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

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