Re: [PATCH v5 5/6] kprobes: Use text_alloc() and text_free()

[Jarkko Sakkinen]

On Sun, Jul 26, 2020 at 11:14:08AM +0300, Mike Rapoport wrote:
> On Sat, Jul 25, 2020 at 06:16:48AM +0300, Jarkko Sakkinen wrote:
> > > I've read the observations in the other threads, but this #ifdef 
> > > jungle is silly, it's a de-facto open coded text_alloc() with a 
> > > module_alloc() fallback...
> > 
> > In the previous version I had:
> > 
> >   https://lore.kernel.org/lkml/20200717030422.679972-4-jarkko.sakkinen@xxxxxxxxxxxxxxx/
> > 
> > and I had just calls to text_alloc() and text_free() in corresponding
> > snippet to the above.
> > 
> > I got this feedback from Mike:
> > 
> >   https://lore.kernel.org/lkml/20200718162359.GA2919062@xxxxxxxxxx/
> > 
> > I'm not still sure that I fully understand this feedback as I don't see
> > any inherent and obvious difference to the v4. In that version fallbacks
> > are to module_alloc() and module_memfree() and text_alloc() and
> > text_memfree() can be overridden by arch.
> 
> Let me try to elaborate.
> 
> There are several subsystems that need to allocate memory for executable
> text. As it happens, they use module_alloc() with some abilities for
> architectures to override this behaviour.
> 
> For many architectures, it would be enough to rename modules_alloc() to
> text_alloc(), make it built-in and this way allow removing dependency on
> MODULES.
> 
> Yet, some architectures have different restrictions for code allocation
> for different subsystems so it would make sense to have more than one
> variant of text_alloc() and a single config option ARCH_HAS_TEXT_ALLOC
> won't be sufficient.
> 
> I liked Mark's suggestion to have text_alloc_<something>() and proposed
> a way to introduce text_alloc_kprobes() along with
> HAVE_KPROBES_TEXT_ALLOC to enable arch overrides of this function.
> 
> The major difference between your v4 and my suggestion is that I'm not
> trying to impose a single ARCH_HAS_TEXT_ALLOC as an alternative to
> MODULES but rather to use per subsystem config option, e.g.
> HAVE_KPROBES_TEXT_ALLOC.
> 
> Another thing, which might be worth doing regardless of the outcome of
> this discussion is to rename alloc_insn_pages() to text_alloc_kprobes()
> because the former is way too generic and does not emphasize that the 
> instruction page is actually used by kprobes only.

What if we in kernel/kprobes.c just:

#ifndef CONFIG_ARCH_HAS_TEXT_ALLOC
void __weak *alloc_insn_page(void)
{
	return module_alloc(PAGE_SIZE);
}

void __weak free_insn_page(void *page)
{
	module_memfree(page);
}
#endif

In Kconfig (as in v5):

config KPROBES
	bool "Kprobes"
	depends on MODULES || ARCH_HAS_TEXT_ALLOC

I checked architectures that override alloc_insn_page(). With the
exception of x86, they do not call module_alloc().

If no rename was done, then with this approach a more consistent.
config flag name would be CONFIG_ARCH_HAS_ALLOC_INSN_PAGE.

I'd call the function just as kprobes_alloc_page(). Then the
config flag would become CONFIG_HAS_KPROBES_ALLOC_PAGE.

> -- 
> Sincerely yours,
> Mike.

Thanks for the feedback!

/Jarkko