Re: [PATCH v2 10/11] x86/alternatives: Simplify ALTERNATIVE_n()

[Peter Zijlstra]

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

>  #
>  #   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