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

Miklos Szeredi
2019-09-18T09:07:31Z

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);