linux-kernel - Re: [PATCH] ftrace: shut up -Wcast-function-type warning for ftrace_ops_no

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Message-ID: <YFmbHI4hnKg5UH2M@hirez.programming.kicks-ass.net>
Date:   Tue, 23 Mar 2021 08:39:08 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Tobias Klauser <tklauser@...tanz.ch>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Chengming Zhou <zhouchengming@...edance.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ftrace: shut up -Wcast-function-type warning for
 ftrace_ops_no_ops

On Mon, Mar 22, 2021 at 10:49:58PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> With 'make W=1', gcc warns about casts between incompatible function
> types:
> 
> kernel/trace/ftrace.c:128:31: error: cast between incompatible function types from 'void (*)(long unsigned int,  long unsigned int)' to 'void (*)(long unsigned int,  long unsigned int,  struct ftrace_ops *, struct ftrace_regs *)' [-Werror=cast-function-type]
>   128 | #define ftrace_ops_list_func ((ftrace_func_t)ftrace_ops_no_ops)
>       |                               ^
> 
> As the commet here explains, this one was intentional, so shut up the
> warning harder by using a double cast.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  kernel/trace/ftrace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 4d8e35575549..d8fc87a17421 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -125,7 +125,7 @@ static void ftrace_ops_list_func(unsigned long ip, unsigned long parent_ip,
>  #else
>  /* See comment below, where ftrace_ops_list_func is defined */
>  static void ftrace_ops_no_ops(unsigned long ip, unsigned long parent_ip);
> -#define ftrace_ops_list_func ((ftrace_func_t)ftrace_ops_no_ops)
> +#define ftrace_ops_list_func ((ftrace_func_t)(void *)ftrace_ops_no_ops)

So now we're making perfectly fine code worse, because GCC is stupid?

Please just kill the warning, like we do with so many other warnings.
It's broken!