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

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Message-ID: <87va2f1int.fsf@intel.com>
Date:   Wed, 23 Jan 2019 17:46:14 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Edwin Zimmerman <edwin@...mainstreet.net>,
        'Greg KH' <gregkh@...uxfoundation.org>,
        'Kees Cook' <keescook@...omium.org>
Cc:     dev@...nvswitch.org, 'Ard Biesheuvel' <ard.biesheuvel@...aro.org>,
        netdev@...r.kernel.org, intel-gfx@...ts.freedesktop.org,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-mm@...ck.org,
        linux-security-module@...r.kernel.org,
        kernel-hardening@...ts.openwall.com,
        intel-wired-lan@...ts.osuosl.org, linux-fsdevel@...r.kernel.org,
        xen-devel@...ts.xenproject.org,
        'Laura Abbott' <labbott@...hat.com>,
        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 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?
>> >
>> > 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.

BR,
Jani.

-- 
Jani Nikula, Intel Open Source Graphics Center