linux-kernel - Re: [PATCH] signal: fix building with clang

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Date:   Thu, 7 Mar 2019 16:28:05 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-arch@...r.kernel.org, Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Christian Brauner <christian@...uner.io>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        linux-mips@...r.kernel.org
Subject: Re: [PATCH] signal: fix building with clang

On 03/07, Arnd Bergmann wrote:
>
> clang warns about the sigset_t manipulating functions (sigaddset, sigdelset,
> sigisemptyset, ...) because it performs semantic analysis before discarding
> dead code, unlike gcc that does this in the reverse order.
>
> The result is a long list of warnings like:
>
> In file included from arch/arm64/include/asm/ftrace.h:21:
> include/linux/compat.h:489:10: error: array index 3 is past the end of the array (which contains 2 elements) [-Werror,-Warray-bounds]
>         case 2: v.sig[3] = (set->sig[1] >> 32); v.sig[2] = set->sig[1];

stupid question... I have no idea if this can work or not, but may be we can just do

	--- x/Makefile
	+++ x/Makefile
	@@ -701,6 +701,7 @@ KBUILD_CPPFLAGS += $(call cc-option,-Qun
	 KBUILD_CFLAGS += $(call cc-disable-warning, format-invalid-specifier)
	 KBUILD_CFLAGS += $(call cc-disable-warning, gnu)
	 KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
	+KBUILD_CFLAGS += $(call cc-disable-warning, array-bounds)
	 # Quiet clang warning: comparison of unsigned expression < 0 is always false
	 KBUILD_CFLAGS += $(call cc-disable-warning, tautological-compare)
	 # CLANG uses a _MergedGlobals as optimization, but this breaks modpost, as the

?

> As a (rather ugly) workaround,

Yes :/

But I am not going to argue, just a couple of questions.

> I turn the nice switch()/case statements
> into preprocessor conditionals, and where that is not possible, use the
> '%' operator

I can't say what looks worse... to me it would be either use ifdef's or %'s
everywhere in signal.h, with this patch the code doesn't look consistent.
But I won't insist.


>  static inline int sigisemptyset(sigset_t *set)
>  {
> -	switch (_NSIG_WORDS) {
> -	case 4:
> -		return (set->sig[3] | set->sig[2] |
> -			set->sig[1] | set->sig[0]) == 0;
> -	case 2:
> -		return (set->sig[1] | set->sig[0]) == 0;
> -	case 1:
> -		return set->sig[0] == 0;
> -	default:
> -		BUILD_BUG();
> -		return 0;
> -	}
> +#if _NSIG_WORDS == 4
> +	return (set->sig[3] | set->sig[2] |
> +		set->sig[1] | set->sig[0]) == 0;
> +#elif _NSIG_WORDS == 2
> +	return (set->sig[1] | set->sig[0]) == 0;
> +#elif _NSIG_WORDS == 1
> +	return set->sig[0] == 0;
> +#else
> +	BUILD_BUG();
> +#endif
>  }

Or perhaps we can simply rewrite this and other helpers?

I don't think that, say,

	static inline int sigisemptyset(sigset_t *set)
	{
		for (i = 0; i < ARRAY_SIZE(set->sig); ++i)
			set->sig[i] = 0;
	}

will make asm worse...

Oleg.