netdev - Re: [PATCH 1/3] treewide: Lift switch variables out of switches

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Message-ID: <CAG48ez2vfXkr9dozJiGmze8k49VOXfs=K7M8bv0aQsDDpzrEFQ@mail.gmail.com>
Date:   Wed, 23 Jan 2019 13:09:13 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Kees Cook <keescook@...omium.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Laura Abbott <labbott@...hat.com>,
        Alexander Popov <alex.popov@...ux.com>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        intel-wired-lan@...ts.osuosl.org,
        Network Development <netdev@...r.kernel.org>,
        linux-usb@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>, dev@...nvswitch.org,
        linux-kbuild@...r.kernel.org,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: [PATCH 1/3] treewide: Lift switch variables out of switches

On Wed, Jan 23, 2019 at 1:04 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> On Wed, Jan 23, 2019 at 03:03:47AM -0800, Kees Cook wrote:
> > Variables declared in a switch statement before any case statements
> > cannot be initialized, so move all instances out of the switches.
> > After this, future always-initialized stack variables will work
> > and not throw warnings like this:
> >
> > fs/fcntl.c: In function ‘send_sigio_to_task’:
> > fs/fcntl.c:738:13: warning: statement will never be executed [-Wswitch-unreachable]
> >    siginfo_t si;
> >              ^~
>
> That's a pain, so this means we can't have any new variables in { }
> scope except for at the top of a function?

AFAICS this only applies to switch statements (because they jump to a
case and don't execute stuff at the start of the block), not blocks
after if/while/... .

> That's going to be a hard thing to keep from happening over time, as
> this is valid C :(