linux-kernel - Re: [PATCH 12/13] x86/jitalloc: prepare to allocate exectuatble memory as ROX

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Date:   Thu, 1 Jun 2023 14:38:04 -0400
From:   Kent Overstreet <kent.overstreet@...ux.dev>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
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Subject: Re: [PATCH 12/13] x86/jitalloc: prepare to allocate exectuatble
 memory as ROX

On Thu, Jun 01, 2023 at 06:13:44PM +0000, Edgecombe, Rick P wrote:
> > text_poke() _does_ create a separate RW mapping.
> 
> Sorry, I meant a separate RW allocation.

Ah yes, that makes sense


> 
> > 
> > The thing that sucks about text_poke() is that it always does a full
> > TLB
> > flush, and AFAICT that's not remotely needed. What it really wants to
> > be
> > doing is conceptually just
> > 
> > kmap_local()
> > mempcy()
> > kunmap_loca()
> > flush_icache();
> > 
> > ...except that kmap_local() won't actually create a new mapping on
> > non-highmem architectures, so text_poke() open codes it.
> 
> Text poke creates only a local CPU RW mapping. It's more secure because
> other threads can't write to it.

*nod*, same as kmap_local

> It also only needs to flush the local core when it's done since it's
> not using a shared MM.
 
Ahh! Thanks for that; perhaps the comment in text_poke() about IPIs
could be a bit clearer.

What is it (if anything) you don't like about text_poke() then? It looks
like it's doing broadly similar things to kmap_local(), so should be
in the same ballpark from a performance POV?