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

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Message-ID: <CAK8P3a2fuD-UBJET_OBKekCxrTDpnAxb0Bpu2LCCXaVT3pXTMQ@mail.gmail.com>
Date:   Thu, 7 Mar 2019 16:37:42 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Linux Kernel Mailing List <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 <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 Thu, Mar 7, 2019 at 4:28 PM Oleg Nesterov <oleg@...hat.com> wrote:
>
> 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
>
> ?

I would definitely not go that far, since the warning is generally
rather useful,
but we could try something more localized

__diag_push();
__diag_ignore(clang, 7, "-Warray-bounds");
...
__diag_pop();

> > 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.

I tried using just #ifdefs, but that did not work inside of macros.
We could use % everywhere, or possible wrap it inside of another
macro.

        Arnd