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

[Miklos Szeredi]

On Thu, Oct 17, 2019 at 12:05 PM Stefan Hajnoczi <address@hidden> wrote:
>
> On Wed, Oct 16, 2019 at 07:37:52PM +0900, Misono Tomohiro wrote:
> > Hello,
> >
> > I test xattr operation on virtiofs using xfstest generic/062
> > (with -o xattr option and XFS backend) and see some problems.
> >
> > These patches fixes the two of the problems.
> >
> > The remaining problems are:
> >  1. we cannot xattr to block device created by mknod
> >     which does not have actual device (since open in virtiofsd fails)
> >  2. we cannot xattr to symbolic link
> >
> > I don't think 1 is a big problem but can we fix 2?
>
> Sorry, I don't know the answer.  Maybe it would be necessary to add a
> new O_SYMLINK open flag to open(2) so that fgetxattr()/fsetxattr()
> operations can be performed.  A kernel change like that would take some
> time to get accepted upstream and shipped by distros, but it might be
> the only way since the current syscall interface doesn't seem to offer a
> way to do this.

The real problem is that open() on a non-regular, non-directory file
may have side effects (unless O_PATH is used).  These patches try to
paper over that, but the fact is: opening special files from a file
server is forbidden.

I see why this is being done, and it's not easy to fix properly
without the ..at() versions of these syscalls.  One idea is to fork()
+ fchdir(lo->proc_self_fd) + ..xattr().  Another related idea is to do
a unshare(CLONE_FS) after each thread's startup (will pthread library
balk?  I don't know) so that it's safe to do fchdir(lo->proc_self_fd)
+ ...xattr() + fchdir(lo->root_fd).

Thanks,
Miklos