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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 linux vnet ibm com>

+    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?

Reviewed-by: Eric Blake <eblake redhat com>

Thanks for reviewing!

Corey Bryant

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