Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20190123110349.35882-1-keescook@chromium.org> <20190123110349.35882-2-keescook@chromium.org>
 <20190123115829.GA31385@kroah.com>
In-Reply-To: <20190123115829.GA31385@kroah.com>
From:   Jann Horn <jannh@google.com>
Date:   Wed, 23 Jan 2019 13:09:13 +0100
Message-ID: <CAG48ez2vfXkr9dozJiGmze8k49VOXfs=K7M8bv0aQsDDpzrEFQ@mail.gmail.com>
Subject: Re: [PATCH 1/3] treewide: Lift switch variables out of switches
To:     Greg KH <gregkh@linuxfoundation.org>
Cc:     Kees Cook <keescook@chromium.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Laura Abbott <labbott@redhat.com>,
        Alexander Popov <alex.popov@linux.com>,
        xen-devel <xen-devel@lists.xenproject.org>,
        dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org,
        intel-wired-lan@lists.osuosl.org,
        Network Development <netdev@vger.kernel.org>,
        linux-usb@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Linux-MM <linux-mm@kvack.org>, dev@openvswitch.org,
        linux-kbuild@vger.kernel.org,
        linux-security-module <linux-security-module@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jan 23, 2019 at 1:04 PM Greg KH <gregkh@linuxfoundation.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 =E2=80=98send_sigio_to_task=E2=80=99:
> > 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 :(