Re: [XEN PATCH 4/6] x86emul: address violations of MISRA C Rule 20.7

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.06.2024 11:52, Nicola Vetrini wrote:
> On 2024-06-12 11:19, Jan Beulich wrote:
>> On 11.06.2024 17:53, Nicola Vetrini wrote:
>>> MISRA C Rule 20.7 states: "Expressions resulting from the expansion
>>> of macro parameters shall be enclosed in parentheses". Therefore, some
>>> macro definitions should gain additional parentheses to ensure that 
>>> all
>>> current and future users will be safe with respect to expansions that
>>> can possibly alter the semantics of the passed-in macro parameter.
>>>
>>> No functional change.
>>>
>>> Signed-off-by: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>
>>> ---
>>> These local helpers could in principle be deviated, but the 
>>> readability
>>> and functionality are essentially unchanged by complying with the 
>>> rule,
>>> so I decided to modify the macro definition as the preferred option.
>>
>> Well, yes, but ...
>>
>>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>>> @@ -2255,7 +2255,7 @@ x86_emulate(
>>>          switch ( modrm_reg & 7 )
>>>          {
>>>  #define GRP2(name, ext) \
>>> -        case ext: \
>>> +        case (ext): \
>>>              if ( ops->rmw && dst.type == OP_MEM ) \
>>>                  state->rmw = rmw_##name; \
>>>              else \
>>> @@ -8611,7 +8611,7 @@ int x86_emul_rmw(
>>>              unsigned long dummy;
>>>
>>>  #define XADD(sz, cst, mod) \
>>> -        case sz: \
>>> +        case (sz): \
>>>              asm ( "" \
>>>                    COND_LOCK(xadd) " %"#mod"[reg], %[mem]; " \
>>>                    _POST_EFLAGS("[efl]", "[msk]", "[tmp]") \
>>
>> ... this is really nitpicky of the rule / tool. What halfway realistic
>> ways do you see to actually misuse these macros? What follows the 
>> "case"
>> keyword is just an expression; no precedence related issues are 
>> possible.
>>
> 
> I do share the view: no real danger is possible in sensible uses. Often 
> MISRA rules are stricter than necessary to have a simple formulation, by 
> avoiding too many special cases.
> 
> However, if a deviation is formulated, then it needs to be maintained, 
> for no real readability benefit in this case, in my opinion. I can be 
> convinced otherwise, of course.

Well, aiui you're thinking of a per-macro deviation here. Whereas I'd be
thinking of deviating the pattern.

Jan


F