1 // SPDX-License-Identifier: GPL-2.0-or-later
2 /*
3  * data_breakpoint.c - Sample HW Breakpoint file to watch kernel data address
4  *
5  * usage: insmod data_breakpoint.ko ksym=<ksym_name>
6  *
7  * This file is a kernel module that places a breakpoint over ksym_name kernel
8  * variable using Hardware Breakpoint register. The corresponding handler which
9  * prints a backtrace is invoked every time a write operation is performed on
10  * that variable.
11  *
12  * Copyright (C) IBM Corporation, 2009
13  *
14  * Author: K.Prasad <prasad@linux.vnet.ibm.com>
15  */
16 #include <linux/module.h>	/* Needed by all modules */
17 #include <linux/kernel.h>	/* Needed for KERN_INFO */
18 #include <linux/init.h>		/* Needed for the macros */
19 #include <linux/kallsyms.h>
20 
21 #include <linux/perf_event.h>
22 #include <linux/hw_breakpoint.h>
23 
24 struct perf_event * __percpu *sample_hbp;
25 
26 static char ksym_name[KSYM_NAME_LEN] = "pid_max";
27 module_param_string(ksym, ksym_name, KSYM_NAME_LEN, S_IRUGO);
28 MODULE_PARM_DESC(ksym, "Kernel symbol to monitor; this module will report any"
29 			" write operations on the kernel symbol");
30 
31 static void sample_hbp_handler(struct perf_event *bp,
32 			       struct perf_sample_data *data,
33 			       struct pt_regs *regs)
34 {
35 	printk(KERN_INFO "%s value is changed\n", ksym_name);
36 	dump_stack();
37 	printk(KERN_INFO "Dump stack from sample_hbp_handler\n");
38 }
39 
40 static int __init hw_break_module_init(void)
41 {
42 	int ret;
43 	struct perf_event_attr attr;
44 
45 	hw_breakpoint_init(&attr);
46 	attr.bp_addr = kallsyms_lookup_name(ksym_name);
47 	attr.bp_len = HW_BREAKPOINT_LEN_4;
48 	attr.bp_type = HW_BREAKPOINT_W | HW_BREAKPOINT_R;
49 
50 	sample_hbp = register_wide_hw_breakpoint(&attr, sample_hbp_handler, NULL);
51 	if (IS_ERR((void __force *)sample_hbp)) {
52 		ret = PTR_ERR((void __force *)sample_hbp);
53 		goto fail;
54 	}
55 
56 	printk(KERN_INFO "HW Breakpoint for %s write installed\n", ksym_name);
57 
58 	return 0;
59 
60 fail:
61 	printk(KERN_INFO "Breakpoint registration failed\n");
62 
63 	return ret;
64 }
65 
66 static void __exit hw_break_module_exit(void)
67 {
68 	unregister_wide_hw_breakpoint(sample_hbp);
69 	printk(KERN_INFO "HW Breakpoint for %s write uninstalled\n", ksym_name);
70 }
71 
72 module_init(hw_break_module_init);
73 module_exit(hw_break_module_exit);
74 
75 MODULE_LICENSE("GPL");
76 MODULE_AUTHOR("K.Prasad");
77 MODULE_DESCRIPTION("ksym breakpoint");
78