linux-kernel - Re: [PATCH v5 5/6] kprobes: Use text_alloc() and text

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Date:   Tue, 18 Aug 2020 19:30:33 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Andi Kleen <ak@...ux.intel.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jessica Yu <jeyu@...nel.org>, Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH v5 5/6] kprobes: Use text_alloc() and text_free()

On Tue, Aug 18, 2020 at 02:51:41PM +0300, Mike Rapoport wrote:
> On Tue, Aug 18, 2020 at 08:30:29AM +0300, Jarkko Sakkinen wrote:
> > On Sun, Jul 26, 2020 at 11:14:08AM +0300, Mike Rapoport wrote:
> > > > 
> > > > 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.
> > > 
> > > 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
> 
> I don't think that CONFIG_ARCH_HAS_TEXT_ALLOC will work for all
> architectures.
> 
> If an architecture has different restrictions for allocation of text
> for, say, modules, kprobes, bfp, it won't be able to use a single
> ARCH_HAS_TEXT_ALLOC. Which means that this architecture is stuck with
> dependency of kprobes on MODULES until the next rework.

I was thinking to name it as CONFIG_HAS_KPROBES_ALLOC_PAGE, alogn the
lines described below, so it is merely a glitch in my example.

> 
> > 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
> 

> -- 
> Sincerely yours,
> Mike.

BR,
/Jarkko