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_SIMPLE_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("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