lib/test_fpu.c

   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");