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

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Message-ID: <YFmdJlESrCh4iC9A@hirez.programming.kicks-ass.net>
Date:   Tue, 23 Mar 2021 08:47:50 +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>,
        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 05:29:21PM -0400, Steven Rostedt wrote:
> On Mon, 22 Mar 2021 22:18:17 +0100
> Arnd Bergmann <arnd@...nel.org> wrote:
> 
> > 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.
> 
> Static calls (and so do tracepoints) currently rely on these kind of
> "undefined behavior" in C. This isn't the only UB that it relies on.

Right, most of the kernel lives in UB. That's what we have -fwrapv
-fno-strict-aliassing and lots of other bits for, to 'fix' the stupid C
standard.

This is one more of them, so just ignore the warning and make it go
away:

 -Wno-cast-function-type

seems to be the magic knob.