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

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Date:   Wed, 23 Jan 2019 13:12:26 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Jann Horn <jannh@...gle.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...hat.com>,
        Alexander Popov <alex.popov@...ux.com>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        intel-gfx@...ts.freedesktop.org, intel-wired-lan@...ts.osuosl.org,
        Network Development <netdev@...r.kernel.org>,
        linux-usb <linux-usb@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Linux-MM <linux-mm@...ck.org>,
        dev@...nvswitch.org,
        Linux Kbuild mailing list <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, 23 Jan 2019 at 13:09, Jann Horn <jannh@...gle.com> wrote:
>
> 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/... .
>

I guess that means it may apply to other cases where you do a 'goto'
into the middle of a for() loop, for instance (at the first
iteration), which is also a valid pattern.

Is there any way to tag these assignments so the diagnostic disregards them?