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

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Date:   Thu, 24 Jan 2019 07:55:51 +1300
From:   Kees Cook <keescook@...omium.org>
To:     Jani Nikula <jani.nikula@...ux.intel.com>
Cc:     Edwin Zimmerman <edwin@...mainstreet.net>,
        Greg KH <gregkh@...uxfoundation.org>, dev@...nvswitch.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Network Development <netdev@...r.kernel.org>,
        intel-gfx@...ts.freedesktop.org, linux-usb@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Maling list - DRI developers 
        <dri-devel@...ts.freedesktop.org>, Linux-MM <linux-mm@...ck.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        intel-wired-lan@...ts.osuosl.org,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>,
        Laura Abbott <labbott@...hat.com>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        Alexander Popov <alex.popov@...ux.com>
Subject: Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches

On Thu, Jan 24, 2019 at 4:44 AM Jani Nikula <jani.nikula@...ux.intel.com> wrote:
>
> On Wed, 23 Jan 2019, Edwin Zimmerman <edwin@...mainstreet.net> wrote:
> > On Wed, 23 Jan 2019, Jani Nikula <jani.nikula@...ux.intel.com> wrote:
> >> On Wed, 23 Jan 2019, 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?

Just in case this wasn't clear: no, it's just the switch statement
before the first "case". I cannot imagine how bad it would be if we
couldn't have block-scoped variables! Heh. :)

> >> >
> >> > That's going to be a hard thing to keep from happening over time, as
> >> > this is valid C :(
> >>
> >> Not all valid C is meant to be used! ;)
> >
> > Very true.  The other thing to keep in mind is the burden of enforcing
> > a prohibition on a valid C construct like this.  It seems to me that
> > patch reviewers and maintainers have enough to do without forcing them
> > to watch for variable declarations in switch statements.  Automating
> > this prohibition, should it be accepted, seems like a good idea to me.
>
> Considering that the treewide diffstat to fix this is:
>
>  18 files changed, 45 insertions(+), 46 deletions(-)
>
> and using the gcc plugin in question will trigger the switch-unreachable
> warning, I think we're good. There'll probably be the occasional
> declarations that pass through, and will get fixed afterwards.

Yeah, that was my thinking as well: it's a rare use, and we get a
warning when it comes up.

Thanks!

-- 
Kees Cook