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_SIMPLE_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("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