1 // SPDX-License-Identifier: GPL-2.0+
2 /*
3  * Test cases for using floating point operations inside a kernel module.
4  *
5  * This tests kernel_fpu_begin() and kernel_fpu_end() functions, especially
6  * when userland has modified the floating point control registers. The kernel
7  * state might depend on the state set by the userland thread that was active
8  * before a syscall.
9  *
10  * To facilitate the test, this module registers file
11  * /sys/kernel/debug/selftest_helpers/test_fpu, which when read causes a
12  * sequence of floating point operations. If the operations fail, either the
13  * read returns error status or the kernel crashes.
14  * If the operations succeed, the read returns "1\n".
15  */
16 
17 #include <linux/module.h>
18 #include <linux/kernel.h>
19 #include <linux/debugfs.h>
20 #include <asm/fpu/api.h>
21 
test_fpu(void)22 static int test_fpu(void)
23 {
24 	/*
25 	 * This sequence of operations tests that rounding mode is
26 	 * to nearest and that denormal numbers are supported.
27 	 * Volatile variables are used to avoid compiler optimizing
28 	 * the calculations away.
29 	 */
30 	volatile double a, b, c, d, e, f, g;
31 
32 	a = 4.0;
33 	b = 1e-15;
34 	c = 1e-310;
35 
36 	/* Sets precision flag */
37 	d = a + b;
38 
39 	/* Result depends on rounding mode */
40 	e = a + b / 2;
41 
42 	/* Denormal and very large values */
43 	f = b / c;
44 
45 	/* Depends on denormal support */
46 	g = a + c * f;
47 
48 	if (d > a && e > a && g > a)
49 		return 0;
50 	else
51 		return -EINVAL;
52 }
53 
test_fpu_get(void * data,u64 * val)54 static int test_fpu_get(void *data, u64 *val)
55 {
56 	int status = -EINVAL;
57 
58 	kernel_fpu_begin();
59 	status = test_fpu();
60 	kernel_fpu_end();
61 
62 	*val = 1;
63 	return status;
64 }
65 
66 DEFINE_DEBUGFS_ATTRIBUTE(test_fpu_fops, test_fpu_get, NULL, "%lld\n");
67 static struct dentry *selftest_dir;
68 
test_fpu_init(void)69 static int __init test_fpu_init(void)
70 {
71 	selftest_dir = debugfs_create_dir("selftest_helpers", NULL);
72 	if (!selftest_dir)
73 		return -ENOMEM;
74 
75 	debugfs_create_file_unsafe("test_fpu", 0444, selftest_dir, NULL,
76 				   &test_fpu_fops);
77 
78 	return 0;
79 }
80 
test_fpu_exit(void)81 static void __exit test_fpu_exit(void)
82 {
83 	debugfs_remove(selftest_dir);
84 }
85 
86 module_init(test_fpu_init);
87 module_exit(test_fpu_exit);
88 
89 MODULE_LICENSE("GPL");
90