Subject: Re: [RFC PATCH 6/7] execmem: add support for cache of large ROX pages

From: Mike Rapoport <[email-masked]>
Date: Thu, 18 Apr 2024 13:23:55 +0300

On Tue, Apr 16, 2024 at 09:52:34AM +0200, Peter Zijlstra wrote:
> On Mon, Apr 15, 2024 at 08:00:26PM +0300, Mike Rapoport wrote:
> > On Mon, Apr 15, 2024 at 12:47:50PM +0200, Peter Zijlstra wrote:
> > > On Thu, Apr 11, 2024 at 07:05:25PM +0300, Mike Rapoport wrote:
> > > 
> > > > To populate the cache, a writable large page is allocated from vmalloc with
> > > > VM_ALLOW_HUGE_VMAP, filled with invalid instructions and then remapped as
> > > > ROX.
> > > 
> > > > +static void execmem_invalidate(void *ptr, size_t size, bool writable)
> > > > +{
> > > > +	if (execmem_info->invalidate)
> > > > +		execmem_info->invalidate(ptr, size, writable);
> > > > +	else
> > > > +		memset(ptr, 0, size);
> > > > +}
> > > 
> > > +static void execmem_invalidate(void *ptr, size_t size, bool writeable)
> > > +{
> > > +       /* fill memory with INT3 instructions */
> > > +       if (writeable)
> > > +               memset(ptr, 0xcc, size);
> > > +       else
> > > +               text_poke_set(ptr, 0xcc, size);
> > > +}
> > > 
> > > Thing is, 0xcc (aka INT3_INSN_OPCODE) is not an invalid instruction.
> > > It raises #BP not #UD.
> > 
> > Do you mean that _invalidate is a poor name choice or that it's necessary
> > to use an instruction that raises #UD?
> 
> Poor naming, mostly. #BP handler will still scream bloody murder if the
> site is otherwise unclaimed.
> 
> It just isn't an invalid instruction.

Well, execmem_fill_with_insns_screaming_bloody_murder seems too long, how
about execmem_fill_trapping_insns?

-- 
Sincerely yours,
Mike.