Re: [PATCH v2 10/20] x86: avoid W^X being broken during modules loading

Borislav Petkov
2019-03-07T17:06:30Z

On Thu, Mar 07, 2019 at 08:53:34AM -0800, hpa@zytor.com wrote:
> If we *do*, what is the issue here? Although boot_cpu_has() isn't
> slow (it should in general be possible to reduce to one testb
> instruction followed by a conditional jump) it seems that "avoiding an
> alternatives slot" *should* be a *very* weak reason, and seems to me
> to look like papering over some other problem.

Forget the current thread: this is simply trying to document when to use
static_cpu_has() and when to use boot_cpu_has(). I get asked about it at
least once a month.

And then it is replacing clear slow paths using static_cpu_has() with
boot_cpu_has() because there's purely no need to patch there. And having
a RIP-relative MOV and a JMP is good enough for slow paths.

Makes sense?

--
Regards/Gruss,
Boris.

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