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

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Message-ID: <YFmaXxwcAb9bPchv@hirez.programming.kicks-ass.net>
Date:   Tue, 23 Mar 2021 08:35:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnd Bergmann <arnd@...nel.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 10:18:17PM +0100, Arnd Bergmann wrote:

> > 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.

Then please kill the warning, it's bullshit.

> > 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?

No, it's in Linus' tree, see commit:

  c8e2fe13d1d1 ("x86/perf: Use RET0 as default for guest_get_msrs to handle "no PMU" case")