drive file=/dev/fdset/2,index=0,media=disk >> >> This example adds dups of fds 3 and 4, and the accompanying opaque >> strings to the fd set with ID=2. qemu_open() already knows how >> to handle a filename of this format. qemu_open() searches the >> corresponding fd set for an fd and when it finds a match, QEMU >> goes on to use a dup of that fd just like it would have used an >> fd that it opened itself. >> >> Signed-off-by: Corey Bryant <coreyb(a)linux.vnet.ibm.com&gt; > >> + >> + if (fcntl(fd, F_GETFD) & FD_CLOEXEC) { >> + qerror_report(ERROR_CLASS_GENERIC_ERROR, >> + "fd is not valid or already in use"); >> + return -1; >> + } > > Hmm, I was about to call you on the fact that you didn't check whether > fcntl() succeeded; but then realized that in the failure case it is > required by POSIX to return -1 which happens to include the FD_CLOEXEC > bit, so you actually ended up with a sneaky optimization that does the > right thing for both open and closed fds. Yep it works for both cases. I have to admit I stumbled into this at first and then decided to leave it this way since it worked. :)

> Perhaps a comment in the code is warranted (after all, it is not > immediately apparent from reading just this if statement why it works); > maybe "/* All fds inherited across exec() necessarily have FD_CLOEXEC > clear, while qemu sets FD_CLOEXEC on all other fds opened from command > line arguments */". But I'm not going to require a v5 just for a > comment addition. I agree, a comment would be useful. Maybe Kevin can add if this series gets pushed?