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

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Date:   Mon, 22 Mar 2021 21:47:34 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     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>,
        Arnd Bergmann <arnd@...db.de>,
        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@...r.kernel.org
Subject: Re: [PATCH] static_call: fix function type mismatch

On Mon, Mar 22, 2021 at 03:32:14PM -0400, Steven Rostedt wrote:
> On Mon, 22 Mar 2021 18:06:37 +0100
> Arnd Bergmann <arnd@...nel.org> wrote:
> 
> > From: Arnd Bergmann <arnd@...db.de>
> > 
> > The __static_call_return0() function is declared to return a 'long',
> > while it aliases a couple of functions that all return 'int'. When
> > building with 'make W=1', gcc warns about this:
> > 
> > kernel/sched/core.c:5420:37: error: cast between incompatible function types from 'long int (*)(void)' to 'int (*)(void)' [-Werror=cast-function-type]
> >  5420 |   static_call_update(might_resched, (typeof(&__cond_resched)) __static_call_return0);
> > 
> > Change the function to return 'int' as well, but remove the cast to
> > ensure we get a warning if any of the types ever change.
> 
> I think the answer is the other way around. That is, to make the functions
> it references return long instead. __static_call_return0 is part of the
> dynamic call infrastructure. Perhaps it is currently only used by functions
> that return int, but what happens when it is used for a function that
> returns a pointer?

Steve is correct. Also, why is that warning correct? On x86 we return in
RAX, and using int will simply not inspect the upper 32 bits there.

And I'm fairly sure I had a pointer user somewhere recently.