[libvirt] [PATCH v3 3/5] osdep: Enable qemu_open to dup pre-opened fd

[Corey Bryant]

On 06/18/2012 04:10 AM, Kevin Wolf wrote:
> Am 15.06.2012 22:00, schrieb Eric Blake:
>> On 06/15/2012 01:19 PM, Corey Bryant wrote:
>>
>>>>> There are some flags that I don't think we'll be able to change.  For
>>>>> example: O_RDONLY, O_WRONLY, O_RDWR.  I assume libvirt would open all
>>>>> files O_RDWR.
>>>>
>>>> I think we need to check all of them and fail qemu_open() if they don't
>>>> match. Those that qemu can change, should be just changed, of course.
>>>>
>>>
>>> Ok.  I remember a scenario where QEMU opens a file read-only (perhaps to
>>> check headers and determine the file format) before re-opening it
>>> read-write.  Perhaps this is only when format= isn't specified with
>>> -drive.  I'm thinking we may need to change flags to read-write where
>>> they used to be read-only, in some circumstances.
>>
>> In those situations, libvirt would pass fd with O_RDWR, and qemu_open()
>> would be fine requesting O_RDONLY the first time (subset is okay), and
>> O_RDWR the second time.  Where you have to error out is where libvirt
>> passes O_RDONLY but qemu wants O_RDWR, and so forth.
>
> Let's try it with requiring an exact match first. If you pass the
> format, I think the probing is completely avoided indeed, and having
> read-only images really opened O_RDONLY protects against stupid mistakes.
>
> Or if we really need to open the file for probing, maybe we could add a
> flag that relaxes the check and that isn't used in the real bdrv_open().
>
> Kevin
>

I haven't heard any objection to this so I'll be checking for exact 
match, and implementing a flag to relax the check only if it's necessary.

-- 
Regards,
Corey