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From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 14 Aug 2015 12:14:17 -0700
Message-ID: <CALCETrWRhq3T0Gdu0O-hYGzcrBXKZ=KKmA3Vd2YWV6ra5RccTg@mail.gmail.com>
Subject: Re: [GIT PULL] x86 fixes
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@kernel.org>, Juergen Gross <jgross@suse.com>,
        Andy Lutomirski <luto@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Brian Gerst <brgerst@gmail.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>
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On Fri, Aug 14, 2015 at 11:57 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Aug 14, 2015 at 11:46 AM, Andy Lutomirski <luto@amacapital.net> wrote:
>>
>> I think it's only slightly broken.
>>
>> This bit:
>>
>>         if ((FPU_CS & 4) != 4) {    /* Must be in the LDT */
>>             /* Can only handle segmented addressing via the LDT
>>                for now, and it must be 16 bit */
>>             printk("FPU emulator: Unsupported addressing mode\n");
>>             math_abort(FPU_info, SIGILL);
>>         }
>>
>>         code_descriptor = FPU_get_ldt_descriptor(FPU_CS);
>>
>> is buggy, but no buggier than the old code.
>
> That code seems fine to me (and explicitly errors out when it's not in
> the LDT). FPU_CS is actually the CS selector value.
>
> So testing that for being in the LDT by checking bit #2, and then
> using FPU_get_ldt_descriptor() on it actually seems *correct*.
>

By "buggy" I meant that it aborted if it was in the GDT but wasn't
flat.  This'll break if anyone does an emulated FP op on TLS data.

--Andy