selftests/fpu: Add an FPU selftest

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 (amluto).
  - Document stuff so that we don't forget.
  - Fix:
     ld: lib/test_fpu.o: in function `test_fpu_get':
     >> test_fpu.c:(.text+0x16e): undefined reference to `__sanitizer_cov_trace_cmpd'
     >> ld: test_fpu.c:(.text+0x1a7): undefined reference to `__sanitizer_cov_trace_cmpd'
     ld: test_fpu.c:(.text+0x1e0): undefined reference to `__sanitizer_cov_trace_cmpd'
  ]

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Petteri Aimonen <jpa@git.mail.kapsi.fi>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/20200624114646.28953-3-bp@alien8.de

1 files changed, 24 insertions, 0 deletions

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/