xref: /openbmc/linux/lib/test_fpu.c (revision 9e255e2b)
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  
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  
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  
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  
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