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

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.

>> >> In which scenario would any client break if we set FD_CLOEXEC? I don't >> think compatibility means we can't fix any bugs. >> > > I don't know if it breaks any client. Maybe it's not a compatibility > error. It dopes change behavior down the line though. If you think

> it's ok to set FD_CLOEXEC for getfd too, then I'm happy to do it. The only case that a client might break is if there were a way to pass an fd into qemu and then intentionally see that fd in a child process of qemu. But in the case of 'migrate fd:nnn', you aren't spawning a child process, and even in the case of 'migrate exec:command' (which libvirt no longer uses if fd:nnn works), I don't see how the client could have ever intentionally tried to use 'getfd' in advance to pass an extra fd for use inside the 'exec:command' child. Besides, before 'pass-fd' was around, how would the management app triggering the 'exec:command' even know what fd number would accidentally be inherited into the exec:command child? I think it is pretty much a straight bug-fix for 'getfd' to always set FD_CLOEXEC, and preferably set it atomically via MSG_CMSG_CLOEXEC.