[Virtio-fs] Question on ACLs support in virtiofs

[Vivek Goyal]

On Tue, Feb 16, 2021 at 04:11:20PM +0100, Miklos Szeredi wrote:
> On Mon, Feb 15, 2021 at 9:52 PM Vivek Goyal <vgoyal at redhat.com> wrote:
> >
> > On Fri, Feb 12, 2021 at 10:30:13AM +0000, Luis Henriques wrote:
> > > Hi!
> > >
> > > I've recently executed the generic fstests on virtiofs and decided to have
> > > a closer look at generic/099 failure.  In a nutshell, here's the sequence
> > > of commands that reproduce that failure:
> > >
> > > # umask 0
> > > # mkdir acldir
> > > # chacl -b "u::rwx,g::rwx,o::rwx" "u::r-x,g::r--,o::---" acldir
> > > # touch acldir/file1
> > > # umask 722
> > > # touch acldir/file2
> > > # ls -l acldir
> > > total 0
> > > -r--r----- 1 root root 0 Feb 12 10:04 file1
> > > ----r----- 1 root root 0 Feb 12 10:05 file2
> > >
> > > The failure is that setting umask to 722 shouldn't affect the new file2
> > > because acldir has a default ACL (from umask(2): "... if the parent
> > > directory has a default ACL (see acl(5)), the umask is ignored...").
> > >
> > > So... I tried to have look at the code, and initially I thought that the
> > > problem was in (kernel) function fuse_create_open(), where we have this:
> > >
> > >       if (!fm->fc->dont_mask)
> > >               mode &= ~current_umask();
> > >
> > > but then I went down the rabbit hole, into the user-space code, and
> > > couldn't reach a conclusion.  Maybe the issue is that there's in fact no
> > > support for this POSIX ACLs in virtiofs/FUSE?  Any ideas?
> >
> > Hi,
> >
> > [ CC Miklos and linux-fsdevel ]
> >
> > I debugged into this a little. There are many knobs and it is little
> > confusing that what are right set of fixes.
> >
> > So what's happening in this case is that fc->dont_mask is not set. That
> > means fuse client is modifying mode using umask. First time you
> > touch file, umask is 0, so there is no modification. But next time,
> > you set umask to 722, and fuse modifies mode before sending file
> > create request to server. virtiofs server is already running with
> > umask 0, so it does not touch the mode.
> >
> > So that means, that in case of default acl, fuse client should not
> > be modifying mode using umask. But question is when should fuse
> > skip applying umask.
> >
> > I see that fuse always sets SB_POSIXACL. That means VFS is not
> > going to apply umask and all the umask handling is with-in fuse.
> >
> > sb->s_flags |= SB_POSIXACL;
> >
> > Currently fuse sets fc->dont_mask in two conditions.
> >
> > - If the caller mounted with flag MS_POSIXACL, then fc->dont_mask is set.
> > - If fuse server opted in for option FUSE_DONT_MASK, then fc->dont_mask
> >   is set.
> >
> > I see that for virtiofs, both the conditions are not true out of the
> > box. In fact looks like ACL support is not fully enabled, because
> > I don't see fuse server opting in for FUSE_POSIX_ACL.
> >
> > I suspect that we probably should provide an option in virtiofsd to
> > enable/disable acl support.
> 
> Sounds good.
> 
> > Setting FUSE_DONT_MASK is tricky. If we leave it to fuse, that means
> > fuse will have to query acl to figure out if default acl is set or
> > not on parent dir. And that data could be stale and there could be
> > races w.r.t setting acls from other client.
> >
> > If we do set FUSE_DONT_MASK, that means in file creation path virtiofsd
> > server will have to switch its umask to one provided in request. Given
> > its a per process property, we will have to have some locks to make
> > sure other create requests are not progressing in parallel. And that
> > hope host does the right thing. That is apply umask if parent dir does
> > not have default acl otherwise apply umask (as set by virtiofsd process).
> >
> > Miklos, does above sound reasonable. You might have more thoughts on
> > how to handle this best in fuse/virtiofs.
> 
> fv_queue_worker() does unshare(CLONE_FS) for the fchdir() call in
> xattr ops, which means that umask is now a per-thread propery in
> virtiofsd.

Aha.. I forgot about that. Thanks. 
> 
> So setting umask before create ops sounds like a good solution.

I will give it a try along with an option to enable/disable acl
support in virtiofsd. 

Vivek