Re: [PATCH] arch/x86: reset MXCSR to default in kernel_fpu_begin()

[Shuah Khan]

On 6/2/20 11:03 AM, Andy Lutomirski wrote:
> On Tue, Jun 2, 2020 at 3:56 AM Borislav Petkov <bp@xxxxxxxxx> wrote:
> > Hi,
> >
> > On Tue, Jun 02, 2020 at 01:29:51PM +0300, Petteri Aimonen wrote:
> > > The kernel module is not actually x86-specific, even though it is
> > > currently only enabled for x86. amdgpu driver already does kernel mode
> > > floating point operations on PPC64 also, and the same module could be
> > > used to test the same thing there.
> >
> > Then make it generic please and put the user portion in, say,
> > tools/testing/selftests/fpu/ and we can ask ppc people to test it too.
> > People might wanna add more stuff to it in the future, which would be
> > good.
> >
> > > To deterministically trigger the bug, the syscall has to come from the
> > > same thread that has modified MXCSR. Going through /usr/sbin/modprobe
> > > won't work, and manually doing the necessary syscalls for module loading
> > > seems too complicated.
> >
> > Ok, fair enough. But put that file in debugfs pls.
>
> I think I agree.  While it would be delightful to have general
> selftest tooling for kernel modules, we don't have that right now, and
> having the test just work with an appropriately configured kernel
> would be nice.

Let's extend it to do what we want it to do. I will happy to take
patches. If you have some concrete ideas on what we can add, please
do a short summary of what is missing. I will find a way to get this
done.

> How about putting the file you frob in
> /sys/kernel/debug/selftest_helpers/something_or_other.  The idea would
> be that /sys/kernel/debug/selftest_helpers would be a general place
> for kernel helpers needed to make selftests work.

Is this a workaround for the lack of selftest tooling for kernel
modules? In which case, let's us focus on fix selftest tooling.

thanks,
-- Shuah