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Re: [PATCH] x86: Implement __WARN using UD0

From hpa@zytor.com
Newsgroups linux.kernel
Subject Re: [PATCH] x86: Implement __WARN using UD0
Date 2017-02-23 16:40 +0100
Message-ID <te3ES-38C-7@gated-at.bofh.it> (permalink)
References <te1D4-1LK-9@gated-at.bofh.it> <te2pr-2oI-5@gated-at.bofh.it> <te3lw-323-19@gated-at.bofh.it> <te3vb-35q-3@gated-at.bofh.it>
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On February 23, 2017 7:23:09 AM PST, Peter Zijlstra <peterz@infradead.org> wrote:
>On Thu, Feb 23, 2017 at 07:09:05AM -0800, hpa@zytor.com wrote:
>> Well, it only matters if the instruction extends past a segment
>> boundary or page.  However, the CPU instruction decoder will consume
>a
>> modrm for UD1, and so using just the two opcode bytes may cause a #PF
>> or #GP when a #UD was intended.
>
>It also matters if you want the decoded instruction stream to make
>sense.
>
>If for instance I use UD1 without the ModRM byte for WARN, objtool gets
>mighty confused because the instruction stream doesn't decode properly.
>
>objtool will also consume the extra byte and then the next instruction
>is offset and decodes wrong and it stresses out.
>
>Similarly, if you were to do objdump (and objdump were to actually
>correctly decode UD1) then the resulting asm would make no sense.
>
>The kernel will work 'fine', because even without ModRM it will #UD,
>and
>the #UD handler will IP+=2 and all is well, but it becomes impossible
>to
>actually decode the function..

Well, once you are using invalid instructions, it depends not on what the CPU decodes but what your own handler expects.  Consider Microsoft's use of C4 C4 /ib as a meta-instruction (called BOP, "BIOS operation")... that format has nothing to do with the CPU, but if you want to disassemble the resulting code you need to know about how they encode BOP.

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

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Thread

[PATCH] x86: Implement __WARN using UD0 Peter Zijlstra <peterz@infradead.org> - 2017-02-23 14:30 +0100
  Re: [PATCH] x86: Implement __WARN using UD0 Arjan van de Ven <arjan@linux.intel.com> - 2017-02-23 15:20 +0100
    Re: [PATCH] x86: Implement __WARN using UD0 Peter Zijlstra <peterz@infradead.org> - 2017-02-23 16:00 +0100
      Re: [PATCH] x86: Implement __WARN using UD0 Ingo Molnar <mingo@kernel.org> - 2017-02-24 09:00 +0100
        Re: [PATCH] x86: Implement __WARN using UD0 Peter Zijlstra <peterz@infradead.org> - 2017-02-24 09:40 +0100
          Re: [PATCH] x86: Implement __WARN using UD0 Ingo Molnar <mingo@kernel.org> - 2017-02-24 10:20 +0100
            Re: [PATCH] x86: Implement __WARN using UD0 Borislav Petkov <bp@alien8.de> - 2017-02-24 10:50 +0100
              Re: [PATCH] x86: Implement __WARN using UD0 "H. Peter Anvin" <hpa@zytor.com> - 2017-02-24 12:50 +0100
            Re: [PATCH] x86: Implement __WARN using UD0 Peter Zijlstra <peterz@infradead.org> - 2017-02-24 12:50 +0100
              Re: [PATCH] x86: Implement __WARN using UD0 Borislav Petkov <bp@alien8.de> - 2017-02-25 11:50 +0100
                Re: [PATCH] x86: Implement __WARN using UD0 hpa@zytor.com - 2017-02-25 19:00 +0100
                Re: [PATCH] x86: Implement __WARN using UD0 Borislav Petkov <bp@alien8.de> - 2017-02-25 20:40 +0100
                Re: [PATCH] x86: Implement __WARN using UD0 Borislav Petkov <bp@alien8.de> - 2017-02-25 21:40 +0100
                Re: [PATCH] x86: Implement __WARN using UD0 hpa@zytor.com - 2017-02-25 21:50 +0100
          Re: [PATCH] x86: Implement __WARN using UD0 hpa@zytor.com - 2017-02-24 10:20 +0100
          Re: [PATCH] x86: Implement __WARN using UD0 hpa@zytor.com - 2017-02-24 10:50 +0100
  Re: [PATCH] x86: Implement __WARN using UD0 Peter Zijlstra <peterz@infradead.org> - 2017-02-23 15:20 +0100
    Re: [PATCH] x86: Implement __WARN using UD0 Peter Zijlstra <peterz@infradead.org> - 2017-02-23 16:00 +0100
    Re: [PATCH] x86: Implement __WARN using UD0 hpa@zytor.com - 2017-02-23 16:20 +0100
      Re: [PATCH] x86: Implement __WARN using UD0 Peter Zijlstra <peterz@infradead.org> - 2017-02-23 16:30 +0100
        Re: [PATCH] x86: Implement __WARN using UD0 hpa@zytor.com - 2017-02-23 16:40 +0100
          Re: [PATCH] x86: Implement __WARN using UD0 Borislav Petkov <bp@alien8.de> - 2017-02-23 17:10 +0100
  Re: [PATCH] x86: Implement __WARN using UD0 Josh Poimboeuf <jpoimboe@redhat.com> - 2017-02-23 15:20 +0100
    Re: [PATCH] x86: Implement __WARN using UD0 Borislav Petkov <bp@alien8.de> - 2017-02-23 15:30 +0100
    Re: [PATCH] x86: Implement __WARN using UD0 Peter Zijlstra <peterz@infradead.org> - 2017-02-23 15:40 +0100
  [PATCH -v2] x86: Implement __WARN using UD0 Peter Zijlstra <peterz@infradead.org> - 2017-02-24 12:20 +0100
    [RFC][PATCH] bug: Add _ONCE logic to report_bug() Peter Zijlstra <peterz@infradead.org> - 2017-02-25 10:10 +0100
      Re: [RFC][PATCH] bug: Add _ONCE logic to report_bug() Ingo Molnar <mingo@kernel.org> - 2017-02-25 10:20 +0100
        Re: [RFC][PATCH] bug: Add _ONCE logic to report_bug() Peter Zijlstra <peterz@infradead.org> - 2017-02-25 11:50 +0100

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