linux-kernel - Re: [PATCH] static_call: fix function type mismatch

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Date:   Wed, 24 Mar 2021 16:40:04 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Arnd Bergmann <arnd@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] static_call: fix function type mismatch

On Wed, Mar 24, 2021 at 3:53 PM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
>
> On 24/03/2021 23.34, Sami Tolvanen wrote:
> > On Wed, Mar 24, 2021 at 2:51 PM Rasmus Villemoes
> > <linux@...musvillemoes.dk> wrote:
> >>
> >> On 24/03/2021 18.33, Peter Zijlstra wrote:
> >>> On Wed, Mar 24, 2021 at 05:45:52PM +0100, Rasmus Villemoes wrote:
> >>>> Sorry, I think I misread the code. The static calls are indeed
> >>>> initialized with a function with the right prototype. Try adding
> >>>> "preempt=full" on the command line so that we exercise these lines
> >>>>
> >>>>                static_call_update(cond_resched,
> >>>> (typeof(&__cond_resched)) __static_call_return0);
> >>>>                 static_call_update(might_resched,
> >>>> (typeof(&__cond_resched)) __static_call_return0);
> >>>>
> >>>> I would expect that to blow up, since we end up calling a long (*)(void)
> >>>> function using a function pointer of type int (*)(void).
> >>>
> >>> Note that on x86 there won't actually be any calling of function
> >>> pointers. See what arch/x86/kernel/static_call.c does :-)
> >>
> >> I know, but so far x86 is the only one with HAVE_STATIC_CALL, so for
> >> arm64 which is where CFI seems to be targeted initially, static_calls
> >> are function pointers. And unless CFI ignores the return type, I'd
> >> really expect the above to fail.
> >
> > I think you're correct, this would trip CFI without HAVE_STATIC_CALL.
> > However, arm64 also doesn't support PREEMPT_DYNAMIC at the moment, so
> > this isn't currently a problem there.
>
> Well, there's PREEMPT_DYNAMIC and HAVE_PREEMPT_DYNAMIC. The former
> doesn't depend on the latter (and the latter does depend on
> HAVE_STATIC_CALL, so effectively not for anything but x86). You should
> be able to select both PREEMPT_DYNAMIC and CFI_CLANG, and test if
> booting with preempt=full does give the fireworks one expects.

Actually, it looks like I can't select PREEMPT_DYNAMIC, and tweaking
Kconfig to force enable it on arm64 results in a build error
("implicit declaration of function 'static_call_mod'").

Sami