Re: module: Remove const attribute from alias for MODULE_DEVICE_TABLE

[Kees Cook]

On Sun, Aug 27, 2017 at 4:52 PM, Stefan Agner <stefan@xxxxxxxx> wrote:
> On 2017-07-24 18:27, Matthias Kaehlcke wrote:
>> MODULE_DEVICE_TABLE(type, name) creates an alias of type 'extern const
>> typeof(name)'. If 'name' is already constant the 'const' attribute is
>> specified twice, which is not allowed in C89 (see discussion at
>> https://lkml.org/lkml/2017/5/23/1440). Since the kernel is built with
>> -std=gnu89 clang generates warnings like this:
>>
>> drivers/thermal/x86_pkg_temp_thermal.c:509:1: warning: duplicate 'const'
>>   declaration specifier
>>       [-Wduplicate-decl-specifier]
>> MODULE_DEVICE_TABLE(x86cpu, pkg_temp_thermal_ids);
>> ^
>> ./include/linux/module.h:212:8: note: expanded from macro 'MODULE_DEVICE_TABLE'
>> extern const typeof(name) __mod_##type##__##name##_device_table
>>
>> Remove the const attribute from the alias to avoid the duplicate
>> specifier. After all it is only an alias and the attribute shouldn't
>> have any effect.
>
> Unfortunately, it has effect where const is missing in the original
> variable declaration:
>
> Before this patch:
> 13:10 $ size drivers/net/wireless/ath/ath6kl/ath6kl_usb.ko
>    text    data     bss     dec     hex filename
>    8825     728      40 9593 2579
> drivers/net/wireless/ath/ath6kl/ath6kl_usb.ko
>
> After this patch:
> 13:12 $ size drivers/net/wireless/ath/ath6kl/ath6kl_usb.ko
>    text    data     bss     dec     hex filename
>    8747     800      40    9587    2573
> drivers/net/wireless/ath/ath6kl/ath6kl_usb.ko
>
>
> Ideally we would fix all MODULE_DEVICE_TABLE usage sites. This would
> also made it clearly visible that the device tables are const.
>
> I created a semantic patch, it turns out that 620 sites are affected
> (out of 4499)...
>
> //
> // Cocinelle Semantic Patch to constify module device tables
> //
> // Author: Stefan Agner <stefan@xxxxxxxx>
> //
> @ module_device_table @
> declarer name MODULE_DEVICE_TABLE;
> identifier moduletype;
> identifier name;
> @@
> MODULE_DEVICE_TABLE(moduletype, name);
>
> @ add_const depends on module_device_table disable optional_qualifier @
> identifier module_device_table.name;
> type T;
> @@
> +const
>  T name[] = {
>  ...
>  };
>
> Thoughts?
>
> --
> Stefan
>
>
>>
>> Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
>> ---
>>  include/linux/module.h | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/module.h b/include/linux/module.h
>> index e7bdd549e527..fe5aa3736707 100644
>> --- a/include/linux/module.h
>> +++ b/include/linux/module.h
>> @@ -209,7 +209,7 @@ extern void cleanup_module(void);
>>  #ifdef MODULE
>>  /* Creates an alias so file2alias.c can find device table. */
>>  #define MODULE_DEVICE_TABLE(type, name)                                      \
>> -extern const typeof(name) __mod_##type##__##name##_device_table              \
>> +extern typeof(name) __mod_##type##__##name##_device_table            \
>>    __attribute__ ((unused, alias(__stringify(name))))
>>  #else  /* !MODULE */
>>  #define MODULE_DEVICE_TABLE(type, name)

Perhaps the reverse is the better solution? Leave "const" in
MODULE_DEVICE_TABLE and remove the redundant usage. This means new
cases of missing the const will never happen (which was the intent
originally of putting const into the MODULE_DEVICE_TABLE macro, I
assume).

-Kees

-- 
Kees Cook
Pixel Security