Re: [PATCH v10 03/15] x86/alternatives: Disable LASS when patching kernel alternatives

Borislav Petkov
2025-10-22T09:41:03Z

On Wed, Oct 22, 2025 at 10:25:41AM +0200, Peter Zijlstra wrote:
> Initially the suggestion was to use stac/clac directly iirc; but that
> looses the information these are for LASS only. Hence the LASS specific
> ones.

Yap.

> (its an unfortunate arch detail that LASS and SMAP both use the AC flag
> and all that)

That is an implementation detail and users of the interface shouldn't care.

> But that's not the same, stac() and clac() are FEATURE_SMAP, these are
> FEATURE_LASS.

So?

Are you thinking of toggling features and then something else getting disabled
in the process?

> If you really want the _disable _enable naming that's fine with me, but
> then perhaps we should also s/clac/smap_disable/ and s/stac/smap_enable/
> for consistency.

So the enable/disable thing is I think what makes this a lot more
understandable when you read it this way: "disable linear address separation
around this code". And that is regardless of how the underlying machinery does
that toggling of LASS.

As to stac/clac - I wouldn't touch them. They've been there forever so it'll
only be unnecessary churn.

Btw, if you need an example which already does that:

arch/x86/include/asm/uaccess.h:37:#define __uaccess_begin() stac()
arch/x86/include/asm/uaccess.h:38:#define __uaccess_end() clac()

So the lass_{enable,disable} will be yet another incarnation of this pattern.

Thx.

--
Regards/Gruss,
Boris.