9pfs does not honor open file handles on unlinked files

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1336794

This was originally filed over here:
https://bugzilla.redhat.com/show_bug.cgi?id=1114221

The open-unlink-fstat idiom used in some places to create an anonymous
private temporary file does not work in a QEMU guest over a virtio-9p
filesystem.

Version-Release number of selected component (if applicable):

qemu-kvm-1.6.2-6.fc20.x86_64
qemu-system-x86-1.6.2-6.fc20.x86_64
(those are fedora RPMs)

How reproducible:

Always. See this example C program:

https://bugzilla.redhat.com/attachment.cgi?id=913069

Steps to Reproduce:
1. Export a filesystem with virt-manager for the guest.
      (type: mount, driver: default, mode: passthrough)
2. Start guest and mount that filesystem
      (mount -t 9p -o trans=virtio,version=9p2000.L  ...)
3. Run a program that uses open-unlink-fstat
      (in my case it was trying to compile Perl 5.20)

Actual results:

fstat fails:

open("/home/tst/filename", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
unlink("/home/tst/filename")            = 0
fstat(3, 0x23aa1a8)                     = -1 ENOENT (No such file or
directory)
close(3)

Expected results:

open("/home/tst/filename", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
unlink("/home/tst/filename")            = 0
fstat(3, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
close(3)

Additional info:

There was a patch put into the kernel back in '07 to handle this very
problem for other filesystems; maybe its helpful:

      http://lwn.net/Articles/251228/

There is also a thread on LKML from last December specifically about this
very problem:

      https://lkml.org/lkml/2013/12/31/163

There was a discussion on the QEMU list back in '11 that doesn't seem to
have come to a conclusion, but did provide the test program that i've
attached to this report:

      http://marc.info/?l=qemu-devel&m=130443605720648&w=2