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

[Edgecombe, Rick P]

On Thu, 2023-06-01 at 14:38 -0400, Kent Overstreet wrote:
> 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's only used and flushed locally, but it is accessible to all CPU's,
right?

> 
> > 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?

The way text_poke() is used here, it is creating a new writable alias
and flushing it for *each* write to the module (like for each write of
an individual relocation, etc). I was just thinking it might warrant
some batching or something.