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

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

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?

>
> Series:
> Reviewed-by: Eric Blake <eblake@redhat.com>
>

Thanks for reviewing!

-- 
Regards,
Corey Bryant