linux-kernel - Re: [PATCH 01/17] kbuild: Disable -Waddress-of-packed-member for gcc 9

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Message-ID: <20190322163954.GG7905@worktop.programming.kicks-ass.net>
Date:   Fri, 22 Mar 2019 17:39:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Andi Kleen <andi@...stfloor.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>
Subject: Re: [PATCH 01/17] kbuild: Disable -Waddress-of-packed-member for gcc
 9

On Fri, Mar 22, 2019 at 02:58:51PM +0100, Arnd Bergmann wrote:
> On Thu, Mar 21, 2019 at 11:00 PM Andi Kleen <andi@...stfloor.org> wrote:
> >
> > From: Andi Kleen <ak@...ux.intel.com>
> >
> > This warning is very noisy in a default build with gcc 9.
> > Move it into W=2 only.
> >
> > Cc: arnd@...db.de
> > Cc: Masahiro Yamada <yamada.masahiro@...ionext.com>
> > Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> 
> I think W=2 is too aggressive. On many architectures, this finds
> real bugs and the false positives tend to be trivial to fix
> (by removing the bogus __packed annotation), which improves
> the generated code in the process.
> 
> Moving it to W=1 for the moment is probably fine, but ideally
> I think we should fix the kernel to behave according to the
> C standard.

Lol... we're actively moving away from the C standard on many places.

Why does the silly compiler think it is a problem to take the address of
a member of a packed structure? That sounds like something that's
perfectly fine and useful.