net: bpfilter: restart bpfilter_umh when error occurred

The bpfilter_umh will be stopped via __stop_umh() when the bpfilter
error occurred.
The bpfilter_umh() couldn't start again because there is no restart
routine.

The section of the bpfilter_umh_{start/end} is no longer .init.rodata
because these area should be reused in the restart routine. hence
the section name is changed to .bpfilter_umh.

The bpfilter_ops->start() is restart callback. it will be called when
bpfilter_umh is stopped.
The stop bit means bpfilter_umh is stopped. this bit is set by both
start and stop routine.

Before this patch,
Test commands:
   $ iptables -vnL
   $ kill -9 <pid of bpfilter_umh>
   $ iptables -vnL
   [  480.045136] bpfilter: write fail -32
   $ iptables -vnL

All iptables commands will fail.

After this patch,
Test commands:
   $ iptables -vnL
   $ kill -9 <pid of bpfilter_umh>
   $ iptables -vnL
   $ iptables -vnL

Now, all iptables commands will work.

Fixes: d2ba09c17a ("net: add skeleton of bpfilter kernel module")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/bpfilter.h

@@ -15,6 +15,8 @@ struct bpfilter_umh_ops {
 	int (*sockopt)(struct sock *sk, int optname,
 		       char __user *optval,
 		       unsigned int optlen, bool is_set);
+	int (*start)(void);
+	bool stop;
 };
 extern struct bpfilter_umh_ops bpfilter_ops;
 #endif

net/bpfilter/bpfilter_kern.c

@@ -16,13 +16,14 @@ extern char bpfilter_umh_end;
 /* since ip_getsockopt() can run in parallel, serialize access to umh */
 static DEFINE_MUTEX(bpfilter_lock);
 
-static void shutdown_umh(struct umh_info *info)
+static void shutdown_umh(void)
 {
 	struct task_struct *tsk;
 
-	if (!info->pid)
+	if (bpfilter_ops.stop)
 		return;
-	tsk = get_pid_task(find_vpid(info->pid), PIDTYPE_PID);
+
+	tsk = get_pid_task(find_vpid(bpfilter_ops.info.pid), PIDTYPE_PID);
 	if (tsk) {
 		force_sig(SIGKILL, tsk);
 		put_task_struct(tsk);
@@ -31,10 +32,8 @@ static void shutdown_umh(struct umh_info *info)
 
 static void __stop_umh(void)
 {
-	if (IS_ENABLED(CONFIG_INET)) {
-		bpfilter_ops.sockopt = NULL;
-		shutdown_umh(&bpfilter_ops.info);
-	}
+	if (IS_ENABLED(CONFIG_INET))
+		shutdown_umh();
 }
 
 static void stop_umh(void)
@@ -85,7 +84,7 @@ static int __bpfilter_process_sockopt(struct sock *sk, int optname,
 	return ret;
 }
 
-static int __init load_umh(void)
+static int start_umh(void)
 {
 	int err;
 
@@ -95,6 +94,7 @@ static int __init load_umh(void)
 				 &bpfilter_ops.info);
 	if (err)
 		return err;
+	bpfilter_ops.stop = false;
 	pr_info("Loaded bpfilter_umh pid %d\n", bpfilter_ops.info.pid);
 
 	/* health check that usermode process started correctly */
@@ -102,14 +102,31 @@ static int __init load_umh(void)
 		stop_umh();
 		return -EFAULT;
 	}
-	if (IS_ENABLED(CONFIG_INET))
-		bpfilter_ops.sockopt = &__bpfilter_process_sockopt;
 
 	return 0;
 }
 
+static int __init load_umh(void)
+{
+	int err;
+
+	if (!bpfilter_ops.stop)
+		return -EFAULT;
+	err = start_umh();
+	if (!err && IS_ENABLED(CONFIG_INET)) {
+		bpfilter_ops.sockopt = &__bpfilter_process_sockopt;
+		bpfilter_ops.start = &start_umh;
+	}
+
+	return err;
+}
+
 static void __exit fini_umh(void)
 {
+	if (IS_ENABLED(CONFIG_INET)) {
+		bpfilter_ops.start = NULL;
+		bpfilter_ops.sockopt = NULL;
+	}
 	stop_umh();
 }
 module_init(load_umh);

net/bpfilter/bpfilter_umh_blob.S

@@ -1,5 +1,5 @@
 /* SPDX-License-Identifier: GPL-2.0 */
-	.section .init.rodata, "a"
+	.section .bpfilter_umh, "a"
 	.global bpfilter_umh_start
 bpfilter_umh_start:
 	.incbin "net/bpfilter/bpfilter_umh"

net/ipv4/bpfilter/sockopt.c

@@ -14,6 +14,7 @@ EXPORT_SYMBOL_GPL(bpfilter_ops);
 
 static void bpfilter_umh_cleanup(struct umh_info *info)
 {
+	bpfilter_ops.stop = true;
 	fput(info->pipe_to_umh);
 	fput(info->pipe_from_umh);
 	info->pid = 0;
@@ -23,14 +24,21 @@ static int bpfilter_mbox_request(struct sock *sk, int optname,
 				 char __user *optval,
 				 unsigned int optlen, bool is_set)
 {
+	int err;
+
 	if (!bpfilter_ops.sockopt) {
-		int err = request_module("bpfilter");
+		err = request_module("bpfilter");
 
 		if (err)
 			return err;
 		if (!bpfilter_ops.sockopt)
 			return -ECHILD;
 	}
+	if (bpfilter_ops.stop) {
+		err = bpfilter_ops.start();
+		if (err)
+			return err;
+	}
 	return bpfilter_ops.sockopt(sk, optname, optval, optlen, is_set);
 }
 
@@ -53,6 +61,7 @@ int bpfilter_ip_get_sockopt(struct sock *sk, int optname, char __user *optval,
 
 static int __init bpfilter_sockopt_init(void)
 {
+	bpfilter_ops.stop = true;
 	bpfilter_ops.info.cmdline = "bpfilter_umh";
 	bpfilter_ops.info.cleanup = &bpfilter_umh_cleanup;