[PATCH net V2] net: shaper: Reject reparenting of existing nodes

[Mohsin Bashir <mohsin.bashr@gmail.com>]

From: Mohsin Bashir <hmohsin@meta.com>

When an existing node-scope shaper is moved to a different parent
via the group operation, the framework fails to update the leaves
count on both the old and new parent shapers. Only newly created
nodes (handle.id == NET_SHAPER_ID_UNSPEC) trigger the parent
leaves increment at line 1039.

This causes the parent's leaves counter to diverge from the
actual number of children in the xarray. When the node is later
deleted, pre_del_node() allocates an array sized by the stale
leaves count, but the xarray iteration finds more children than
expected, hitting the WARN_ON_ONCE guard and returning -EINVAL.

Rather than adding reparenting support with complex leaves count
bookkeeping, reject group calls that attempt to change an existing
node's parent. Updates to an existing node's rate or leaves under
the same parent remain permitted. We expect that for any modification
of the topology user should always create new groups and let the
kernel garbage collect the leaf-less nodes.

Fixes: 5d5d4700e75d ("net-shapers: implement NL group operation")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Mohsin Bashir <hmohsin@meta.com>
---
Changelog:

V2:
- Add missing maintainer
- Nit: Remove null at the end of fixes tag
- Move reparenting check after parent resolution so updates
  that omit the parent argument are not incorrectly rejected

V1: https://lore.kernel.org/netdev/20260505174321.1794689-1-mohsin.bashr@gmail.com/
---
 net/shaper/shaper.c | 30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/net/shaper/shaper.c b/net/shaper/shaper.c
index 94bc9c7382ea..1069fa4eb9f6 100644
--- a/net/shaper/shaper.c
+++ b/net/shaper/shaper.c
@@ -964,15 +964,22 @@ static int __net_shaper_group(struct net_shaper_binding *binding,
 	int i, ret;
 
 	if (node->handle.scope == NET_SHAPER_SCOPE_NODE) {
+		struct net_shaper *cur = NULL;
+
 		new_node = node->handle.id == NET_SHAPER_ID_UNSPEC;
 
-		if (!new_node && !net_shaper_lookup(binding, &node->handle)) {
-			/* The related attribute is not available when
-			 * reaching here from the delete() op.
-			 */
-			NL_SET_ERR_MSG_FMT(extack, "Node shaper %d:%d does not exists",
-					   node->handle.scope, node->handle.id);
-			return -ENOENT;
+		if (!new_node) {
+			cur = net_shaper_lookup(binding, &node->handle);
+			if (!cur) {
+				/* The related attribute is not available
+				 * when reaching here from the delete() op.
+				 */
+				NL_SET_ERR_MSG_FMT(extack,
+						   "Node shaper %d:%d does not exist",
+						   node->handle.scope,
+						   node->handle.id);
+				return -ENOENT;
+			}
 		}
 
 		/* When unspecified, the node parent scope is inherited from
@@ -986,6 +993,15 @@ static int __net_shaper_group(struct net_shaper_binding *binding,
 				return ret;
 		}
 
+		if (cur && net_shaper_handle_cmp(&cur->parent,
+						 &node->parent)) {
+			NL_SET_ERR_MSG_FMT(extack,
+					   "Cannot reparent node shaper %d:%d",
+					   node->handle.scope,
+					   node->handle.id);
+			return -EOPNOTSUPP;
+		}
+
 	} else {
 		net_shaper_default_parent(&node->handle, &node->parent);
 	}
-- 
2.52.0