[Virtio-fs] [PATCH 0/2] virtiofsd: Two fix for xattr operation

[Stefan Hajnoczi]

On Wed, Oct 23, 2019 at 11:42:50AM +0000, misono.tomohiro at fujitsu.com wrote:
> > -----Original Message-----
> > From: Stefan Hajnoczi [mailto:stefanha at redhat.com]
> > Sent: Monday, October 21, 2019 6:41 PM
> > To: Misono, Tomohiro/味曽野 智礼 <misono.tomohiro at fujitsu.com>
> > Cc: 'Miklos Szeredi' <mszeredi at redhat.com>; virtio-fs at redhat.com; qemu-devel at nongnu.org
> > Subject: Re: [Virtio-fs] [PATCH 0/2] virtiofsd: Two fix for xattr operation
> > 
> > On Fri, Oct 18, 2019 at 08:51:20AM +0000, misono.tomohiro at fujitsu.com wrote:
> > > > Doing unshare(CLONE_FS) after thread startup seems safe, though must
> > > > be careful to change the working directory to the root of the mount
> > > > *before* starting any threads.
> > >
> > > I think working directry is changed in setup_sandbox() -> setup_mount_namespace() -> setup_pivot_root().
> > > So, can we just add unshare(CLONE_FS) in fv_queue_worker()?
> > 
> > fv_queue_worker() is the thread pool worker function that is called for each request.  Calling unshare(CLONE_FS) for each request
> > is not necessary and will reduce performance.
> > 
> > A thread-local variable can be used to avoid repeated calls to
> > unshare(CLONE_FS) from the same worker thread:
> > 
> >   static __thread bool clone_fs_called;
> > 
> >   static void fv_queue_worker(gpointer data, gpointer user_data)
> >   {
> >       ...
> >       if (!clone_fs_called) {
> >           int ret;
> > 
> > 	  ret = unshare(CLONE_FS);
> > 	  assert(ret == 0); /* errors not expected according to man page */
> > 
> > 	  clone_fs_called = true;
> >       }
> > 
> > Another issue is the seccomp policy.  Since worker threads are spawned at runtime it is necessary to add the unshare(2) syscall
> > to the seccomp whitelist in contrib/virtiofsd/seccomp.c.
> > 
> 
> Thanks for suggesting.
> 
> I tried above code with fchdir() + ...xattr() + fchdir() in lo_...xattr
> and it solves all the problem about xattr I see.
> 
> However, it seems the fix causes some performance issue in stress test
> as ACL check issues getxattr() and a lot of fchdir() happens. So, I may
> try to combine the old method for regular file and this method for special
> files.

Sounds good.

Stefan
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