linux-kernel - Re: [PATCH 2/4] tools/nolibc: avoid unused parameter warnings for ENOSYS fallbacks

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Message-ID: <20230917094827.GA11081@1wt.eu>
Date:   Sun, 17 Sep 2023 11:48:27 +0200
From:   Willy Tarreau <w@....eu>
To:     Thomas Weißschuh <linux@...ssschuh.net>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 2/4] tools/nolibc: avoid unused parameter warnings for
 ENOSYS fallbacks

On Sun, Sep 17, 2023 at 07:49:57AM +0200, Thomas Weißschuh wrote:
> On 2023-09-17 04:58:51+0200, Willy Tarreau wrote:
> > On Thu, Sep 14, 2023 at 06:01:18PM +0200, Thomas Weißschuh wrote:
> > > The ENOSYS fallback code does not use its functions parameters.
> > > This can lead to compiler warnings about unused parameters.
> > > 
> > > Explicitly avoid these warnings.
> > 
> > Just out of curiosity, did you find a valid case for enabling this
> > warning or were you trying various combinations ? I'm asking because
> > I've never seen it enabled anywhere given that it's probably the most 
> > useless and unusable warning: as soon as you're dealing with function
> > pointers, you start to have multiple functions with a similar
> > prototype, some of which just don't need certain arguments, and the
> > only way to shut the warning is to significantly uglify the code.
> 
> nolibc-test uses it currently and I also used it in some projects.

OK then let's handle it.

> >   @@ -934,6 +960,11 @@ int sys_select(int nfds, fd_set *rfds, fd_set *wfds, fd_set *efds, struct timeva
> >    #endif
> >    	return my_syscall5(__NR__newselect, nfds, rfds, wfds, efds, timeout);
> >    #else
> >   +	return no_syscall5(nfds, rfds, wfds, efds, timeout);
> >   -	return -ENOSYS;
> >    #endif
> > 
> > What do you think ?
> 
> The idea sounds good. But "no_syscall5" sounds a bit non-obvious to me.

Of course, I was just trying to illustrate. I'm never good at giving
names.

> Maybe the macro-equivalent of this?
> 
> static inline int __nolibc_enosys(...)
> {
> 	return -ENOSYS;
> }
> 
> The only-vararg function unfortunately needs C23 so we can't use it.
>
> It's clear to the users that this is about ENOSYS and we don't need a
> bunch of new macros similar.

I like it, I didn't think about varargs, it's an excellent idea! Let's
just do simpler, start with a first arg "syscall_num" that we may later
reuse for debugging, and just mark this one unused:

  static inline int __nolibc_enosys(int syscall_num, ...)
  {
	(void)syscall_num;
  	return -ENOSYS;
  }

Willy