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, 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.

BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center

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