linux-kernel - Re: [linus:master] BUILD REGRESSION a2e5790d841658485d642196dbb0927303d6c22f

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Message-ID: <20180208094651.GA7964@pd.tnic>
Date:   Thu, 8 Feb 2018 10:46:51 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        kbuild test robot <fengguang.wu@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>
Subject: Re: [linus:master] BUILD REGRESSION
 a2e5790d841658485d642196dbb0927303d6c22f

On Thu, Feb 08, 2018 at 10:13:02AM +0100, Peter Zijlstra wrote:
> diff --git a/arch/x86/include/asm/bug.h b/arch/x86/include/asm/bug.h
> index 34d99af43994..f0d5b4a1512d 100644
> --- a/arch/x86/include/asm/bug.h
> +++ b/arch/x86/include/asm/bug.h
> @@ -12,16 +12,21 @@
>   * (binutils knows about "ud1" but {en,de}codes it as 2 bytes, whereas
>   *  our kernel decoder thinks it takes a ModRM byte, which seems consistent
>   *  with various things like the Intel SDM instruction encoding rules)
> + *
> + * And now someone went and taught binutils about UD0 as taking a ModR/M too
> + * and it generates crap when disassembling the kernel. Stick a pointless 0xD6
> + * ModR/M on, which the old binutils decodes as (bad) and the new binutils sees
> + * as a valid single byte ModR/M.
>   */
>  
> -#define ASM_UD0		".byte 0x0f, 0xff"
> +#define ASM_UD0		".byte 0x0f, 0xff, 0xd6"

Yeah, we probably should say that it was SALC:

http://ref.x86asm.net/geek.html#xD6

but undefined after P4.

Not that it matters a whole lot after an UD0. And it still won't matter
if it gets redefined to some new fancy prefix.

-- 
Regards/Gruss,
    Boris.

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