[PATCH v2 03/18] mm/ksw: add /proc/kstackwatch interface

Jinchao Wang
2025-09-04T00:22:19Z

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