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From: Jani Nikula <jani.nikula@linux.intel.com>
Subject: Re: [Intel-gfx] [PATCH 1/3] treewide: Lift switch variables out of switches
In-Reply-To: <20190123115829.GA31385@kroah.com>
References: <20190123110349.35882-1-keescook@chromium.org> <20190123110349.35882-2-keescook@chromium.org> <20190123115829.GA31385@kroah.com>
Date: Wed, 23 Jan 2019 16:17:30 +0200
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To: Greg KH <gregkh@linuxfoundation.org>, Kees Cook <keescook@chromium.org>
Cc: dev@openvswitch.org, Ard Biesheuvel <ard.biesheuvel@linaro.org>, netdev@vger.kernel.org, intel-gfx@lists.freedesktop.org, linux-usb@vger.kernel.org, linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linux-mm@kvack.org, linux-security-module@vger.kernel.org, kernel-hardening@lists.openwall.com, intel-wired-lan@lists.osuosl.org, linux-fsdevel@vger.kernel.org, xen-devel@lists.xenproject.org, Laura Abbott <labbott@redhat.com>, linux-kbuild@vger.kernel.org, Alexander Popov <alex.popov@linux.com>

On Wed, 23 Jan 2019, 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:
>>=20
>> 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-u=
nreachable]
>>    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 =3D 0; /* not valid C */
	case 0:
        	/* ... */
	}

but can have e.g.:

	switch (i) {
	case 0:
		{
			int j =3D 0;
	        	/* ... */
		}
	}

I think Kees' approach of moving such variable declarations to the
enclosing block scope is better than adding another nesting block.

BR,
Jani.


--=20
Jani Nikula, Intel Open Source Graphics Center