Re: [XEN RFC] x86/uaccess: remove __{put,get}_user_bad()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.12.2023 13:01, Nicola Vetrini wrote:
> Hi Andrew,
> 
> On 2023-12-21 12:03, Andrew Cooper wrote:
>> On 21/12/2023 10:58 am, Jan Beulich wrote:
>>> On 21.12.2023 11:53, Federico Serafini wrote:
>>>> Remove declarations of __put_user_bad() and __get_user_bad()
>>>> since they have no definition.
>>>> Replace their uses with a break statement to address violations of
>>>> MISRA C:2012 Rule 16.3 ("An unconditional `break' statement shall
>>>> terminate every switch-clause").
>>>> No functional change.
>>>>
>>>> Signed-off-by: Federico Serafini <federico.serafini@xxxxxxxxxxx>
>>>> ---
>>>> Several violations of Rule 16.3 come from uses of macros
>>>> get_unsafe_size() and put_unsafe_size().
>>>> Looking at the macro definitions I found __get_user_bad() and 
>>>> __put_user_bad().
>>>> I was wondering if instead of just adding the break statement I can 
>>>> also remove
>>>> such functions which seem to not have a definition.
>>> No, you can't. Try introducing a caller which "accidentally" uses the
>>> wrong size. Without your change you'll observe the build failing (in
>>> a somewhat obscure way, but still), while with your change bad code
>>> will silently be generated.
>>
>> The construct here is deliberate.  It's a build time assertion that bad
>> sizes aren't used.
>>
>> __bitop_bad_size() and __xsm_action_mismatch_detected() are the same
>> pattern in other areas of code too, with the latter being more explicit
>> because of how it's wrapped by LINKER_BUG_ON().
>>
>>
>> It is slightly horrible, and not the most obvious construct for
>> newcomers.  If there's an alternative way to get a build assertion, we
>> could consider switching to a new pattern.
> 
> would you be in favour of a solution with a BUILD_BUG_ON in the default 
> branch followed by a break?
> 
> default:
>      BUILD_BUG_ON(!size || size >=8 || (size & (size - 1)));
>      break;

I don't think this would compile - BUILD_BUG_ON() wants a compile-time
constant passed.

Jan