Re: [Qemu-devel] [qemu-devel]The problem of QMP command getfd.

[Li Zhang]

On 2013年01月28日 21:28, Stefan Hajnoczi wrote:
> On Mon, Jan 28, 2013 at 03:27:01PM +0800, Li Zhang wrote:
> > Hi all,
> >
> > I am trying to executing qmp command getfd according to qmp-commands.hx.
> >
> > { "execute": "getfd", "arguments": { "fdname": "fd1" } }
> >
> > Every time, it returns the error.
> >
> > {
> >      "error": {
> >          "class": "GenericError",
> >          "desc": "No file descriptor supplied via SCM_RIGHTS"
> >      }
> > }
> >
> > This error is related with SCM_RIGHTS.
> > I don't know about SCM at all.
> >
> > Can anyone give some hints about this?
> The "getfd" command allows the client to pass a file descriptor to QEMU
> and give it a name.  Later that file descriptor can be used by QEMU.
>
> The QMP client must send '{ "execute": "getfd", "arguments": { "fdname":
> "fd1" } }' together with SCM_RIGHTS CMSG.  You are getting the error
> because you sent the "getfd" command but forgot to include a file
> descriptor using SCM_RIGHTS.
>
> SCM_RIGHTS is a feature of UNIX domain sockets.  It allows one process
> to pass a file descriptor to another process through the UNIX domain
> socket.  See "man 7 unix" and "man 3 cmsg" for details.
>
> Look at monitor.c:qmp_getfd() to understand how this works.  The QEMU
> code to receive a passed file descriptor is in
> qemu-char.c:unix_process_msgfd().
Thanks a lot for your detailed explanation and suggestions.
I will look into the code to understand how it works.

Thanks
--Li
> Stefan