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

[Eric Blake <eblake@redhat.com>]

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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.

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@redhat.com>

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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