linux-kernel - Re: [PATCH 11/15] static_call: Add inline static call infrastructure

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Message-ID: <20191001120023.GQ4519@hirez.programming.kicks-ass.net>
Date:   Tue, 1 Oct 2019 14:00:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Nadav Amit <namit@...are.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jason Baron <jbaron@...mai.com>, Jiri Kosina <jkosina@...e.cz>,
        David Laight <David.Laight@...LAB.COM>,
        Borislav Petkov <bp@...en8.de>,
        Julia Cartwright <julia@...com>, Jessica Yu <jeyu@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Edward Cree <ecree@...arflare.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH 11/15] static_call: Add inline static call infrastructure

On Mon, Jun 10, 2019 at 12:19:29PM -0500, Josh Poimboeuf wrote:
> On Fri, Jun 07, 2019 at 10:37:56AM +0200, Peter Zijlstra wrote:
> > > > +}
> > > > +
> > > > +static int static_call_module_notify(struct notifier_block *nb,
> > > > +				     unsigned long val, void *data)
> > > > +{
> > > > +	struct module *mod = data;
> > > > +	int ret = 0;
> > > > +
> > > > +	cpus_read_lock();
> > > > +	static_call_lock();
> > > > +
> > > > +	switch (val) {
> > > > +	case MODULE_STATE_COMING:
> > > > +		module_disable_ro(mod);
> > > > +		ret = static_call_add_module(mod);
> > > > +		module_enable_ro(mod, false);
> > > 
> > > Doesn’t it cause some pages to be W+X ?
> 
> How so?

This is after complete_formation() which does RO,X. If we then disable
RO we end up with W+X pages, which is bad.

That said, alternatives, ftrace, dynamic_debug all run before
complete_formation() specifically such that they can directly poke text.

Possibly we should add a notifier callback for MODULE_STATE_UNFORMED,
but that is for another day.

> >> Can it be avoided?
> > 
> > I don't know why it does this, jump_labels doesn't seem to need this,
> > and I'm not seeing what static_call needs differently.
> 
> I forgot why I did this, but it's probably for the case where there's a
> static call site in module init code.  It deserves a comment.
> 
> Theoretically, jump labels need this to.
> 
> BTW, there's a change coming that will require the text_mutex before
> calling module_{disable,enable}_ro().

I can't find why it would need this (and I'm going to remove it).
Specifically complete_formation() does enable_ro(.after_init=false),
which leaves .ro_after_init writable so
{jump_label,static_call}_sort_entries() will work.

But both jump_label and static_call then use the full text_poke(), not
text_poke_early(), for modules.