[PATCH 03/17] mm/ksw: add /proc/kstackwatch interface

[Jinchao Wang <wangjinchao600@gmail.com>]

Provide the /proc/kstackwatch file to read or update the KSW configuration.
Writing a valid config string starts watching; empty input stops watching.
Invalid input stops watching and reports an error.

Allocate a ksw_config struct on module init and free it on exit.
Manage watching state with ksw_start_watching() and ksw_stop_watching().

Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/kstackwatch/kernel.c | 140 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 140 insertions(+)

diff --git a/mm/kstackwatch/kernel.c b/mm/kstackwatch/kernel.c
index 4a6dc49449fe..95ade95abde1 100644
--- a/mm/kstackwatch/kernel.c
+++ b/mm/kstackwatch/kernel.c
@@ -1,7 +1,10 @@
 // SPDX-License-Identifier: GPL-2.0
 #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"
 
@@ -9,6 +12,29 @@ MODULE_AUTHOR("Jinchao Wang");
 MODULE_DESCRIPTION("Kernel Stack Watch");
 MODULE_LICENSE("GPL");
 
+struct ksw_config *ksw_config;
+bool watching_active;
+
+bool panic_on_catch;
+module_param(panic_on_catch, bool, 0644);
+MODULE_PARM_DESC(panic_on_catch,
+		 "Trigger a kernel panic immediately on corruption catch");
+
+static int ksw_start_watching(void)
+{
+	watching_active = true;
+
+	pr_info("KSW: start watching %s\n", ksw_config->config_str);
+	return 0;
+}
+
+static void ksw_stop_watching(void)
+{
+	watching_active = false;
+
+	pr_info("KSW: stop watching %s\n", ksw_config->config_str);
+}
+
 /*
  * Format of the configuration string:
  *    function+ip_offset[+depth] [local_var_offset:local_var_len]
@@ -74,8 +100,114 @@ static int ksw_parse_config(char *buf, struct ksw_config *config)
 	return 0;
 }
 
+/**
+ * kstackwatch_proc_write - Handle writes to the /proc/kstackwatch file
+ * @file: file struct for the proc entry
+ * @buffer: user-space buffer containing the command string
+ * @count: the number of bytes from the user-space buffer
+ * @pos: file offset
+ *
+ * This function processes user input to control the kernel stack watcher
+ * Before attempting to process any new configuration. It handles three
+ * cases based on the input string, In all three cases, the system will
+ * clear its state first, with subsequent actions determined by the input:
+ *
+ * 1. An empty or whitespace-only string
+ *    Return a success code
+ *
+ * 2. An invalid configuration string
+ *    Return a error code
+ *
+ * 3. A valid configuration string
+ *    Start a new stack watch, return a success code
+ *
+ * Return: The number of bytes successfully processed on success,
+ * or a negative error code on failure.
+ */
+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;
+
+	if (watching_active)
+		ksw_stop_watching();
+	memset(ksw_config, 0, sizeof(*ksw_config));
+
+	input[count] = '\0';
+	strim(input);
+
+	/* case 1 */
+	if (!strlen(input)) {
+		pr_info("KSW: config cleared\n");
+		return count;
+	}
+
+	ret = ksw_parse_config(input, ksw_config);
+	if (ret) {
+		/* case 2 */
+		pr_err("KSW: Failed to parse config %d\n", ret);
+		return ret;
+	}
+
+	/* case 3 */
+	ret = ksw_start_watching();
+	if (ret) {
+		pr_err("KSW: Failed to start watching with %d\n", ret);
+		return ret;
+	}
+
+	return count;
+}
+
+static int kstackwatch_proc_show(struct seq_file *m, void *v)
+{
+	if (watching_active) {
+		seq_printf(m, "%s\n", ksw_config->config_str);
+	} else {
+		seq_puts(m, "not watching\n");
+		seq_puts(m, "\nusage:\n");
+		seq_puts(
+			m,
+			" echo 'function+ip_offset[+depth] [local_var_offset:local_var_len]' > /proc/kstackwatch\n");
+		seq_puts(m, "  Watch the canary without the local var part.");
+	}
+
+	return 0;
+}
+
+static int kstackwatch_proc_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, kstackwatch_proc_show, NULL);
+}
+
+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 = single_release,
+};
+
 static int __init kstackwatch_init(void)
 {
+	ksw_config = kmalloc(sizeof(*ksw_config), GFP_KERNEL);
+	if (!ksw_config)
+		return -ENOMEM;
+
+	/* Create proc interface */
+	if (!proc_create("kstackwatch", 0644, NULL, &kstackwatch_proc_ops)) {
+		pr_err("KSW: create proc kstackwatch fail");
+		return -ENOMEM;
+	}
+
 	pr_info("KSW: module loaded\n");
 	pr_info("KSW: usage:\n");
 	pr_info("KSW: echo 'function+ip_offset[+depth] [local_var_offset:local_var_len]' > /proc/kstackwatch\n");
@@ -85,6 +217,14 @@ static int __init kstackwatch_init(void)
 
 static void __exit kstackwatch_exit(void)
 {
+	/* Cleanup active watching */
+	if (watching_active)
+		ksw_stop_watching();
+
+	/* Remove proc interface */
+	remove_proc_entry("kstackwatch", NULL);
+	kfree(ksw_config);
+
 	pr_info("KSW: Module unloaded\n");
 }
 
-- 
2.43.0