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Re: [PATCH 1/3] virtiofsd: Add an option to enable/disable posix acls

From: Miklos Szeredi
Subject: Re: [PATCH 1/3] virtiofsd: Add an option to enable/disable posix acls
Date: Wed, 17 Feb 2021 16:23:54 +0100

On Wed, Feb 17, 2021 at 4:07 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> On Wed, Feb 17, 2021 at 09:53:04AM +0100, Miklos Szeredi wrote:
> > On Wed, Feb 17, 2021 at 12:36 AM Vivek Goyal <vgoyal@redhat.com> wrote:
> > >
> > > fuse has an option FUSE_POSIX_ACL which needs to be opted in by fuse
> > > server to enable posix acls.
> > >
> > > Add virtiofsd option "-o posix_acl/no_posix_acl" to let users 
> > > enable/disable
> > > posix acl support. By default it is disabled as of now.
> >
> > If I read the code correctly, then no_posix_acl will still result in
> > system.posix_acl_* xattr ops being passed through to virtiofsd, which
> > will forward them to the underlying fs, resulting in posix acls
> > appearing to work, but doing so incorrectly (i.e. no change from
> > previous behavior).
> Yes, and this is confuing me a lot. fuse server has not indicated
> support for POSIX_ACL, still user can get and set ACLs. fuse_xattr_get()
> and fuse_xattr_set() must be kicking in.
> I do see that we have fuse_no_acl_xattr_handlers and that should
> be able to block setting/getting acls if acl support is not there
> but we register it only if we are not mounted in init_user_ns.
>         if (sb->s_user_ns != &init_user_ns)
>                 sb->s_xattr = fuse_no_acl_xattr_handlers;
> So question is, should fuse client be fixed as well to block setting
> and getting acls if fuse server does not support ACL? Or we now need
> to keep it around for backward compatibility.

Yes, this is a compatibility thing.   User namespaces don't work
without actual ACL ops, so this was disabled in that case and no
backward compatibility worries in that case.   We should have disabled
this for virtiofs from the start, but at this point we are again stuck
with a backward compatibility issue.

Alternatively make posix_acl the default, hence fixing the bad
behavior is unlikely to cause a regression.

> > Possibly better would be to have three different
> > modes of operation:
> >
> > 1) no option: default fall back to broken acl support for backward
> > compat (this could be removed in the future)
> What about FUSE_DONT_MASK in this mode. ACLs are not enabled but
> user can get/set these. Should that mean we still honor default
> acl and not apply umask?
> Probably I should opt for FUSE_DONT_MASK only if posix_acl support is
> enabled. Given this does not work even today (atleast for virtiofs), so
> it is not a backward compatibility issue. And its confusing anyway.
> > 2) no_posix_acl: really disable acl support
> That is block getting and setting system.posix_acl xattr. Will do that.
> I think we will have to block it even if somebody has remapped xattrs
> in virtiofsd.



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