1 /*
2  * data_breakpoint.c - Sample HW Breakpoint file to watch kernel data address
3  *
4  * This program is free software; you can redistribute it and/or modify
5  * it under the terms of the GNU General Public License as published by
6  * the Free Software Foundation; either version 2 of the License, or
7  * (at your option) any later version.
8  *
9  * This program is distributed in the hope that it will be useful,
10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
12  * GNU General Public License for more details.
13  *
14  * You should have received a copy of the GNU General Public License
15  * along with this program; if not, write to the Free Software
16  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
17  *
18  * usage: insmod data_breakpoint.ko ksym=<ksym_name>
19  *
20  * This file is a kernel module that places a breakpoint over ksym_name kernel
21  * variable using Hardware Breakpoint register. The corresponding handler which
22  * prints a backtrace is invoked every time a write operation is performed on
23  * that variable.
24  *
25  * Copyright (C) IBM Corporation, 2009
26  *
27  * Author: K.Prasad <prasad@linux.vnet.ibm.com>
28  */
29 #include <linux/module.h>	/* Needed by all modules */
30 #include <linux/kernel.h>	/* Needed for KERN_INFO */
31 #include <linux/init.h>		/* Needed for the macros */
32 #include <linux/kallsyms.h>
33 
34 #include <linux/perf_event.h>
35 #include <linux/hw_breakpoint.h>
36 
37 struct perf_event * __percpu *sample_hbp;
38 
39 static char ksym_name[KSYM_NAME_LEN] = "pid_max";
40 module_param_string(ksym, ksym_name, KSYM_NAME_LEN, S_IRUGO);
41 MODULE_PARM_DESC(ksym, "Kernel symbol to monitor; this module will report any"
42 			" write operations on the kernel symbol");
43 
44 static void sample_hbp_handler(struct perf_event *bp,
45 			       struct perf_sample_data *data,
46 			       struct pt_regs *regs)
47 {
48 	printk(KERN_INFO "%s value is changed\n", ksym_name);
49 	dump_stack();
50 	printk(KERN_INFO "Dump stack from sample_hbp_handler\n");
51 }
52 
53 static int __init hw_break_module_init(void)
54 {
55 	int ret;
56 	struct perf_event_attr attr;
57 
58 	hw_breakpoint_init(&attr);
59 	attr.bp_addr = kallsyms_lookup_name(ksym_name);
60 	attr.bp_len = HW_BREAKPOINT_LEN_4;
61 	attr.bp_type = HW_BREAKPOINT_W | HW_BREAKPOINT_R;
62 
63 	sample_hbp = register_wide_hw_breakpoint(&attr, sample_hbp_handler, NULL);
64 	if (IS_ERR((void __force *)sample_hbp)) {
65 		ret = PTR_ERR((void __force *)sample_hbp);
66 		goto fail;
67 	}
68 
69 	printk(KERN_INFO "HW Breakpoint for %s write installed\n", ksym_name);
70 
71 	return 0;
72 
73 fail:
74 	printk(KERN_INFO "Breakpoint registration failed\n");
75 
76 	return ret;
77 }
78 
79 static void __exit hw_break_module_exit(void)
80 {
81 	unregister_wide_hw_breakpoint(sample_hbp);
82 	printk(KERN_INFO "HW Breakpoint for %s write uninstalled\n", ksym_name);
83 }
84 
85 module_init(hw_break_module_init);
86 module_exit(hw_break_module_exit);
87 
88 MODULE_LICENSE("GPL");
89 MODULE_AUTHOR("K.Prasad");
90 MODULE_DESCRIPTION("ksym breakpoint");
91