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

[Eric Blake <eblake@redhat.com>]

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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.

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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