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Date: Wed, 23 Jan 2019 16:23:39 +0200
From: Jani Nikula <jani.nikula@...ux.intel.com>
To: 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, 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! ;)
>
> Anyway, I think you're mistaking the limitation to arbitrary blocks
> while it's only about the switch block IIUC.
>
> Can't have:
>
> switch (i) {
> int j;
> case 0:
> /* ... */
> }
>
> because it can't be turned into:
>
> switch (i) {
> int j = 0; /* not valid C */
> case 0:
> /* ... */
> }
>
> but can have e.g.:
>
> switch (i) {
> case 0:
> {
> int j = 0;
> /* ... */
> }
> }
>
> I think Kees' approach of moving such variable declarations to the
> enclosing block scope is better than adding another nesting block.
PS. The patch is
Reviewed-by: Jani Nikula <jani.nikula@...el.com>
and the drivers/gpu/drm/i915/* parts are
Acked-by: Jani Nikula <jani.nikula@...el.com>
for merging via whichever tree is appropriate. (There'll be minor
conflicts with in-flight work in our -next tree, but no biggie.)
--
Jani Nikula, Intel Open Source Graphics Center
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