linux-kernel - Re: Floating point usage inside kernel

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Date:	Sun, 27 Nov 2011 20:00:58 -0800
From:	Andy Lutomirski <luto@....EDU>
To:	Nuno Santos <nsantos@...gma.com>
CC:	Jiri Slaby <jirislaby@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: Floating point usage inside kernel

On 11/25/2011 08:16 AM, Nuno Santos wrote:
> Hi Jiri,
> 
> Thanks for your reply.
> 
> The algorithm is a matrix transform computation to make a affine a
> geometric transform.
> 
> Basicly it is based on the following functions:
> 
> http://pastebin.com/YHzYuLPU
> 
> Curiously I ran a test before writing this email and guest what.... is
> working!! :)
> 
> But now that I have opened this question here I should ask... is it safe?

[...]

> I'm not protecting the call to the function with kernel_fpu_begin and
> kernel_fpu_end

It's not safe even if you do protect it with kernel_fpu_begin.  On
x86-64, a userspace process that mucks with MXCSR can trivially cause
you to oops even if you use kernel_fpu_begin.  (I'm pretty sure x86-32
has a similar issue, and I imagine that other architectures have related
issues.)  This might change someday, but there are exactly zero use
cases outside staging, so don't hold your breath.

--Andy
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