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Subject[PATCH v2 2/2] selftests/fpu: Add an FPU selftest
Date
From: Petteri Aimonen <jpa@git.mail.kapsi.fi>

Add a selftest for the usage of FPU code in kernel mode.

Currently only implemented for x86. In the future, kernel FPU testing
could be unified between the different architectures supporting it.

[ bp:

- Split out from a conglomerate patch, put comments over statements.
- run the test only on debugfs write.
- Add bare-minimum run_test_fpu.sh, run 1000 iterations on all CPUs
by default.
- Add conditionally -msse2 so that clang doesn't generate library
calls.
- Use cc-option to detect gcc 7.1 not supporting -mpreferred-stack-boundary=3.
- Document stuff so that we don't forget.
]

Signed-off-by: Petteri Aimonen <jpa@git.mail.kapsi.fi>
Signed-off-by: Borislav Petkov <bp@suse.de>
---
lib/Kconfig.debug | 11 +++
lib/Makefile | 24 ++++++
lib/test_fpu.c | 89 +++++++++++++++++++++
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/fpu/.gitignore | 2 +
tools/testing/selftests/fpu/Makefile | 9 +++
tools/testing/selftests/fpu/run_test_fpu.sh | 46 +++++++++++
tools/testing/selftests/fpu/test_fpu.c | 61 ++++++++++++++
8 files changed, 243 insertions(+)
create mode 100644 lib/test_fpu.c
create mode 100644 tools/testing/selftests/fpu/.gitignore
create mode 100644 tools/testing/selftests/fpu/Makefile
create mode 100755 tools/testing/selftests/fpu/run_test_fpu.sh
create mode 100644 tools/testing/selftests/fpu/test_fpu.c

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index d74ac0fd6b2d..a1963a493920 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2308,6 +2308,17 @@ config TEST_HMM

If unsure, say N.

+config TEST_FPU
+ tristate "Test floating point operations in kernel space"
+ depends on X86
+ help
+ Enable this option to add /sys/kernel/debug/selftest_helpers/test_fpu
+ which will trigger a sequence of floating point operations. This is used
+ for self-testing floating point control register setting in
+ kernel_fpu_begin().
+
+ If unsure, say N.
+
endif # RUNTIME_TESTING_MENU

config MEMTEST
diff --git a/lib/Makefile b/lib/Makefile
index b1c42c10073b..d0f17a74afaf 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -99,6 +99,30 @@ obj-$(CONFIG_TEST_MEMINIT) += test_meminit.o
obj-$(CONFIG_TEST_LOCKUP) += test_lockup.o
obj-$(CONFIG_TEST_HMM) += test_hmm.o

+#
+# CFLAGS for compiling floating point code inside the kernel. x86/Makefile turns
+# off the generation of FPU/SSE* instructions for kernel proper but FPU_FLAGS
+# get appended last to CFLAGS and thus override those previous compiler options.
+#
+FPU_CFLAGS := -mhard-float -msse -msse2
+ifdef CONFIG_CC_IS_GCC
+# Stack alignment mismatch, proceed with caution.
+# GCC < 7.1 cannot compile code using `double` and -mpreferred-stack-boundary=3
+# (8B stack alignment).
+# See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
+#
+# The "-msse" in the first argument is there so that the
+# -mpreferred-stack-boundary=3 build error:
+#
+# -mpreferred-stack-boundary=3 is not between 4 and 12
+#
+# can be triggered. Otherwise gcc doesn't complain.
+FPU_CFLAGS += $(call cc-option,-msse -mpreferred-stack-boundary=3,-mpreferred-stack-boundary=4)
+endif
+
+obj-$(CONFIG_TEST_FPU) += test_fpu.o
+CFLAGS_test_fpu.o += $(FPU_CFLAGS)
+
obj-$(CONFIG_TEST_LIVEPATCH) += livepatch/

obj-$(CONFIG_KUNIT) += kunit/
diff --git a/lib/test_fpu.c b/lib/test_fpu.c
new file mode 100644
index 000000000000..c33764aa3eb8
--- /dev/null
+++ b/lib/test_fpu.c
@@ -0,0 +1,89 @@
+// 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");
diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 1195bd85af38..227ca78a5b7f 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -15,6 +15,7 @@ TARGETS += filesystems
TARGETS += filesystems/binderfs
TARGETS += filesystems/epoll
TARGETS += firmware
+TARGETS += fpu
TARGETS += ftrace
TARGETS += futex
TARGETS += gpio
diff --git a/tools/testing/selftests/fpu/.gitignore b/tools/testing/selftests/fpu/.gitignore
new file mode 100644
index 000000000000..d6d12ac1d9c3
--- /dev/null
+++ b/tools/testing/selftests/fpu/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0+
+test_fpu
diff --git a/tools/testing/selftests/fpu/Makefile b/tools/testing/selftests/fpu/Makefile
new file mode 100644
index 000000000000..ea62c176ede7
--- /dev/null
+++ b/tools/testing/selftests/fpu/Makefile
@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: GPL-2.0+
+
+LDLIBS := -lm
+
+TEST_GEN_PROGS := test_fpu
+
+TEST_PROGS := run_test_fpu.sh
+
+include ../lib.mk
diff --git a/tools/testing/selftests/fpu/run_test_fpu.sh b/tools/testing/selftests/fpu/run_test_fpu.sh
new file mode 100755
index 000000000000..d77be93ec139
--- /dev/null
+++ b/tools/testing/selftests/fpu/run_test_fpu.sh
@@ -0,0 +1,46 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Load kernel module for FPU tests
+
+uid=$(id -u)
+if [ $uid -ne 0 ]; then
+ echo "$0: Must be run as root"
+ exit 1
+fi
+
+if ! which modprobe > /dev/null 2>&1; then
+ echo "$0: You need modprobe installed"
+ exit 4
+fi
+
+if ! modinfo test_fpu > /dev/null 2>&1; then
+ echo "$0: You must have the following enabled in your kernel:"
+ echo "CONFIG_TEST_FPU=m"
+ exit 4
+fi
+
+NR_CPUS=$(getconf _NPROCESSORS_ONLN)
+if [ ! $NR_CPUS ]; then
+ NR_CPUS=1
+fi
+
+modprobe test_fpu
+
+if [ ! -e /sys/kernel/debug/selftest_helpers/test_fpu ]; then
+ mount -t debugfs none /sys/kernel/debug
+
+ if [ ! -e /sys/kernel/debug/selftest_helpers/test_fpu ]; then
+ echo "$0: Error mounting debugfs"
+ exit 4
+ fi
+fi
+
+echo "Running 1000 iterations on all CPUs... "
+for i in $(seq 1 1000); do
+ for c in $(seq 1 $NR_CPUS); do
+ ./test_fpu &
+ done
+done
+
+rmmod test_fpu
diff --git a/tools/testing/selftests/fpu/test_fpu.c b/tools/testing/selftests/fpu/test_fpu.c
new file mode 100644
index 000000000000..200238522a9d
--- /dev/null
+++ b/tools/testing/selftests/fpu/test_fpu.c
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0+
+/* This testcase operates with the test_fpu kernel driver.
+ * It modifies the FPU control register in user mode and calls the kernel
+ * module to perform floating point operations in the kernel. The control
+ * register value should be independent between kernel and user mode.
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <errno.h>
+#include <string.h>
+#include <fenv.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+const char *test_fpu_path = "/sys/kernel/debug/selftest_helpers/test_fpu";
+
+int main(void)
+{
+ char dummy[1];
+ int fd = open(test_fpu_path, O_RDONLY);
+
+ if (fd < 0) {
+ printf("[SKIP]\tcan't access %s: %s\n",
+ test_fpu_path, strerror(errno));
+ return 0;
+ }
+
+ if (read(fd, dummy, 1) < 0) {
+ printf("[FAIL]\taccess with default rounding mode failed\n");
+ return 1;
+ }
+
+ fesetround(FE_DOWNWARD);
+ if (read(fd, dummy, 1) < 0) {
+ printf("[FAIL]\taccess with downward rounding mode failed\n");
+ return 2;
+ }
+ if (fegetround() != FE_DOWNWARD) {
+ printf("[FAIL]\tusermode rounding mode clobbered\n");
+ return 3;
+ }
+
+ /* Note: the tests up to this point are quite safe and will only return
+ * an error. But the exception mask setting can cause misbehaving kernel
+ * to crash.
+ */
+ feclearexcept(FE_ALL_EXCEPT);
+ feenableexcept(FE_ALL_EXCEPT);
+ if (read(fd, dummy, 1) < 0) {
+ printf("[FAIL]\taccess with fpu exceptions unmasked failed\n");
+ return 4;
+ }
+ if (fegetexcept() != FE_ALL_EXCEPT) {
+ printf("[FAIL]\tusermode fpu exception mask clobbered\n");
+ return 5;
+ }
+
+ printf("[OK]\ttest_fpu\n");
+ return 0;
+}
--
2.21.0
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