Provide the debugfs config file to read or update the configuration.
Only a single process can open this file at a time, enforced using atomic
config_file_busy, to prevent concurrent access.

ksw_get_config() exposes the configuration pointer as const.

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/kstackwatch/kernel.c      | 104 +++++++++++++++++++++++++++++++++--
 mm/kstackwatch/kstackwatch.h |   3 +
 2 files changed, 103 insertions(+), 4 deletions(-)

diff --git a/mm/kstackwatch/kernel.c b/mm/kstackwatch/kernel.c
index 3b7009033dd4..898ebb2966fe 100644
--- a/mm/kstackwatch/kernel.c
+++ b/mm/kstackwatch/kernel.c
@@ -1,13 +1,18 @@
 // SPDX-License-Identifier: GPL-2.0
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
+#include <linux/debugfs.h>
 #include <linux/kstrtox.h>
 #include <linux/module.h>
 #include <linux/string.h>
+#include <linux/uaccess.h>
 
 #include "kstackwatch.h"
 
+static atomic_t dbgfs_config_busy = ATOMIC_INIT(0);
 static struct ksw_config *ksw_config;
+static struct dentry *dbgfs_config;
+static struct dentry *dbgfs_dir;
 
 struct param_map {
 	const char *name;       /* long name */
@@ -74,7 +79,7 @@ static int ksw_parse_param(struct ksw_config *config, const char *key,
  * - sp_offset  |so (u16) : offset from stack pointer at func_offset
  * - watch_len  |wl (u16) : watch length (1,2,4,8)
  */
-static int __maybe_unused ksw_parse_config(char *buf, struct ksw_config *config)
+static int ksw_parse_config(char *buf, struct ksw_config *config)
 {
 	char *part, *key, *val;
 	int ret;
@@ -109,20 +114,111 @@ static int __maybe_unused ksw_parse_config(char *buf, struct ksw_config *config)
 	return 0;
 }
 
+static ssize_t ksw_dbgfs_read(struct file *file, char __user *buf, size_t count,
+			      loff_t *ppos)
+{
+	return simple_read_from_buffer(buf, count, ppos, ksw_config->user_input,
+		ksw_config->user_input ? strlen(ksw_config->user_input) : 0);
+}
+
+static ssize_t ksw_dbgfs_write(struct file *file, const char __user *buffer,
+			       size_t count, loff_t *ppos)
+{
+	char input[MAX_CONFIG_STR_LEN];
+	int ret;
+
+	if (count == 0 || count >= sizeof(input))
+		return -EINVAL;
+
+	if (copy_from_user(input, buffer, count))
+		return -EFAULT;
+
+	input[count] = '\0';
+	strim(input);
+
+	if (!strlen(input)) {
+		pr_info("config cleared\n");
+		return count;
+	}
+
+	ret = ksw_parse_config(input, ksw_config);
+	if (ret) {
+		pr_err("Failed to parse config %d\n", ret);
+		return ret;
+	}
+
+	return count;
+}
+
+static int ksw_dbgfs_open(struct inode *inode, struct file *file)
+{
+	if (atomic_cmpxchg(&dbgfs_config_busy, 0, 1))
+		return -EBUSY;
+	return 0;
+}
+
+static int ksw_dbgfs_release(struct inode *inode, struct file *file)
+{
+	atomic_set(&dbgfs_config_busy, 0);
+	return 0;
+}
+
+static const struct file_operations kstackwatch_fops = {
+	.owner = THIS_MODULE,
+	.open = ksw_dbgfs_open,
+	.read = ksw_dbgfs_read,
+	.write = ksw_dbgfs_write,
+	.release = ksw_dbgfs_release,
+	.llseek = default_llseek,
+};
+
+const struct ksw_config *ksw_get_config(void)
+{
+	return ksw_config;
+}
+
 static int __init kstackwatch_init(void)
 {
+	int ret = 0;
+
 	ksw_config = kzalloc(sizeof(*ksw_config), GFP_KERNEL);
-	if (!ksw_config)
-		return -ENOMEM;
+	if (!ksw_config) {
+		ret = -ENOMEM;
+		goto err_alloc;
+	}
+
+	dbgfs_dir = debugfs_create_dir("kstackwatch", NULL);
+	if (!dbgfs_dir) {
+		ret = -ENOMEM;
+		goto err_dir;
+	}
+
+	dbgfs_config = debugfs_create_file("config", 0600, dbgfs_dir, NULL,
+				       &kstackwatch_fops);
+	if (!dbgfs_config) {
+		ret = -ENOMEM;
+		goto err_file;
+	}
 
 	pr_info("module loaded\n");
 	return 0;
+
+err_file:
+	debugfs_remove_recursive(dbgfs_dir);
+	dbgfs_dir = NULL;
+err_dir:
+	kfree(ksw_config);
+	ksw_config = NULL;
+err_alloc:
+	return ret;
 }
 
 static void __exit kstackwatch_exit(void)
 {
+	debugfs_remove_recursive(dbgfs_dir);
+	kfree(ksw_config->func_name);
+	kfree(ksw_config->user_input);
 	kfree(ksw_config);
-
 	pr_info("module unloaded\n");
 }
 
diff --git a/mm/kstackwatch/kstackwatch.h b/mm/kstackwatch/kstackwatch.h
index a7bad207f863..983125d5cf18 100644
--- a/mm/kstackwatch/kstackwatch.h
+++ b/mm/kstackwatch/kstackwatch.h
@@ -29,4 +29,7 @@ struct ksw_config {
 	char *user_input;
 };
 
+// singleton, only modified in kernel.c
+const struct ksw_config *ksw_get_config(void);
+
 #endif /* _KSTACKWATCH_H */
-- 
2.43.0