Re: [PATCH v4 1/2] compiler_types: Introduce __flex_counter() and family

[Przemek Kitszel <przemyslaw.kitszel@intel.com>]

On 3/17/25 10:26, Przemek Kitszel wrote:
> On 3/15/25 04:15, Kees Cook wrote:
>> Introduce __flex_counter() which wraps __builtin_counted_by_ref(),
>> as newly introduced by GCC[1] and Clang[2]. Use of __flex_counter()
>> allows access to the counter member of a struct's flexible array member
>> when it has been annotated with __counted_by().
>>
>> Introduce typeof_flex_counter(), can_set_flex_counter(), and
>> set_flex_counter() to provide the needed _Generic() wrappers to get sane
>> results out of __flex_counter().
>>
>> For example, with:
>>
>>     struct foo {
>>         int counter;
>>         short array[] __counted_by(counter);
>>     } *p;
>>
>> __flex_counter(p->array) will resolve to: &p->counter
>>
>> typeof_flex_counter(p->array) will resolve to "int". (If p->array was not
>> annotated, it would resolve to "size_t".)
>>
>> can_set_flex_counter(p->array, COUNT) is the same as:
>>
>>     COUNT <= type_max(p->counter) && COUNT >= type_min(p->counter)
>>
>> (If p->array was not annotated it would return true since everything
>> fits in size_t.)
>>
>> set_flex_counter(p->array, COUNT) is the same as:
>>
>>     p->counter = COUNT;
>>
>> (It is a no-op if p->array is not annotated with __counted_by().)
>>
>> Signed-off-by: Kees Cook <kees@kernel.org>
> 
> I agree that there is no suitable fallback handy, but I see counter
> as integral part of the struct (in contrast to being merely annotation),
> IOW, without set_flex_counter() doing the assignment, someone will
> reference it later anyway, without any warning when kzalloc()'d
> 
> So, maybe BUILD_BUG() instead of no-op?

I get that so far this is only used as an internal helper (in the next
patch), so for me it would be also fine to just add __ prefix: 
__set_flex_counter(), at least until the following is true:
  "manual initialization of the flexible array counter is still
required (at some point) after allocation as not all compiler versions
support the __counted_by annotation yet"

> 
>> +#define set_flex_counter(FAM, COUNT)                \
>> +({                                \
>> +    *_Generic(__flex_counter(FAM),                \
>> +          void *:  &(size_t){ 0 },            \
>> +          default: __flex_counter(FAM)) = (COUNT);    \
>> +})
>> +
>>   #endif /* __LINUX_OVERFLOW_H */
>