linux-kernel - Re: violating function pointer signature

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Message-ID: <20201118185540.GL2672@gate.crashing.org>
Date:   Wed, 18 Nov 2020 12:55:40 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Florian Weimer <fw@...eb.enyo.de>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Matt Mullins <mmullins@...x.us>,
        Ingo Molnar <mingo@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-toolchains@...r.kernel.org
Subject: Re: violating function pointer signature

On Wed, Nov 18, 2020 at 07:31:50PM +0100, Florian Weimer wrote:
> * Segher Boessenkool:
> 
> > On Wed, Nov 18, 2020 at 12:17:30PM -0500, Steven Rostedt wrote:
> >> I could change the stub from (void) to () if that would be better.
> >
> > Don't?  In a function definition they mean exactly the same thing (and
> > the kernel uses (void) everywhere else, which many people find clearer).
> 
> And I think () functions expected a caller-provided parameter save
> area on powerpc64le, while (void) functions do not.

Like I said (but you cut off, didn't realise it matters I guess):

> > In a function declaration that is not part of a definition it means no
> > information about the arguments is specified, a quite different thing.

Since the caller does not know if the callee will need a save area, it
has to assume it does.  Similar is true for many ABIs.

> It does not
> matter for an empty function, but GCC prefers to use the parameter
> save area instead of setting up a stack frame if it is present.  So
> you get stack corruption if you call a () function as a (void)
> function.  (The other way round is fine.)

If you have no prototype for a function, you have to assume worst case,
yes.  Calling things "a () function" can mean two things (a declaration
that is or isn't a definition, two very different things), so it helps
to be explicit about it.

Just use (void) and do not worry :-)


Segher