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

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Date:   Wed, 24 Mar 2021 13:46:06 +0100
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH] static_call: fix function type mismatch

On 23/03/2021 08.47, Peter Zijlstra wrote:
> 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.
> 

That can be done for now, but I think something has to be done if CFI is
to become a thing.

Sami, what happens if you try to boot a
CONFIG_CFI_CLANG+CONFIG_PREEMPT_DYNAMIC kernel?

Rasmus