Re: [PATCH] overlayfs: ignore empty NFSv4 ACLs in ext4 upperdir

[Miklos Szeredi]

On Fri, May 10, 2019 at 04:09:41PM -0400, J. Bruce Fields wrote:
> On Tue, May 07, 2019 at 10:24:58AM +1000, NeilBrown wrote:
> > Interesting perspective .... though doesn't NFSv4 explicitly allow
> > client-side ACL enforcement in the case of delegations?
> 
> Not really.  What you're probably thinking of is the single ACE that the
> server can return on granting a delegation, that tells the client it can
> skip the ACCESS check for users matching that ACE.  It's unclear how
> useful that is.  It's currently unused by the Linux client and server.
> 
> > Not sure how relevant that is....
> > 
> > It seems to me we have two options:
> >  1/ declare the NFSv4 doesn't work as a lower layer for overlayfs and
> >     recommend people use NFSv3, or
> >  2/ Modify overlayfs to work with NFSv4 by ignoring nfsv4 ACLs either
> >  2a/ always - and ignore all other acls and probably all system. xattrs,
> >  or
> >  2b/ based on a mount option that might be
> >       2bi/ general "noacl" or might be
> >       2bii/ explicit "noxattr=system.nfs4acl"
> >  
> > I think that continuing to discuss the miniature of the options isn't
> > going to help.  No solution is perfect - we just need to clearly
> > document the implications of whatever we come up with.
> > 
> > I lean towards 2a, but I be happy with with any '2' and '1' won't kill
> > me.
> 
> I guess I'd also lean towards 2a.
> 
> I don't think it applies to posix acls, as overlayfs is capable of
> copying those up and evaluating them on its own.

POSIX acls are evaluated and copied up.

I guess same goes for "security.*" attributes, that are evaluated on MAC checks.

I think it would be safe to ignore failure to copy up anything else.  That seems
a bit saner than just blacklisting nfs4_acl...

Something like the following untested patch.

Thanks,
Miklos

---
 fs/overlayfs/copy_up.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

--- a/fs/overlayfs/copy_up.c
+++ b/fs/overlayfs/copy_up.c
@@ -36,6 +36,13 @@ static int ovl_ccup_get(char *buf, const
 module_param_call(check_copy_up, ovl_ccup_set, ovl_ccup_get, NULL, 0644);
 MODULE_PARM_DESC(check_copy_up, "Obsolete; does nothing");
 
+static bool ovl_must_copy_xattr(const char *name)
+{
+	return !strcmp(name, XATTR_POSIX_ACL_ACCESS) ||
+	       !strcmp(name, XATTR_POSIX_ACL_DEFAULT) ||
+	       !strncmp(name, XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN);
+}
+
 int ovl_copy_xattr(struct dentry *old, struct dentry *new)
 {
 	ssize_t list_size, size, value_size = 0;
@@ -107,8 +114,13 @@ int ovl_copy_xattr(struct dentry *old, s
 			continue; /* Discard */
 		}
 		error = vfs_setxattr(new, name, value, size, 0);
-		if (error)
-			break;
+		if (error) {
+			if (ovl_must_copy_xattr(name))
+				break;
+
+			/* Ignore failure to copy unknown xattrs */
+			error = 0;
+		}
 	}
 	kfree(value);
 out: