Re: [PATCH] nfs: we don't support removing system.nfs4_acl

["bfields@fieldses.org" <bfields@fieldses.org>]

On Sun, Jan 31, 2021 at 08:41:37PM +0000, Trond Myklebust wrote:
> On Thu, 2021-01-28 at 21:50 -0500, bfields@fieldses.org wrote:
> > On Thu, Jan 28, 2021 at 09:35:27PM -0500, bfields@fieldses.org wrote:
> > > Note that this patch doesn't prevent an application from setting a
> > > zero-length ACL.  The xattr format is XDR with the first four bytes
> > > representing the number of ACEs, so you'd set a zero-length ACL by
> > > passing down a 4-byte all-zero buffer as the new value of the
> > > system.nfs4_acl xattr.
> > > 
> > > A zero-length NULL buffer is what's used to implement removexattr:
> > > 
> > > int
> > > __vfs_removexattr(struct dentry *dentry, const char *name)
> > > {
> > >         ...
> > >         return handler->set(handler, dentry, inode, name, NULL, 0,
> > > XATTR_REPLACE);
> > > }
> > > 
> > > That's the case this patch covers.
> > 
> > So, I should have said in the changelog, apologies--the behavior
> > without
> > this patch is that when it gets a removexattr, the client sends a
> > SETATTR with a bitmap claiming there's an ACL attribute, but a
> > zero-length attribute value list, and the server (correctly) returns
> > BADXDR.
> > 
> 
> I don't see anything in the spec that prohibits a zero length array
> size for nfs41_aces<> or states that should return NFS4ERR_BADXDR. Why
> shouldn't we allow that?

Again: I agree.  And we do allow that, both before and after this patch.

There's a difference between a SETATTR with a zero-length body and a
SETATTR with a body containing a zero-length ACL.  The former is bad
protocol, the latter is, I agree, fine.

--b.

> 
> Windows, for instance has explicit support for such an ACL:
> https://docs.microsoft.com/en-us/windows/win32/secauthz/null-dacls-and-empty-dacls
> 
> 
> 
> -- 
> Trond Myklebust
> Linux NFS client maintainer, Hammerspace
> trond.myklebust@hammerspace.com
> 
>