[Virtio-fs] [PATCH] virtiofs: Enable SB_NOSEC flag to improve small write performance

[Vivek Goyal]

On Tue, Jul 21, 2020 at 02:33:41PM +0200, Miklos Szeredi wrote:
> On Mon, Jul 20, 2020 at 5:41 PM Vivek Goyal <vgoyal at redhat.com> wrote:
> >
> > On Fri, Jul 17, 2020 at 10:53:07AM +0200, Miklos Szeredi wrote:
> 
> > I see in VFS that chown() always kills suid/sgid. While truncate() and
> > write(), will suid/sgid only if caller does not have CAP_FSETID.
> >
> > How does this work with FUSE_HANDLE_KILLPRIV. IIUC, file server does not
> > know if caller has CAP_FSETID or not. That means file server will be
> > forced to kill suid/sgid on every write and truncate. And that will fail
> > some of the tests.
> >
> > For WRITE requests now we do have the notion of setting
> > FUSE_WRITE_KILL_PRIV flag to tell server explicitly to kill suid/sgid.
> > Probably we could use that in cached write path as well to figure out
> > whether to kill suid/sgid or not. But truncate() will still continue
> > to be an issue.
> 
> Yes, not doing the same for truncate seems to be an oversight.
> Unfortunate, since we'll need another INIT flag to enable selective
> clearing of suid/sgid on truncate.
> 
> >
> > >
> > > Even writeback_cache could be handled by this addition, since we call
> > > fuse_update_attributes() before generic_file_write_iter() :
> > >
> > > --- a/fs/fuse/dir.c
> > > +++ b/fs/fuse/dir.c
> > > @@ -985,6 +985,7 @@ static int fuse_update_get_attr(struct inode
> > > *inode, struct file *file,
> > >
> > >         if (sync) {
> > >                 forget_all_cached_acls(inode);
> > > +               inode->i_flags &= ~S_NOSEC;
> >
> > Ok, So I was clearing S_NOSEC only if server reports that file has
> > suid/sgid bit set. This change will clear S_NOSEC whenever we fetch
> > attrs from host and will force getxattr() when we call file_remove_privs()
> > and will increase overhead for non cache writeback mode. We probably
> > could keep both. For cache writeback mode, clear it undonditionally
> > otherwise not.
> 
> We clear S_NOSEC because the attribute timeout has expired.  This
> means we need to refresh all metadata, including cached xattr (which
> is what S_NOSEC effectively is).
> 
> > What I don't understand is though that how this change will clear
> > suid/sgid on host in cache=writeback mode. I see fuse_setattr()
> > will not set ATTR_MODE and clear S_ISUID and S_ISGID if
> > fc->handle_killpriv is set. So when server receives setattr request
> > (if it does), then how will it know it is supposed to kill suid/sgid
> > bit. (its not chown, truncate and its not write).
> 
> Depends.  If the attribute timeout is infinity, then that means the
> cache is always up to date.  In that case we only need to clear
> suid/sgid if set in i_mode.  Similarly, the security.capability will
> only be cleared if it was set in the first place (which would clear
> S_NOSEC).
> 
> If the timeout is finite, then that means we need to check if the
> metadata changed after a timeout.  That's the purpose of the
> fuse_update_attributes() call before generic_file_write_iter().
> 
> Does that make it clear?

I understood it partly but one thing is still bothering me. What
happens when cache writeback is set as well as fc->handle_killpriv=1.

When handle_killpriv is set, how suid/sgid will be cleared by
server. Given cache=writeback, write probably got cached in
guest and server probably will not not see a WRITE immideately.
(I am assuming we are relying on a WRITE to clear setuid/setgid when
 handle_killpriv is set). And that means server will not clear
 setuid/setgid till inode is written back at some point of time
 later.

IOW, cache=writeback and fc->handle_killpriv don't seem to go
together (atleast given the current code).

Thanks
Vivek