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

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.

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.

Series: Reviewed-by: Eric Blake <eblake(a)redhat.com&gt;