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

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Date:   Wed, 24 Mar 2021 09:01:27 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        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 Wed, Mar 24, 2021 at 5:46 AM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
>
> 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?

Seems to boot just fine. CFI instrumentation is only for
compiler-generated indirect calls. Casting functions to different
types is fine as long as you don't end up calling them using an
incorrect pointer type.

Sami