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

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Message-ID: <871s53311w.fsf@intel.com>
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