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

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Message-ID: <20210322172921.56350a69@gandalf.local.home>
Date:   Mon, 22 Mar 2021 17:29:21 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.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, 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.

<shameless plug>
Perhaps this might make a good topic to talk about at the tool chain
microconference at Plumbers!
</shameless plug>

-- Steve