[Virtio-fs] [PATCH 0/2] virtiofsd: Two fix for xattr operation
misono.tomohiro at fujitsu.com
misono.tomohiro at fujitsu.com
Wed Oct 23 11:42:50 UTC 2019
> -----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.
Thanks,
Misono
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