[Qemu-devel] [Bug 1336794] Re: 9pfs does not honor open file handles on unlinked files

[Alexander Gretha]

hi greg,

thanks very much for you answer. i saw the proposed kernel patch from eric van 
hensbergen - even tried to build my own kernel with the patch applied, i was 
ready to run this on a custom kernel with a custom built qemu, but although the 
patch can be applied, there have been too many changes in the surrounding code 
for it to be able to work.
the idea of the 9p file sharing in qemu is really nice (and fast). i am (was) 
trying to use it as a persistent storage on a kubernetes cluster and it is much 
better than nfs (performance wise) locking works just dandy.
with 9p i thought i was golden, unfortunately no cigar.
since there are different parties involved (and to get something into the linux 
kernel requires - from what i have read - the patience of a buddhist monk) i 
think it will be very hard to get this picked up.
because of the time frame this will probably not be a solution for me, but i am 
nonetheless willing to invest some time to bringing this forward.
how is a good way to proceed? (sorry, this question might sound dumb, but 
despite being a software developer for most of my working life the ways of the 
open source community have never revealed themselves to me).

-alex

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1336794

Title:
  9pfs does not honor open file handles on unlinked files

Status in QEMU:
  In Progress
Status in Ubuntu:
  Confirmed

Bug description:
  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

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1336794/+subscriptions