Messages in this thread | | | Date | Thu, 7 Sep 2023 17:06:32 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v2 10/11] x86/alternatives: Simplify ALTERNATIVE_n() |
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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.
https://people.kernel.org/tglx/notes-about-netiquette
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