linux-kernel - Re: [PATCH] string.h: Mark 34 functions with __must

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Message-ID: <20191009135522.GA20194@kadam>
Date:   Wed, 9 Oct 2019 16:56:33 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Markus Elfring <Markus.Elfring@....de>,
        kernel-janitors@...r.kernel.org,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Joe Perches <joe@...ches.com>,
        Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] string.h: Mark 34 functions with __must_check

[ I haven't reviewed the original patch ]

On Wed, Oct 09, 2019 at 03:26:18PM +0200, Rasmus Villemoes wrote:
> On 09/10/2019 14.14, Markus Elfring wrote:
> > From: Markus Elfring <elfring@...rs.sourceforge.net>
> > Date: Wed, 9 Oct 2019 13:53:59 +0200
> > 
> > Several functions return values with which useful data processing
> > should be performed. These values must not be ignored then.
> > Thus use the annotation “__must_check” in the shown function declarations.
> 
> This _might_ make sense for those that are basically kmalloc() wrappers
> in one way or another [1]. But what's the point of annotating pure
> functions such as strchr, strstr, memchr etc? Nobody is calling those
> for their side effects (they don't have any...), so obviously the return
> value is used. If somebody does a strcmp() without using the result, so
> what? OK, it's odd code that might be worth flagging, but I don't think
> that's the kind of thing one accidentally adds.


	if (ret) {
		-EINVAL;
	}

People do occasionally make mistakes like this.  It can't hurt to
warn them as early as possible about nonsense code.


> You're also not consistent - strlen() is not annotated. And, for the
> standard C functions, -Wall already seems to warn about an unused
> call:
> 
>  #include <string.h>
> int f(const char *s)
> {
> 	strlen(s);
> 	return 3;
> }
> $ gcc -Wall -o a.o -c a.c
> a.c: In function ‘f’:
> a.c:5:2: warning: statement with no effect [-Wunused-value]
>   strlen(s);
>   ^~~~~~~~~

That's because glibc strlen is annotated with __attribute_pure__ which
means it has no side effects.

regards,
dan carpenter