// SPDX-License-Identifier: GPL-2.0+
  /*
   * Test cases for using floating point operations inside a kernel module.
   *
   * This tests kernel_fpu_begin() and kernel_fpu_end() functions, especially
   * when userland has modified the floating point control registers. The kernel
   * state might depend on the state set by the userland thread that was active
   * before a syscall.
   *
   * To facilitate the test, this module registers file
   * /sys/kernel/debug/selftest_helpers/test_fpu, which when read causes a
   * sequence of floating point operations. If the operations fail, either the
   * read returns error status or the kernel crashes.
   * If the operations succeed, the read returns "1
  ".
   */
  
  #include <linux/module.h>
  #include <linux/kernel.h>
  #include <linux/debugfs.h>
  #include <asm/fpu/api.h>
  
  static int test_fpu(void)
  {
  	/*
  	 * This sequence of operations tests that rounding mode is
  	 * to nearest and that denormal numbers are supported.
  	 * Volatile variables are used to avoid compiler optimizing
  	 * the calculations away.
  	 */
  	volatile double a, b, c, d, e, f, g;
  
  	a = 4.0;
  	b = 1e-15;
  	c = 1e-310;
  
  	/* Sets precision flag */
  	d = a + b;
  
  	/* Result depends on rounding mode */
  	e = a + b / 2;
  
  	/* Denormal and very large values */
  	f = b / c;
  
  	/* Depends on denormal support */
  	g = a + c * f;
  
  	if (d > a && e > a && g > a)
  		return 0;
  	else
  		return -EINVAL;
  }
  
  static int test_fpu_get(void *data, u64 *val)
  {
  	int status = -EINVAL;
  
  	kernel_fpu_begin();
  	status = test_fpu();
  	kernel_fpu_end();
  
  	*val = 1;
  	return status;
  }
  
  DEFINE_SIMPLE_ATTRIBUTE(test_fpu_fops, test_fpu_get, NULL, "%lld
  ");
  static struct dentry *selftest_dir;
  
  static int __init test_fpu_init(void)
  {
  	selftest_dir = debugfs_create_dir("selftest_helpers", NULL);
  	if (!selftest_dir)
  		return -ENOMEM;
  
  	debugfs_create_file("test_fpu", 0444, selftest_dir, NULL,
  			    &test_fpu_fops);
  
  	return 0;
  }
  
  static void __exit test_fpu_exit(void)
  {
  	debugfs_remove(selftest_dir);
  }
  
  module_init(test_fpu_init);
  module_exit(test_fpu_exit);
  
  MODULE_LICENSE("GPL");