net: dsa: replace NETDEV_PRE_CHANGE_HWTSTAMP notifier with a stub

There was a sort of rush surrounding commit 88c0a6b503 ("net: create a
netdev notifier for DSA to reject PTP on DSA master"), due to a desire
to convert DSA's attempt to deny TX timestamping on a DSA master to
something that doesn't block the kernel-wide API conversion from
ndo_eth_ioctl() to ndo_hwtstamp_set().

What was required was a mechanism that did not depend on ndo_eth_ioctl(),
and what was provided was a mechanism that did not depend on
ndo_eth_ioctl(), while at the same time introducing something that
wasn't absolutely necessary - a new netdev notifier.

There have been objections from Jakub Kicinski that using notifiers in
general when they are not absolutely necessary creates complications to
the control flow and difficulties to maintainers who look at the code.
So there is a desire to not use notifiers.

In addition to that, the notifier chain gets called even if there is no
DSA in the system and no one is interested in applying any restriction.

Take the model of udp_tunnel_nic_ops and introduce a stub mechanism,
through which net/core/dev_ioctl.c can call into DSA even when
CONFIG_NET_DSA=m.

Compared to the code that existed prior to the notifier conversion, aka
what was added in commits:
- 4cfab35667 ("net: dsa: Add wrappers for overloaded ndo_ops")
- 3369afba1e ("net: Call into DSA netdevice_ops wrappers")

this is different because we are not overloading any struct
net_device_ops of the DSA master anymore, but rather, we are exposing a
rather specific functionality which is orthogonal to which API is used
to enable it - ndo_eth_ioctl() or ndo_hwtstamp_set().

Also, what is similar is that both approaches use function pointers to
get from built-in code to DSA.

There is no point in replicating the function pointers towards
__dsa_master_hwtstamp_validate() once for every CPU port (dev->dsa_ptr).
Instead, it is sufficient to introduce a singleton struct dsa_stubs,
built into the kernel, which contains a single function pointer to
__dsa_master_hwtstamp_validate().

I find this approach preferable to what we had originally, because
dev->dsa_ptr->netdev_ops->ndo_do_ioctl() used to require going through
struct dsa_port (dev->dsa_ptr), and so, this was incompatible with any
attempts to add any data encapsulation and hide DSA data structures from
the outside world.

Link: https://lore.kernel.org/netdev/20230403083019.120b72fd@kernel.org/
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -2878,7 +2878,6 @@ enum netdev_cmd {
 	NETDEV_OFFLOAD_XSTATS_REPORT_USED,
 	NETDEV_OFFLOAD_XSTATS_REPORT_DELTA,
 	NETDEV_XDP_FEAT_CHANGE,
-	NETDEV_PRE_CHANGE_HWTSTAMP,
 };
 const char *netdev_cmd_to_name(enum netdev_cmd cmd);
 
@@ -2929,11 +2928,6 @@ struct netdev_notifier_pre_changeaddr_info {
 	const unsigned char *dev_addr;
 };
 
-struct netdev_notifier_hwtstamp_info {
-	struct netdev_notifier_info info; /* must be first */
-	struct kernel_hwtstamp_config *config;
-};
-
 enum netdev_offload_xstats_type {
 	NETDEV_OFFLOAD_XSTATS_TYPE_L3 = 1,
 };

include/net/dsa_stubs.h

@@ -0,0 +1,48 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/*
+ * include/net/dsa_stubs.h - Stubs for the Distributed Switch Architecture framework
+ */
+
+#include <linux/mutex.h>
+#include <linux/netdevice.h>
+#include <linux/net_tstamp.h>
+#include <net/dsa.h>
+
+#if IS_ENABLED(CONFIG_NET_DSA)
+
+extern const struct dsa_stubs *dsa_stubs;
+
+struct dsa_stubs {
+	int (*master_hwtstamp_validate)(struct net_device *dev,
+					const struct kernel_hwtstamp_config *config,
+					struct netlink_ext_ack *extack);
+};
+
+static inline int dsa_master_hwtstamp_validate(struct net_device *dev,
+					       const struct kernel_hwtstamp_config *config,
+					       struct netlink_ext_ack *extack)
+{
+	if (!netdev_uses_dsa(dev))
+		return 0;
+
+	/* rtnl_lock() is a sufficient guarantee, because as long as
+	 * netdev_uses_dsa() returns true, the dsa_core module is still
+	 * registered, and so, dsa_unregister_stubs() couldn't have run.
+	 * For netdev_uses_dsa() to start returning false, it would imply that
+	 * dsa_master_teardown() has executed, which requires rtnl_lock().
+	 */
+	ASSERT_RTNL();
+
+	return dsa_stubs->master_hwtstamp_validate(dev, config, extack);
+}
+
+#else
+
+static inline int dsa_master_hwtstamp_validate(struct net_device *dev,
+					       const struct kernel_hwtstamp_config *config,
+					       struct netlink_ext_ack *extack)
+{
+	return 0;
+}
+
+#endif

net/Makefile

@@ -24,7 +24,7 @@ obj-$(CONFIG_PACKET)		+= packet/
 obj-$(CONFIG_NET_KEY)		+= key/
 obj-$(CONFIG_BRIDGE)		+= bridge/
 obj-$(CONFIG_NET_DEVLINK)	+= devlink/
-obj-$(CONFIG_NET_DSA)		+= dsa/
+obj-y				+= dsa/
 obj-$(CONFIG_ATALK)		+= appletalk/
 obj-$(CONFIG_X25)		+= x25/
 obj-$(CONFIG_LAPB)		+= lapb/

net/core/dev.c

@@ -1612,7 +1612,7 @@ const char *netdev_cmd_to_name(enum netdev_cmd cmd)
 	N(SVLAN_FILTER_PUSH_INFO) N(SVLAN_FILTER_DROP_INFO)
 	N(PRE_CHANGEADDR) N(OFFLOAD_XSTATS_ENABLE) N(OFFLOAD_XSTATS_DISABLE)
 	N(OFFLOAD_XSTATS_REPORT_USED) N(OFFLOAD_XSTATS_REPORT_DELTA)
-	N(XDP_FEAT_CHANGE) N(PRE_CHANGE_HWTSTAMP)
+	N(XDP_FEAT_CHANGE)
 	}
 #undef N
 	return "UNKNOWN_NETDEV_EVENT";

net/core/dev_ioctl.c

@@ -7,7 +7,7 @@
 #include <linux/net_tstamp.h>
 #include <linux/wireless.h>
 #include <linux/if_bridge.h>
-#include <net/dsa.h>
+#include <net/dsa_stubs.h>
 #include <net/wext.h>
 
 #include "dev.h"
@@ -259,9 +259,6 @@ static int dev_get_hwtstamp(struct net_device *dev, struct ifreq *ifr)
 
 static int dev_set_hwtstamp(struct net_device *dev, struct ifreq *ifr)
 {
-	struct netdev_notifier_hwtstamp_info info = {
-		.info.dev = dev,
-	};
 	struct kernel_hwtstamp_config kernel_cfg;
 	struct netlink_ext_ack extack = {};
 	struct hwtstamp_config cfg;
@@ -276,12 +273,7 @@ static int dev_set_hwtstamp(struct net_device *dev, struct ifreq *ifr)
 	if (err)
 		return err;
 
-	info.info.extack = &extack;
-	info.config = &kernel_cfg;
-
-	err = call_netdevice_notifiers_info(NETDEV_PRE_CHANGE_HWTSTAMP,
-					    &info.info);
-	err = notifier_to_errno(err);
+	err = dsa_master_hwtstamp_validate(dev, &kernel_cfg, &extack);
 	if (err) {
 		if (extack._msg)
 			netdev_err(dev, "%s\n", extack._msg);

net/dsa/Makefile

@@ -1,4 +1,10 @@
 # SPDX-License-Identifier: GPL-2.0
+
+# the stubs are built-in whenever DSA is built-in or module
+ifdef CONFIG_NET_DSA
+obj-y := stubs.o
+endif
+
 # the core
 obj-$(CONFIG_NET_DSA) += dsa_core.o
 dsa_core-y += \

net/dsa/dsa.c

@@ -17,6 +17,7 @@
 #include <linux/of.h>
 #include <linux/of_mdio.h>
 #include <linux/of_net.h>
+#include <net/dsa_stubs.h>
 #include <net/sch_generic.h>
 
 #include "devlink.h"
@@ -1702,6 +1703,20 @@ bool dsa_mdb_present_in_other_db(struct dsa_switch *ds, int port,
 }
 EXPORT_SYMBOL_GPL(dsa_mdb_present_in_other_db);
 
+static const struct dsa_stubs __dsa_stubs = {
+	.master_hwtstamp_validate = __dsa_master_hwtstamp_validate,
+};
+
+static void dsa_register_stubs(void)
+{
+	dsa_stubs = &__dsa_stubs;
+}
+
+static void dsa_unregister_stubs(void)
+{
+	dsa_stubs = NULL;
+}
+
 static int __init dsa_init_module(void)
 {
 	int rc;
@@ -1721,6 +1736,8 @@ static int __init dsa_init_module(void)
 	if (rc)
 		goto netlink_register_fail;
 
+	dsa_register_stubs();
+
 	return 0;
 
 netlink_register_fail:
@@ -1735,6 +1752,8 @@ module_init(dsa_init_module);
 
 static void __exit dsa_cleanup_module(void)
 {
+	dsa_unregister_stubs();
+
 	rtnl_link_unregister(&dsa_link_ops);
 
 	dsa_slave_unregister_notifier();

net/dsa/master.c

@@ -198,7 +198,7 @@ static void dsa_master_get_strings(struct net_device *dev, uint32_t stringset,
 /* Deny PTP operations on master if there is at least one switch in the tree
  * that is PTP capable.
  */
-int dsa_master_pre_change_hwtstamp(struct net_device *dev,
+int __dsa_master_hwtstamp_validate(struct net_device *dev,
 				   const struct kernel_hwtstamp_config *config,
 				   struct netlink_ext_ack *extack)
 {

net/dsa/master.h

@@ -15,7 +15,7 @@ int dsa_master_lag_setup(struct net_device *lag_dev, struct dsa_port *cpu_dp,
 			 struct netlink_ext_ack *extack);
 void dsa_master_lag_teardown(struct net_device *lag_dev,
 			     struct dsa_port *cpu_dp);
-int dsa_master_pre_change_hwtstamp(struct net_device *dev,
+int __dsa_master_hwtstamp_validate(struct net_device *dev,
 				   const struct kernel_hwtstamp_config *config,
 				   struct netlink_ext_ack *extack);
 

net/dsa/slave.c

@@ -3289,7 +3289,6 @@ static int dsa_master_changeupper(struct net_device *dev,
 static int dsa_slave_netdevice_event(struct notifier_block *nb,
 				     unsigned long event, void *ptr)
 {
-	struct netlink_ext_ack *extack = netdev_notifier_info_to_extack(ptr);
 	struct net_device *dev = netdev_notifier_info_to_dev(ptr);
 
 	switch (event) {
@@ -3419,16 +3418,6 @@ static int dsa_slave_netdevice_event(struct notifier_block *nb,
 
 		return NOTIFY_OK;
 	}
-	case NETDEV_PRE_CHANGE_HWTSTAMP: {
-		struct netdev_notifier_hwtstamp_info *info = ptr;
-		int err;
-
-		if (!netdev_uses_dsa(dev))
-			return NOTIFY_DONE;
-
-		err = dsa_master_pre_change_hwtstamp(dev, info->config, extack);
-		return notifier_from_errno(err);
-	}
 	default:
 		break;
 	}

net/dsa/stubs.c

@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Stubs for DSA functionality called by the core network stack.
+ * These are necessary because CONFIG_NET_DSA can be a module, and built-in
+ * code cannot directly call symbols exported by modules.
+ */
+#include <net/dsa_stubs.h>
+
+const struct dsa_stubs *dsa_stubs;
+EXPORT_SYMBOL_GPL(dsa_stubs);