linux-kernel - Re: [PATCH v2 10/11] x86/alternatives: Simplify ALTERNATIVE

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Message-ID: <20230907111624.GB29900@noisy.programming.kicks-ass.net>
Date:   Thu, 7 Sep 2023 13:16:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, David.Kaplan@....com,
        Andrew.Cooper3@...rix.com, jpoimboe@...nel.org,
        gregkh@...uxfoundation.org, nik.borisov@...e.com
Subject: Re: [PATCH v2 10/11] x86/alternatives: Simplify ALTERNATIVE_n()

On Thu, Sep 07, 2023 at 01:11:00PM +0200, Peter Zijlstra wrote:
> On Thu, Sep 07, 2023 at 01:09:17PM +0200, Peter Zijlstra wrote:
> 
> > Anyway, the 1,3,2 variant spelled out reads like:
> > 
> > #APP
> > # 1563 "../arch/x86/kernel/alternative.c" 1
> > # ALT: oldnstr
> > 661:
> > # ALT: oldnstr
> > 661:
> > push %rbp
> > 662:
> > # ALT: padding
> > .skip -(((665f-664f)-(662b-661b)) > 0) * ((665f-664f)-(662b-661b)),0x90
> > 
> >  #   Which evaluates like:
> >  #     665f-664f = 3
> >  #     662b-661b = 1
> >  #     3-1 > 0 = -1
> >  #     --1 * (3-1) = 2
> >  #
> >  #   so two single byte nops get emitted here.
> > 
> > 663:
> > .pushsection .altinstructions,"a"
> > .long 661b - .
> > .long 664f - .
> > .4byte ( 3*32+21)
> > .byte 663b-661b
> > .byte 665f-664f
> > .popsection
> > .pushsection .altinstr_replacement, "ax"
> > # ALT: replacement
> > 664:
> > mov %rsp,%rbp
> > 665:
> > .popsection
> > 
> > 662:
> > # ALT: padding
> > .skip -(((665f-664f)-(662b-661b)) > 0) * ((665f-664f)-(662b-661b)),0x90
> > 
> >  #   And this evaluates to:
> >  #     665f-664f = 2
> >  #     662b-661b = 3 (because it includes the original 1 byte instruction and 2 bytes padding)
> >  #     3-1 > 0 = 0
> >  #     0 * (3-1) = 0
> 
> copy-paste fail, that needs to read:
> 
> 	3-3 > 0 = 0
> 	0 * (3-3) = 0

I'm a moron ofcourse:

	2-3

> 
> >  #
> >  #   so no extra padding
> > 
> > 663:
> > .pushsection .altinstructions,"a"
> > .long 661b - .
> > .long 664f - .
> > .4byte ( 3*32+21)
> > .byte 663b-661b
> > .byte 665f-664f
> > .popsection
> > .pushsection .altinstr_replacement, "ax"
> > # ALT: replacement
> > 664:
> > push %r12
> > 665:
> > .popsection
> > 
> > # 0 "" 2
> > # ../arch/x86/kernel/alternative.c:1569:        int3_selftest();
> > #NO_APP