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

[address@hidden]

> -----Original Message-----
> From: Stefan Hajnoczi [mailto:address@hidden]
> Sent: Monday, October 21, 2019 6:41 PM
> To: Misono, Tomohiro/味曽野 智礼 <address@hidden>
> Cc: 'Miklos Szeredi' <address@hidden>; address@hidden; address@hidden
> Subject: Re: [Virtio-fs] [PATCH 0/2] virtiofsd: Two fix for xattr operation
> 
> On Fri, Oct 18, 2019 at 08:51:20AM +0000, address@hidden 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.

Thanks,
Misono