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// 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\n".
*/
#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\n");
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");
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