linux-kernel - Re: [PATCH] arch/x86: reset MXCSR to default in kernel_fpu

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Message-ID: <8b1f0bfa-79b0-74e4-0241-8b2a94491807@linuxfoundation.org>
Date:   Tue, 2 Jun 2020 11:27:08 -0600
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Shuah Khan <shuah@...nel.org>
Cc:     Petteri Aimonen <jpa@....mail.kapsi.fi>,
        Dave Hansen <dave.hansen@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>, skhan@...uxfoundation.org
Subject: Re: [PATCH] arch/x86: reset MXCSR to default in kernel_fpu_begin()

On 6/2/20 11:03 AM, Andy Lutomirski wrote:
> On Tue, Jun 2, 2020 at 3:56 AM Borislav Petkov <bp@...en8.de> 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