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

[Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx>]

On Wed, 23 Jan 2019, Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> wrote:
> On Wed, 23 Jan 2019, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxx>

and the drivers/gpu/drm/i915/* parts are

Acked-by: Jani Nikula <jani.nikula@xxxxxxxxx>

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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