Groups > linux.kernel > #1207872

Re: [GIT PULL] x86 fixes

From	Andy Lutomirski <luto@amacapital.net>
Newsgroups	linux.kernel
Subject	Re: [GIT PULL] x86 fixes
Date	2015-08-14 21:20 +0200
Message-ID	<pXsGe-2Nt-19@gated-at.bofh.it> (permalink)
References	<pXhrr-3w7-1@gated-at.bofh.it> <pXrTQ-1Dj-17@gated-at.bofh.it> <pXsdb-206-13@gated-at.bofh.it> <pXsmS-2bn-11@gated-at.bofh.it> <pXswx-2Ca-7@gated-at.bofh.it>
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On Fri, Aug 14, 2015 at 12:06 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Aug 14, 2015 at 11:57 AM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>>
>> 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*.
>>
>> It's the actual instruction data segment handling that looks entirely
>> broken, and was explicitly made *more* broken by that commit.
>
> Note that in practice, it's *probably* true that if CS ends up being
> in the LDT (so we're running something odd like Wine), then *probably*
> the data segments are going to be in the LDT too. So the old code that
> unconditionally looked things up in the LDT probably worked in
> practice, even if it was wrong.
>
> The new code cannot *possibly* work at all, because even if the data
> segment register is in the LDT, it uses the wrong thing to look up the
> LDT entry, so it will get the wrong base.
>
> But as mentioned, it will only *matter* on something like a 486SX, and
> only when the whole "CS/DS didn't match the default flat segments"
> case triggers, so not only do you have to run on a 486SX, you will
> have to run something like Wine on it. So it sounds very very unlikely
> that this bug matters in practice.

Unless I'm missing something, it's literally a one-line fix -- just
put the missing PM_REG_(segment) back in.

--Andy
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Thread

[GIT PULL] x86 fixes Ingo Molnar <mingo@kernel.org> - 2015-08-14 09:20 +0200
  Re: [GIT PULL] x86 fixes Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-14 20:30 +0200
    Re: [GIT PULL] x86 fixes Andy Lutomirski <luto@amacapital.net> - 2015-08-14 20:50 +0200
      Re: [GIT PULL] x86 fixes Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-14 21:00 +0200
        Re: [GIT PULL] x86 fixes Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-14 21:10 +0200
          Re: [GIT PULL] x86 fixes Andy Lutomirski <luto@amacapital.net> - 2015-08-14 21:20 +0200
            Re: [GIT PULL] x86 fixes Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-14 21:40 +0200
        Re: [GIT PULL] x86 fixes Andy Lutomirski <luto@amacapital.net> - 2015-08-14 21:20 +0200
    Re: [GIT PULL] x86 fixes Ingo Molnar <mingo@kernel.org> - 2015-08-17 10:10 +0200
      Re: [GIT PULL] x86 fixes Denys Vlasenko <dvlasenk@redhat.com> - 2015-08-17 13:00 +0200
        Re: [GIT PULL] x86 fixes Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-17 19:00 +0200
          Re: [GIT PULL] x86 fixes Ingo Molnar <mingo@kernel.org> - 2015-08-18 10:00 +0200
      Re: [GIT PULL] x86 fixes Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-17 18:50 +0200
        Re: [GIT PULL] x86 fixes "H. Peter Anvin" <hpa@zytor.com> - 2015-08-17 19:00 +0200
          Re: [GIT PULL] x86 fixes Linus Torvalds <torvalds@linux-foundation.org> - 2015-08-17 19:20 +0200
            Re: [GIT PULL] x86 fixes "H. Peter Anvin" <hpa@zytor.com> - 2015-08-18 00:20 +0200
              Re: [GIT PULL] x86 fixes Ingo Molnar <mingo@kernel.org> - 2015-08-19 08:00 +0200
                Re: [GIT PULL] x86 fixes Ingo Molnar <mingo@kernel.org> - 2015-08-19 08:20 +0200
                Re: [GIT PULL] x86 fixes Ingo Molnar <mingo@kernel.org> - 2015-08-19 09:00 +0200
                Re: [GIT PULL] x86 fixes "H. Peter Anvin" <hpa@zytor.com> - 2015-08-19 12:10 +0200
            Re: [GIT PULL] x86 fixes Bryan O'Donoghue <pure.logic@nexus-software.ie> - 2015-08-18 01:50 +0200
      Re: [GIT PULL] x86 fixes "H. Peter Anvin" <hpa@zytor.com> - 2015-08-17 23:10 +0200
      Re: [GIT PULL] x86 fixes Andy Lutomirski <luto@amacapital.net> - 2015-08-18 02:00 +0200
        Re: [GIT PULL] x86 fixes "H. Peter Anvin" <hpa@zytor.com> - 2015-08-18 02:10 +0200
        Re: [GIT PULL] x86 fixes "H. Peter Anvin" <hpa@zytor.com> - 2015-08-18 02:10 +0200
          Re: [GIT PULL] x86 fixes Andy Lutomirski <luto@amacapital.net> - 2015-08-18 02:20 +0200
            Re: [GIT PULL] x86 fixes "H. Peter Anvin" <hpa@zytor.com> - 2015-08-18 08:00 +0200
            Re: [GIT PULL] x86 fixes "H. Peter Anvin" <hpa@zytor.com> - 2015-08-18 08:10 +0200
        Re: [GIT PULL] x86 fixes Ingo Molnar <mingo@kernel.org> - 2015-08-18 10:00 +0200

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