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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Oct 18, 2019 at 08:51:20AM +0000, misono.tomohiro@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.

Stefan

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