the-tree/lib/test_fpu.c

0001 // SPDX-License-Identifier: GPL-2.0+
0002 /*
0003  * Test cases for using floating point operations inside a kernel module.
0004  *
0005  * This tests kernel_fpu_begin() and kernel_fpu_end() functions, especially
0006  * when userland has modified the floating point control registers. The kernel
0007  * state might depend on the state set by the userland thread that was active
0008  * before a syscall.
0009  *
0010  * To facilitate the test, this module registers file
0011  * /sys/kernel/debug/selftest_helpers/test_fpu, which when read causes a
0012  * sequence of floating point operations. If the operations fail, either the
0013  * read returns error status or the kernel crashes.
0014  * If the operations succeed, the read returns "1\n".
0015  */
0016
0017 #include <linux/module.h>
0018 #include <linux/kernel.h>
0019 #include <linux/debugfs.h>
0020 #include <asm/fpu/api.h>
0021
0022 static int test_fpu(void)
0023 {
0024     /*
0025      * This sequence of operations tests that rounding mode is
0026      * to nearest and that denormal numbers are supported.
0027      * Volatile variables are used to avoid compiler optimizing
0028      * the calculations away.
0029      */
0030     volatile double a, b, c, d, e, f, g;
0031
0032     a = 4.0;
0033     b = 1e-15;
0034     c = 1e-310;
0035
0036     /* Sets precision flag */
0037     d = a + b;
0038
0039     /* Result depends on rounding mode */
0040     e = a + b / 2;
0041
0042     /* Denormal and very large values */
0043     f = b / c;
0044
0045     /* Depends on denormal support */
0046     g = a + c * f;
0047
0048     if (d > a && e > a && g > a)
0049         return 0;
0050     else
0051         return -EINVAL;
0052 }
0053
0054 static int test_fpu_get(void *data, u64 *val)
0055 {
0056     int status = -EINVAL;
0057
0058     kernel_fpu_begin();
0059     status = test_fpu();
0060     kernel_fpu_end();
0061
0062     *val = 1;
0063     return status;
0064 }
0065
0066 DEFINE_DEBUGFS_ATTRIBUTE(test_fpu_fops, test_fpu_get, NULL, "%lld\n");
0067 static struct dentry *selftest_dir;
0068
0069 static int __init test_fpu_init(void)
0070 {
0071     selftest_dir = debugfs_create_dir("selftest_helpers", NULL);
0072     if (!selftest_dir)
0073         return -ENOMEM;
0074
0075     debugfs_create_file_unsafe("test_fpu", 0444, selftest_dir, NULL,
0076                    &test_fpu_fops);
0077
0078     return 0;
0079 }
0080
0081 static void __exit test_fpu_exit(void)
0082 {
0083     debugfs_remove(selftest_dir);
0084 }
0085
0086 module_init(test_fpu_init);
0087 module_exit(test_fpu_exit);
0088
0089 MODULE_LICENSE("GPL");