[libvirt] [PATCH v4 4/4] qemu-config: Add new -add-fd command line option
Corey Bryant
coreyb at linux.vnet.ibm.com
Fri Oct 19 13:12:26 UTC 2012
On 10/19/2012 07:05 AM, Kevin Wolf wrote:
> Am 18.10.2012 23:37, schrieb Corey Bryant:
>>
>>
>> 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 at 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. :)
>
> I wouldn't be surprised to find such subtleties in Fabrice's code, but
> I'm not sure if adding new instances is the best idea ever. :-)
>
>>> 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?
>
> I'll amend the following to this patch, hope you both agree with the change:
>
> diff --git a/vl.c b/vl.c
> index 47095a2..5fb40da 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -792,7 +792,7 @@ static int parse_sandbox(QemuOpts *opts, void *opaque)
>
> static int parse_add_fd(QemuOpts *opts, void *opaque)
> {
> - int fd, dupfd;
> + int fd, dupfd, flags;
> int64_t fdset_id;
> const char *fd_opaque = NULL;
>
> @@ -812,7 +812,12 @@ static int parse_add_fd(QemuOpts *opts, void *opaque)
> return -1;
> }
>
> - if (fcntl(fd, F_GETFD) & FD_CLOEXEC) {
> + /*
> + * All fds inherited across exec() necessarily have FD_CLOEXEC
> + * clear, while qemu sets FD_CLOEXEC on all other fds used internally.
> + */
> + flags = fcntl(fd, F_GETFD);
> + if (flags == -1 || (flags & FD_CLOEXEC)) {
> qerror_report(ERROR_CLASS_GENERIC_ERROR,
> "fd is not valid or already in use");
> return -1;
>
That works for me. Thanks Kevin!
--
Regards,
Corey Bryant
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