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

[Borislav Petkov]

On Thu, Sep 07, 2023 at 01:09:17PM +0200, Peter Zijlstra wrote:
> If you flip the 3 and 2 byte instructions the result is the same. No
> extra padding.
>
> And no, I had not actually tested this before, because clearly this is
> all obvious ;-)

IKR.

So I take that extra padding thing back - we actually *must* have that
padding so that it actually works correctly. I just did a silly example
but nothing says one cannot do one like that today:

		alternative_2("", "pop %%rax", X86_FEATURE_ALWAYS,
			      "call clear_page_orig", X86_FEATURE_ALWAYS);

An order of insns which grows in size: 0, then 1, then 5.

It turns into:

> # arch/x86/mm/init.c:163: 		alternative_2("", "pop %%rax", X86_FEATURE_ALWAYS,
> # 163 "arch/x86/mm/init.c" 1
> 	# ALT: oldnstr
> 661:
> 	# ALT: oldnstr
> 661:
> 	
> 662:
> # ALT: padding
> .skip -(((665f-664f)-(662b-661b)) > 0) * ((665f-664f)-(662b-661b)),0x90

IINM, this turns into:

.skip 1 * (1 - 0) = 1.

because "pop %rax" is one byte. The original insn is of size 0.

So we end up with a 0x90 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:
> 	pop %rax
>  665:
> .popsection
> 
> 662:

<--- X

> # ALT: padding
> .skip -(((665f-664f)-(662b-661b)) > 0) * ((665f-664f)-(662b-661b)),0x90

Now the second guy comes in. That turns into:

.skip 1 * (5 - 1) = 4

Because, IINM, the 662 label above is the *second* one at marker X (we
go backwards) and the 661 is the second one too.

So between those two labels you have the 0x90 - one byte padding from
the first .skip.

And now it adds 4 more bytes to accomodate the CALL.

So we need to have that padding back-to-back in case the second
replacement is longer.

Ok, I guess the only thing that's bothering me is:

>       # ALT: oldnstr
> 661:
>       # ALT: oldnstr 
> 661:

I'll keep on playing with this.

Thx.

-- 
Regards/Gruss,
    Boris.

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