1 /*
2  * livepatch-sample.c - Kernel Live Patching Sample Module
3  *
4  * Copyright (C) 2014 Seth Jennings <sjenning@redhat.com>
5  *
6  * This program is free software; you can redistribute it and/or
7  * modify it under the terms of the GNU General Public License
8  * as published by the Free Software Foundation; either version 2
9  * of the License, or (at your option) any later version.
10  *
11  * This program is distributed in the hope that it will be useful,
12  * but WITHOUT ANY WARRANTY; without even the implied warranty of
13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
14  * GNU General Public License for more details.
15  *
16  * You should have received a copy of the GNU General Public License
17  * along with this program; if not, see <http://www.gnu.org/licenses/>.
18  */
19 
20 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
21 
22 #include <linux/module.h>
23 #include <linux/kernel.h>
24 #include <linux/livepatch.h>
25 
26 /*
27  * This (dumb) live patch overrides the function that prints the
28  * kernel boot cmdline when /proc/cmdline is read.
29  *
30  * Example:
31  *
32  * $ cat /proc/cmdline
33  * <your cmdline>
34  *
35  * $ insmod livepatch-sample.ko
36  * $ cat /proc/cmdline
37  * this has been live patched
38  *
39  * $ echo 0 > /sys/kernel/livepatch/livepatch_sample/enabled
40  * $ cat /proc/cmdline
41  * <your cmdline>
42  */
43 
44 #include <linux/seq_file.h>
45 static int livepatch_cmdline_proc_show(struct seq_file *m, void *v)
46 {
47 	seq_printf(m, "%s\n", "this has been live patched");
48 	return 0;
49 }
50 
51 static struct klp_func funcs[] = {
52 	{
53 		.old_name = "cmdline_proc_show",
54 		.new_func = livepatch_cmdline_proc_show,
55 	}, { }
56 };
57 
58 static struct klp_object objs[] = {
59 	{
60 		/* name being NULL means vmlinux */
61 		.funcs = funcs,
62 	}, { }
63 };
64 
65 static struct klp_patch patch = {
66 	.mod = THIS_MODULE,
67 	.objs = objs,
68 };
69 
70 static int livepatch_init(void)
71 {
72 	int ret;
73 
74 	if (!klp_have_reliable_stack() && !patch.immediate) {
75 		/*
76 		 * WARNING: Be very careful when using 'patch.immediate' in
77 		 * your patches.  It's ok to use it for simple patches like
78 		 * this, but for more complex patches which change function
79 		 * semantics, locking semantics, or data structures, it may not
80 		 * be safe.  Use of this option will also prevent removal of
81 		 * the patch.
82 		 *
83 		 * See Documentation/livepatch/livepatch.txt for more details.
84 		 */
85 		patch.immediate = true;
86 		pr_notice("The consistency model isn't supported for your architecture.  Bypassing safety mechanisms and applying the patch immediately.\n");
87 	}
88 
89 	ret = klp_register_patch(&patch);
90 	if (ret)
91 		return ret;
92 	ret = klp_enable_patch(&patch);
93 	if (ret) {
94 		WARN_ON(klp_unregister_patch(&patch));
95 		return ret;
96 	}
97 	return 0;
98 }
99 
100 static void livepatch_exit(void)
101 {
102 	WARN_ON(klp_unregister_patch(&patch));
103 }
104 
105 module_init(livepatch_init);
106 module_exit(livepatch_exit);
107 MODULE_LICENSE("GPL");
108 MODULE_INFO(livepatch, "Y");
109