Provide the /proc/kstackwatch 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      | 75 +++++++++++++++++++++++++++++++++++-
 mm/kstackwatch/kstackwatch.h |  3 ++
 2 files changed, 77 insertions(+), 1 deletion(-)

diff --git a/mm/kstackwatch/kernel.c b/mm/kstackwatch/kernel.c
index 1502795e02af..8e1dca45003e 100644
--- a/mm/kstackwatch/kernel.c
+++ b/mm/kstackwatch/kernel.c
@@ -3,7 +3,10 @@
 
 #include <linux/kstrtox.h>
 #include <linux/module.h>
+#include <linux/proc_fs.h>
+#include <linux/seq_file.h>
 #include <linux/string.h>
+#include <linux/uaccess.h>
 
 #include "kstackwatch.h"
 
@@ -12,6 +15,7 @@ MODULE_DESCRIPTION("Kernel Stack Watch");
 MODULE_LICENSE("GPL");
 
 static struct ksw_config *ksw_config;
+static atomic_t config_file_busy = ATOMIC_INIT(0);
 
 /*
  * Format of the configuration string:
@@ -23,7 +27,7 @@ static struct ksw_config *ksw_config;
  * - local_var_offset : offset from the stack pointer at function+ip_offset
  * - local_var_len    : length of the local variable(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 *func_part, *local_var_part = NULL;
 	char *token;
@@ -92,18 +96,87 @@ static int __maybe_unused ksw_parse_config(char *buf, struct ksw_config *config)
 	return -EINVAL;
 }
 
+static ssize_t kstackwatch_proc_write(struct file *file,
+				      const char __user *buffer, size_t count,
+				      loff_t *pos)
+{
+	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 kstackwatch_proc_show(struct seq_file *m, void *v)
+{
+	seq_printf(m, "%s\n", ksw_config->config_str);
+	return 0;
+}
+
+static int kstackwatch_proc_open(struct inode *inode, struct file *file)
+{
+	if (atomic_cmpxchg(&config_file_busy, 0, 1))
+		return -EBUSY;
+
+	return single_open(file, kstackwatch_proc_show, NULL);
+}
+
+static int kstackwatch_proc_release(struct inode *inode, struct file *file)
+{
+	atomic_set(&config_file_busy, 0);
+	return single_release(inode, file);
+}
+
+static const struct proc_ops kstackwatch_proc_ops = {
+	.proc_open = kstackwatch_proc_open,
+	.proc_read = seq_read,
+	.proc_write = kstackwatch_proc_write,
+	.proc_lseek = seq_lseek,
+	.proc_release = kstackwatch_proc_release,
+};
+
+const struct ksw_config *ksw_get_config(void)
+{
+	return ksw_config;
+}
 static int __init kstackwatch_init(void)
 {
 	ksw_config = kzalloc(sizeof(*ksw_config), GFP_KERNEL);
 	if (!ksw_config)
 		return -ENOMEM;
 
+	if (!proc_create("kstackwatch", 0600, NULL, &kstackwatch_proc_ops)) {
+		pr_err("create proc kstackwatch fail");
+		kfree(ksw_config);
+		return -ENOMEM;
+	}
+
 	pr_info("module loaded\n");
 	return 0;
 }
 
 static void __exit kstackwatch_exit(void)
 {
+	remove_proc_entry("kstackwatch", NULL);
 	kfree(ksw_config);
 
 	pr_info("module unloaded\n");
diff --git a/mm/kstackwatch/kstackwatch.h b/mm/kstackwatch/kstackwatch.h
index 7c595c5c24d1..277b192f80fa 100644
--- a/mm/kstackwatch/kstackwatch.h
+++ b/mm/kstackwatch/kstackwatch.h
@@ -35,4 +35,7 @@ struct ksw_config {
 	char config_str[MAX_CONFIG_STR_LEN];
 };
 
+// singleton, only modified in kernel.c
+const struct ksw_config *ksw_get_config(void);
+
 #endif /* _KSTACKWATCH_H */
-- 
2.43.0