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