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

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Date:   Mon, 22 Mar 2021 22:18:17 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.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 Mon, Mar 22, 2021 at 9:47 PM Peter Zijlstra <peterz@...radead.org> wrote:
> 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?

I've done a little testing on the replacement patch now, will send in a bit.

> 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.

I think the code works correctly on all architectures we support because
both 'int' and 'long' are returned in a register with any unused bits cleared.
It is however undefined behavior in C because 'int' and 'long' are not
compatible types, and the calling conventions don't have to allow this.

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

I've only tested my series with 5.12-rc so far, but don't get any other
such warnings. Maybe it's in linux-next?

          Arnd