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

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().


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.


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