Subject: Re: [PATCH 2/7] mark root hnode explicitly

From: Al Viro <[email-masked]>
Date: Thu, 6 Sep 2018 11:59:33 +0100

On Thu, Sep 06, 2018 at 06:34:00AM -0400, Jamal Hadi Salim wrote:
> On 2018-09-06 6:28 a.m., Jamal Hadi Salim wrote:
> > On 2018-09-05 3:04 p.m., Al Viro wrote:
> > > From: Al Viro <[email-masked]>
> > > 
> > > ... and disallow deleting or linking to such
> > > 
> > > Signed-off-by: Al Viro <[email-masked]>
> > 
> > Same comment as other one in regards to subject
> > 
> > Since the flag space is coming from htnode which is
> > exposed via uapi it makes sense to keep this one here
> > because it is for private use; but  a comment in
> > include/uapi/linux/pkt_cls.h that this flag or
> > maybe a set of bits is reserved for internal use.
> > Otherwise:
> > 
> > Acked-by: Jamal Hadi Salim <[email-masked]>
> 
> 
> Sorry, additional comment:
> It makes sense to reject user space attempt to
> set TCA_CLS_FLAGS_U32_ROOT

Point, and that one is IMO enough to give up on using ->flags for
that.  How about simply

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index 3f985f29ef30..d14048e38b5c 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -84,6 +84,7 @@ struct tc_u_hnode {
 	int			refcnt;
 	unsigned int		divisor;
 	struct idr		handle_idr;
+	bool			is_root;
 	struct rcu_head		rcu;
 	u32			flags;
 	/* The 'ht' field MUST be the last field in structure to allow for
@@ -377,6 +378,7 @@ static int u32_init(struct tcf_proto *tp)
 	root_ht->refcnt++;
 	root_ht->handle = tp_c ? gen_new_htid(tp_c, root_ht) : 0x80000000;
 	root_ht->prio = tp->prio;
+	root_ht->is_root = true;
 	idr_init(&root_ht->handle_idr);
 
 	if (tp_c == NULL) {
@@ -693,7 +695,7 @@ static int u32_delete(struct tcf_proto *tp, void *arg, bool *last,
 		goto out;
 	}
 
-	if (root_ht == ht) {
+	if (ht->is_root) {
 		NL_SET_ERR_MSG_MOD(extack, "Not allowed to delete root node");
 		return -EINVAL;
 	}
@@ -795,6 +797,10 @@ static int u32_set_parms(struct net *net, struct tcf_proto *tp,
 				NL_SET_ERR_MSG_MOD(extack, "Link hash table not found");
 				return -EINVAL;
 			}
+			if (ht_down->is_root) {
+				NL_SET_ERR_MSG_MOD(extack, "Not linking to root node");
+				return -EINVAL;
+			}
 			ht_down->refcnt++;
 		}